There are two types of standard bipolar transistors, namely PNP & NPN transistors. In this article, one of them, namely PNP, will be discussed in detail.
Definition of PNP Transistor
PNP Transistor Symbol
Diagram
Configuration
Working Principle
Applications
Advantages-Disadvantages
PNP Transistor as a Switch
PNP vs NPN Transistor
PNP Transistor Definition
“A P-N-P Transistor is a BJT type built by merging an N-type semiconductor between two P-type semiconductors.”
PNP Transistor Diagram:
The transistor consists of three section-
E-Emitter
B-Base
C-Collector
On the subject of the working of three terminals of the PNP transistor,
The emitter is used to provide charge carriers into the collector through the Base area.
The Collector region gathers most of the charge carriers emitted in the emitter.
The base used to controls the quantity of current pass through the Emitter to Collector.
PNP Transistor Symbol
PNP Transistor Symbol
Where, E=Emitter, B=Base, C=Collector
The mid-layer (N-type) is termed as the B- Base terminal. The left-sided P-type layer works as an E- Emitter terminal and the right-sided P-type layer known as a C-Collector terminal.
PNP Transistor
In an N-P-N transistor formation, One P-type semiconductor material is fit in between two N-type semiconductors, as explained in the article (Link NPN transistor). Whereas in a P-N-P transistor, one N-type semiconductor is fit in between two P-type semiconductors material.
In a PNP transistor, two types of diodes are used. They are respectively P-N and N-P diode. These P-N junction diodes are called the collector-base or C-B junction and base-emitter or B-E junction.
In the P-type semiconductor material, the charge carriers are holes primarily. So, in this transistor, the current formation is due to the movement of holes only.
The (P-type) Emitter and Collector regions are comparatively doped more than the N-type Base. The regions of the Emitter and Collector regions are wider in comparison to the base.
An adequately more number of free electrons are available in an N-type semiconductor, usually. But, the width of the mid-layer is narrower and lightly doped in this case.
The Emitter-Base intersection is linked to forwarding bias. Along with also the +ve terminal of a voltage supply (VCB) is connected with all the Base terminal (N-type), and the -ve terminal is linked with all the Collector terminal (P-type). Consequently, the Collector-Base intersection is associated with reverse biasing.
As a result of this biasing, the depletion area at the E-B junction is less since it’s linked to forwarding bias. Even though the C-B junction is in reverse bias, the depletion area at the Collector-Base junction is wide enough. The E-B junction is forward biased. Therefore, more hole moves from emitters across the depletion area and acts as an input to the base. Simultaneously, not many electrons carried in an emitter in the base and recombined with the holes.
But the amount of electrons at the base is minimal since it’s a reasonably less doped and narrow area. Therefore, almost all Emitter regions’ holes will pass the depletion region and carried into the Base regions.
The current will pass through the E-B junction. This is Emitter current (IE). So IC, the Collector current will pass through the Collector-Base layers because of holes.
PNP Transistor Circuit
PNP Transistor Circuit
When a PNP transistor is linked with voltage resources, the base current will be carried in the transistor. Even the little quantity of base present controls the circulation of a massive number of current through the emitter to collector supplied the Base voltage is more -ve compared to Emitter voltage.
When VB the base voltage isn’t -ve in comparison to the VE the emitter voltage, the current can’t pass within the circuit. So, it’s necessary to provide a voltage supply in reverse bias > 0.72 Volt.
The resistors RL and RB are connected in the circuit. That to restricts the current to pass through the transistor’s maximum possible height.
The Emitter’s voltage is VEB as input side. Here the emitter current (IE) flows from the input side, and it flows in two directions; one is IB and other is IC.
IE= IB+ IC
But only 2 to 5 % of the total current flows in the IB, so IB is negligible.
Advantages of PNP Transistor
Small in size and could be utilized as a part of IC design.
Comparatively cheap, long-lasting and simpler circuit.
Spontaneous actions available
Low supply voltage requirement and less output impedance.
Produce less noise than NPN Transistors.
Disadvantages of PNP Transistor
Not suitable to operate on high-frequency application.
Perform slowly in comparison to NPN.
Temperature sensitivity and may get damaged during a thermal runaway.
PNP transistors Applications:
PNP transistors are applied as switches, i.e., analog switches, emergency push button etc. They have applications when emergency shutdown required.
These types of transistors are used in current sources circuitry, i.e., by exploiting the characteristics of current flows out of the collector.
The P-N-P type transistors are used in heavy motors to control current flow and various robotic and microcontroller design applications.
PNP Transistor as a Switch
Once the switch is ON, the current will pass through the circuit and also behave as a close circuit. The transistor is an analogue power electronics-based circuitry with changeover characteristics which may function like ordinary switches.
As we have observed in the P-N-P transistor’s working, When the Base voltage isn’t more –ve than the VE, the current will not able to pass through the circuit. Thus, VB is at least 0.72 Volt in reverse bias connection to operate the transistor.
So, if the VB is 0 or > 0.72Volt, the current will not pass and operate as an open switch.
PNP vs NPN Transistor
PNP Transistor
NPN Transistor
PNP stands for Positive-Negative-Positive transistors
An NPN Transistor stands for Negative-Positive-Negative transistor.
A PNP transistor needs negative current flow from the base to emitter.
An NPN transistor needs positive current flow from the base to emitter.
A PNP transistor gets positive voltage at the emitter terminal. This +ve voltage permits the current emitter to collector.
An NPN transistor gets +ve voltage in the collector terminal. This +ve allows the current to flow from collector to emitter.
In the case of PNP transistor, the current directed from the emitter to base. Once the transistor is switched ON, current pass through the emitter to collector. When current is supplied from the transistor base to emitter in an NPN transistor, the transistor base gets a positive voltage, and the emitter receives a negative voltage. Thus the current flows into the base. When there is enough current flowing from base into the emitter, the transistor is switched ON, and it directs the current flow from the collector to emitter instead of base to emitter.
“Transistor is a semiconductor device with three connection parts. This device is mainly used for amplification to switching electronic signals application”.
Transistor Characteristics:
A transistor represents the relation between current and voltages.
It is a two-port network in general
Each of the transistor modes has different input characteristics, output characteristics, and current transfer characteristics.
A transistor has three poles, and each of the poles is made of N-type & P-type substrate mainly.
To know more about PNP and NPN transistors, first, we have to know about P-type and N-type semiconductors.
What is a P-type Semiconductor?
A P-type semiconductor (link) is a type of semiconductor when some impurity (mainly trivalent) is added to the intrinsic or pure semiconductor. In these types, the holes are majority and electronics are minority carriers. The trivalent impurities can be Boron (B), Gallium (Ga), etc.
What is N-type Semiconductor?
An N-type semiconductor is a type of semiconductor when some impurities (mainly pentavalent) are doped to an extrinsic semiconductor. In this, electrons are majority or primary carriers, and holes are minority or secondary carriers.
Some of the examples are Phosphorus (P), Arsenic (As) etc.
In N-type and P-type semiconductors, we observe different types of ‘energy bands’ which plays an important role in the function of a transistor; they are:-
“The Band Gap refers to the energy difference between the top of the valance band and the bottom of the conduction band in an insulator and semiconductor.”
– This is an energy range for solid basically where no electron states can be existent.
Band Gap Diagram
Forbidden Gap
– In a solid, the range of energies than an electron within solid may have an energy band, and a range of energy that it may not have is called the forbidden gap.
In solid states, valance band and conduction bands are the bands closest to the Fermi level (a thermodynamic quantity denoted by µ) and determine the solids’ electrical conductivity.
Valance and conduction Band
To build up a transistor, we need two types of semiconductors, which are:
1. Intrinsic semiconductor
Intrinsic semiconductor
– Materials are in pure form
– Low electrical conductivity
– No. of free electrons in conduction band = No. of the holes in the valance band
– Electrical conductivity be influenced by on the temperature.
2. Extrinsic semiconductor
Extrinsic semiconductor
Extrinsic semiconductors are divided into further two types
n-type
p-type
– Impure material doped with p-type and n-type dopants
– Numbers of holes and electrons are not equal
– High electrical conductivity
– Impurities like Sb, P, ln, Bi are doped with Silicon and Germanium atoms.
Direct and indirect bandgap
In semiconductor electronics, a semiconductor’s bandgap can be classified in basic forms as follow:
Direct bandgap
Indirect bandgap.
Direct Bandgap
Indirect bandgap
Dependent on the band structures, substances have a direct bandgap or indirect bandgap.
The direct band-gap occurs when the momentum of the low-energy level from conductive region and high-energy level from valence region are similar.
The in-direct band-gap occurs when the momentum of the low-energy level from conductive region and high-energy level from valence region are not similar.
When an electron has sufficient energy, they can reach to the conductive band. In this process, photons are being emitted.
For an indirect bandgap material, both photon and phonon has be included in a transition from upper valence band top to the lower conduction band.
The max-energy state in the valence band and the min-energy state in the conduction band are distinguished by the Brillouin zones k-vector or a particular crystal momentum. In the event the k-vectors are distinct, the substance has an “indirect gap”. The bandgap is known as direct if the crystal movement of holes and electrons is the equal in the conduction and valence bands; an e– could emit a photon. A photon can’t be emitted within an “indirect” gap since the electron has to pass through an intermediate one and transfer momentum into the crystal lattice.
What is semimetal material?
In certain substances with a direct gap, the value of the difference is negative. Such substances are called semimetals.
Moss–Burstein Effect
The Moss-Burstein effect or Burstein-Moss shift is the prodigy where the bandgap of a semiconductor may increase.
This is witnessed for a degenerate electron distribution or in some variant of semiconductors.
As per Moss-Burstein shift the Band Gap is
Moss–Burstein Effect
Apparent Band Gap = Actual Band Gap + Moss-Burstein shift
In ostensibly doped semiconductor, the Fermi level is to be found between the valence and conduction bands.
For example, in an n-type semiconductor, as the doping concentration increases, electrons populate in the conduction regions that compels the Fermi level to higher energy label.
The Fermi level is located in the conduction band for degenerate amount of doping. Pauli’s exclusion principle prohibits excitation for these pre occupied states. Thus an increase s observed apparently in the bandgap.
In this article, different transistor types will be discussed, primarily related to bipolar junction transistor (BJT) and field-effect transistor (FET) and their characteristics. However, transistors have been used as an amplifier in different circuits and various stages, mode, configurations, etc. That will also be discussed.
Although there are diverse classification of the amplifier as per different parameters are as follows:
Transistor Amplifier Classification
Transistor AmplifierClass: as per the number of stages
As per the number of stage of amplification, there are two class is available in transistor amplifiers, are
Single-stage Amplifier
− The circuit comprising one transistor circuitry for only step of amplification.
Multi-stage Amplifier
– This circuitry has multiple transistor circuits that be responsible for multi-stage amplification in the course of operation.
Transistor AmplifierClass: as per the input signal
As per the input signal’s level the categorization are as follows:
− If the input signal is very weak to generate minor or insignificant fluctuations in the collector current than quiescent value, then it’s termed as a small-signal amplifier circuit.
Large signal amplifier
− If the fluctuations existing in collector current are to be high enough, then it’s is termed as a large-signal amplifier circuit.
Class as per its output
If the output is considered as parameters, then the amplifier can be of two types. They are – Voltage Amplifiers and Power Amplifiers.
Voltage Amplifier
− It is the amplifier circuit that increases the input signal’s voltage level (V0) is called a Voltage amplifier.
Power Amplifier
− It is the amplifier circuit that increases the input signal’s power level (P0) is called a Power amplifier.
Transistor AmplifierClass: as per the frequency range
As per the signals freq. range, there are two types of an audio amplifier and radio amplifier.
Audio-Amplifier
− The audio amplifier circuit capable of amplifying the input signal in the range marked for audio signals, i.e., Frequency Range: from 20Hz to 20 kHz range.
Radio-Amplifier
−The radio amplifier capable of amplifying the input signal in the radio frequency range or lie in a very high-freq. range.
Transistor AmplifierClass: as per Biasing and mode
As per the biasing and mode of operation, classifications are class A, class B, class C, and Class AB type transistor amplifiers. The condition is as follows:
Class-A Amplifier
− The collector current carried through for the entire cycle (One Cycle) of applied alternative current signal.
Class-B Amplifier
− The collector current pass through for half-cycle (equal to 0.5 Cycle) of applied input alternative current signal.
Class-C Amplifier
− The collector current carried for the less than half the cycle (< 0.5 Cycle) of applied input alternative current signal.
Class AB amplifiers
− Class AB amplifiers: Class AB amplifiers are formed by combining A and B classes. It helps to achieve all the gains as well as it eliminates the negatives.
Transistor AmplifierClass: Based on Configuration
Transistor Amplifier Classes: There are three types on the basis of configurations. They are – Common Emitter, Common Collector and Common Base types.
C E or Common Emitter Amplifier Configuration
− The amplifier circuit formed using a Common Emitter configured transistor combination is called a CE amplifier.
C B or Common Base Amplifier Configuration
− The amplifier circuit formed using a Common Base configured transistor combination is called a CB amplifier.
CC or Common Collector Amplifier Configuration
− The amplifier circuit formed using a Common Collector configured transistor combination is called a CC amplifier.
Transistor AmplifierClass: Based on Coupling method
There are three types on the basis of Method of coupling. They are – Resistor-Capacitor Coupled, Transformer Coupled, and the last one is the Direct Coupled.
Direct-Coupled Amplifier
− If a multi-stage amplifier is coupled directly to the subsequent stage.
RC-Coupled Amplifier
− A Multi-stage amplifier coupled to the subsequent stage using a resistive and capacitive (RC) element via a combination circuit then it is termed as an RC coupled amplifier.
Transformer-coupled Amplifier
− A Multi-stage amplifier coupled to the subsequent stage by means of a transformer based circuit, then it is a transformer coupled amplifier.
The Types of Transistors:
There are several transistors available in the market as per different applications. The important types are as follows.
Transistor Type
Bipolar Junction Transistor (BJT)
“A BJT is a type of transistor, has both electrons and holes. Electrons, as well as the holes, act here as charge carriers.”
Bipolar junction transistor is a current controlled device.
A Bipolar junction transistor (BJT) has two PN junctions for its functioning.
There are two types of standard transistors, which are bipolar; PNP & NPN.
There are three leads in a transistor, labeled as Base (B), Collector(C), and Emitter (E).
PNP Transistor
In P-N-P transistors, two types of diodes are assembled here. They are P-N and N-P.
The transistor consists of three section-
− Base
− Collector
− Emitter
In the PNP configuration, the transistor P junction has many holes, and the intermediate junction called N has efficiency and electrons. Now, the EB junction becomes the reverse biased and the CB junction becomes revere bias.
Due to the connection the bias formed and the holes started flowing from P junction. After that, the flow continues towards the N region. Here recombination takes place. The rest of the holes again flow towards the N. Now, the current through the emitter is known as Emitter Current which goes into two side. One is the Base Current another is the Collector current.
IE=IB+IC
But 2% of the total current flows in the IB, so IB is negligible.
Hence, IE=IC
NPN Transistor
In the NPN configuration of a transistor, two types of diodes are used: N-P & P-N.
As mentioned earlier, a transistor has three Terminals. They are – Collector, Emitter and Base.
Due to the connection the bias formed and the holes started flowing from N junction. After that, the flow continues towards the P region. Here recombination takes place. The rest of the holes again flow towards the P. Now, the current through the emitter is known as Emitter Current which goes into two side. One is the Base Current another is the Collector current.
IE=IB+IC
Field Effect Transistor (FET):
In a field-effect transistor, only an electrical field is used to control the flow of current. They have three terminals, which are Source, Drain, and Gate. FETs are unipolar transistors.
Difference between Step up and step down transformer
Difference between single phase and three phase transformer
Types of Transformers
There are many types of transformers based on classification parameters described below. We will discuss some of the transformer types and their workings. List of transformers we will discuss are the followings –
There are different types of transformer classification parameters based on which we can classify the transformer. Some of them are –
Voltage Class: Transformer can be classified based on the voltage used by them. From a few volts to megavolt amount of voltage can be used by transformers.
Power Rating: Transformers have ratings range from few Volt-Amperes to mega Volt-Amperes.
The number of turns in primary and secondary windings: Step down transformer, step-up transformer.
Construction of core: Depending on the transformer’s core construction, they can be classified into two types. They are shell types and core types.
Cooling Type: Transformers can be classified upon the cooling types. There are several types of transformer – self-cooled, oil-cooled, forced cooled, etc.
Application type: Based on transformer’s various applications like – energy transfer, power distribution, voltage-current stabilizer, isolation, etc., they can be classified into enormous kinds.
Ideal Transformer
Ideal transformers are theoretical transformer that suffer no losses and provides 100% efficiency. An ideal transformer can not be made in reality and present only in imagination.
Real Transformers
Every transformer which we can use in the real world is real transformer.
A real transformer can not achieve 100% efficiency as it will suffer some loss of power. There are many types of transformer power-loss can be found. Some of them are – Eddy current loss, Hysteresis loss, dielectric loss, etc.
Step-up transformers
This types of transformer increases voltage, which is applied to primary windings. The secondary windings supply the higher voltage.
The number of turns of the secondary transformer is higher than the number of turns in the primary windings.
Step-up transformers found its application in transmission line carrying high voltage.
Step-down transformers
This types of transformers does the opposite of a step-up transformer.
Step-down transformers reduce the voltage that is applied to its primary windings. The secondary windings supply the lower voltage. Many home appliances, power distribution systems, and many other electrical fields use this type of transformer.
Power transformers
Power transformer is the one particularised for the distribution of power. They are very high rated transformers and are designed for 100% efficiency. They are extensive and useful for delivering needed and limited power for consumers.
Transformer working on faraday’s law and having two windings are single-phase transformers. The windings are known as primary and secondary windings. Without varying the frequency and power, this transformer transfers AC energy.
Three single-phase transformers are connected to form a three-phase transformer. All three primary windings are combined to form a single primary winding, and also all three secondary windings are combined to form a single secondary winding. Star and delta are the types for primary and secondary connections. The combination of primary and secondary windings are all possible combination of star and delta type.
This types of transformer is generally used for industrial purposes.
Assembling of three single-phase transformer is less costly than buying a three-phase transformer.
A centre tapped transformer works almost in the same way a normal transformer works. The only difference is that its secondary windings has two parts and from that individual voltages can be acquired. The tapping point lies in the centre of the secondary windings and that divides the secondary windings. The tapping point provides a common connection for opposite and equal secondary voltages.
Diagrammatic Representation of a Centre Tapped Transformer
Instrument transformers
Instrument transformer is a special type of transformer used for transforming or isolation of current and voltage. It is a high accuracy device. An instrument transformer’s main use is to isolate high voltage connected primary windings from the meter connected with the secondary windings.
It has two types. The series-connected type is known as the current transformer, while the parallel-connected transformer is known as potential or voltage transformer. Current transformer steps down the current while the voltage transformers do the same for voltage of a supplied power.
Some advantages of using Instrument transformer are that – large current and voltage of Alternating Current power can be measured by using a low power rated instrument transformer, many measuring instruments can be connected using a single instrument transformer to power system, measuring instruments can also be standardized.
Another special type of transformer is the pulse transformer. It is used for transmitting rectangular electrical pulses. It transmits pulse of voltages between load and the windings. It has high open-circuit inductance, distributed capacitance, and low leakage induction. Depending on the types, it has several applications. Small versions are used in digital logic circuits. Medium versions are used in power-controlling systems. In contrast, larger versions are used in the power distribution system. Various pulse transformers have a wide range of applications like radar, power semiconductors, and high energy power applications.
There are some parameters which measures the performance of a pulse transformer. Some of them are – repetition rate, pulse width, duty cycle, current, frequency, input – output voltages, etc.
The main advantages of a pulse transformers includes that they are small in size, less costly, provides a high isolation voltage and operates at high frequency. The disadvantage includes – saturation current of the core can get reduced because of the direct current through the primary windings.
The transformers used in the radio frequency domain is known as RF transformer. This devices transfers energy in circuits with the help of electromagnetic induction. Steel as a core structure is prohibited in this type of transformer. It has several types too. Air core(low inductance, PCB use), Ferrite core(baluns for TV and radios), and transmission line transformers are some types. Low power circuit is ideal for the use of this transformers. Some important specifications of a RF transformers are – range of operating frequency, bandwidth, unbalance amplitude and phase, operating temperatures etc.
Audio Transformers
The transformers used in audio circuits are known as audio transformer. Audio transformer has various applications.
Previously audio transformers were made to isolate different telephone systems while keeping their power supplies isolated. Carrying an audio signal is its main objective. It can be used to match impedance like low impedance loudspeaker can be matched with high impedance amplifiers.
Audio transformers also do interconnection of professional audio system components, elimination of buzz and hum. Loudspeaker transformer, inter-stage and coupling transformers, small-signal transformers are some of its types.
A transactor is a combined device of the reactor (inductor or choke coil) and a transformer. Air – core present in the device used to limit the coupling between windings.
Difference between Step up and Step down transformer
Subject of Comparison
Step down transformer
Step up transformer
Number of turns in windings
Higher no. of turns in primary windings, lower no. of turns in secondary windings.
Lower no. of turns in primary windings, higher no. of turns in secondary windings.
Working
Reduce the input voltage applied in the primary windings.
Increases the input voltage applied in the primary windings.
Voltage -Current
High input voltage, Low output voltage and high current in the secondary side.
Low input voltage, high output voltage and low current in the secondary side.
Size of conductor
Secondary windings are made up of thick insulated copper wire.
Primary windings are made up of thick insulated copper wire.
Power Rating
Comparatively lower than Step-up transformer. The range lies under 110 volts.
Comparatively higher than step down transformers. Rated above 11,000 volts.
Uses
Many home appliances, voltage converters.
Power distribution system, X-Ray machines etc.
Types of Transformer, Table – 1
Difference between Single phase and three phase transformer
““Any changes in the coil of wires’ magnetic-fields, will cause an induction of emf. The magnitude of the induced potential is identical to the flux’s changing rate.
It can be written as –
E = – N * dϕ/dt
E is the induced emf, & N, ϕ is the number of turns and the magnetic flux produced, respectively.
The negative sign represents that the change in the magnetic field’s direction is opposite to induced emf. It is also known as Lenz’s law.
Now, we know that transformers have two windings. The alternating power is applied to the primary windings. The flow of current causes generation of a magnetic field around it. This property is known as mutual inductance. Now the current flows according to Faraday’s Law. The maximum strength of the magnetic field will be equal to d ‘phi’/dt. Magnetic lines of force now expand outside from the coil. The soft iron core concentrates the field lines and forms a path. The magnetic fluxes connect the primary windings as well as the secondary windings.
Now, as the flux also passes through the secondary windings, a voltage generates there. The induced emf’s magnitude will be given according to Faraday’s law. It will be = N * dϕ /dt.
The frequency and power of the supplied voltage never change in the whole process.
f is the frequency of the flux and N is the number of turns
Now, E = N * dϕ/dt
or, E = N * d (ϕm sin (2 * π * f * t)) /dt
or, E = N * 2 * π * f * ϕm * cos (2 * π * f * t)
For E =Emax, cos (2 * π * f * t) = 1
Emax = N * 2 * π * f * ϕ m
Now, Erms = Emax / √ 2
Erms = N * 2 * π * f * ϕm / √2
Erms = 4.44 f * N * ϕm
This is known as Transformer EMF Equation.
Losses of a Transformer
Loss in an electrical device or circuit means loss of power. A real transformer has different types of losses, but an ideal transformers never suffers a loss. There are several types of loss inside a transformer. Some of them are –
A. Core Loss / Iron Loss
B. Copper loss or Ohmic Loss
C. Stray Loss
D. Dielectric Loss
A. Core loss / Iron Losses:
The loss occurs due to alternating flux, inside the iron core is known as Core Loss or Iron loss. This type of losses are known as No Load Losses.
There are two categories of core loss. They are –
i) Hysteresis Loss
ii) Eddy Current Loss
i) Hysteresis Loss –
An alternative magnetic force generates in the core of the transformer. That magnetizing force causes a hysteresis loop and that causes hysteresis loss.
Ph = η * Bmax * n * f * V
Ph = Hysteresis Loss
η = Steinmetz hysteresis coefficient
Bmax = Maximum flux density
n = Steinmetz exponenet
f represents the magnetic reversal per second
V = volume of magnetic material
Hysteresis loss contributes 50% of no load loss.
ii) Eddy Current Loss –
Faraday’s Laws are behind the cause of Eddy Current Loss. The magnetic-fluxes cause a potential in the core. Now, due to this emf, current flows. This current is termed as Eddy Current and it is an undesired current. Loss due to this current is Eddy Current Loss.
The eddy current loss is expressed as –
Pe = Ke * Bmax2 * f * V * t2
Pe = Eddy Current loss
Ke = Eddy current constant
Bmax refers to the maximum flux density and f is the frequency of the magnetic reversal per second.
V = volume of magnetic material
t = magnetic thickness
B. Copper loss or Ohomic Loss:
This type of loss occurs due to the windings’ wire resistance. If Ip, Rp is current and resistance of primary winding and Is, Rs is current and resistance of secondary windings, then the loss will be given by the equation –
Po = Ip2Rp + Is2Rs
As the wires are of coppers’, the loss is termed as Copper loss. This type of loss is also known as Load Losses because this loss occurs only when load is connected with the secondary windings.
C. Stray Loss:
The reason behind such losses is the leakage field. It is a negligible loss.
D. Dielectric Loss:
The transformer’s insulator causes this type of loss.
There are also losses due to distorted voltage and currents.
The efficiency is the ratio of the produced power in the input to the supplied power of the output. It is represented as – η.
η = Output power /Input Power * 100%
In an ideal transformer, η comes as 1, which means Output power is equal to the input power. But in reality, a transformer suffers losses.
Loss = Input Power – Output Power
Or, Output Power = Input Power – Loss
So, Efficiency –
η = (Input Power – Loss) / Input Power * 100%
η = 1 – loss/ Input Power * 100%
Frequently Asked Questions
1. How is a transformer rated?
Transformers are rated in volt-amperes or kilo-volt-amperes (kVA). This rating indicates that the primary windings and the secondary windings are designed to tolerate the rated power.
2. How many types of Transformers are there?
There are many types of transformers based on different parameters. Some of them are –
3. A transformer has a turn’s ratio of 16 to 4 or 4. If the transformer secondary voltage is 220 V, determine the primary voltage.
We know that
Turns ratio =NpNs =VpVs
Here, Np = 16
Ns =4
Vs = 220 v
we have to find Vp
so Vp = Np*Vs/Ns = 16 * 220 / 4
Vp = 480 volt.
So the primary voltage was 480 volt.
4. What is the Reversibility of Transformer Operation?
Reversibility of Transformer Operation means using the transformer from backward. That is, giving the secondary windings an input voltage and connecting load at the primary windings.
5. Do the transformers perform in DC voltage?
No, a transformer does not perform in DC voltage. Applying Dc voltage will cause over hitting of the primary windings as the signal finds it a short-circuit.
6. What is Impedance matching?
The concept of impedance matching is that when a source voltage is connected to load, the load get the maximum power if the impedance of load is equal to the impedance of the impedance of the fixed internal source .It is one of the application of transformers.
7. A single phase transformer is with a rating of 2 kilo volt ampere has a 400v at primary windings and a 150v at secondary windings. Find out the primary and secondary full load current of the transformer.
Primary full-load current = 2kVA x 1000 / 400 V = 5 A
Secondary full-load current = 2kVA x 1000 / 150 V = 13.33 A
8. A transformer has 500 turns in the primary windings and 20 turns in the secondary windings. Find out –
a) The secondary voltage if the secondary circuit is open and the primary voltage is 100 v
b) Find out the current in primary and secondary windings when the secondary winding is connected to a resistance load of 16 ohms.
We know that turns ratio is given by
Turns ratio = Np/Ns = Vp/Vs
Np is the number of turns in primary windings.
Ns is the number of turns in secondary windings.
Vp is the voltage at primary side.
Vs is the voltage at secondary side.
Now we can write
Vs = (Ns * Vp) / Np
Or, Vs = (20*100)/500 V
Or, Vs = 4 V
Now for the second case, we know that power remains unchanged while transferring energy through a transformer.
We can write,
Pp = Ps
Where Pp is the power in primary side and Ps is the power from secondary side.
Pp = Vp * Ip
Ps = Vs * Is
Ip is the current in primary side and Is is the current in secondary side.
So, Vp *Ip = Vs * Is
Or, Ip = (Vs * Is) / Vp
Or, Ip = ((Vs*(Vs/Rs) / VpFrom ohm’s law V= IR, thus I = V/R, Here Rs is the resistance of the secondary coil.
Or, Ip = (Vs * Vs )/ (Vs * Rs)
Or, Ip = 4*4 / 100*16, Substituting the values and Rs = 16 ohm was given in the question.
So, Ip = 10 mili – ampere.
And, Is = Vs/ Rs
Is = 4/16 A = 0.25 A
To know more about transformers and electronics click here
As the name suggests, an Electrical transformer transfers energy. A formal transformer definition will be –
“It is a device that transfers electrical energy between electrical circuits.”
It is a passive device. It uses Faraday’s law to transfer energy without any metallic contact. Electrical transformers are one of the useful and needed device for the power distribution.
History related to transformers
Miksa Deri, Otto Blathy and Karoly Zipernowsky are considered to be first designer of the first transformer. They also implemented transformer for commercial systems. Though the Law of induction was given by Faraday in 1830’s and Rev. The induction coil was invented by Nicholas Callan in the year 1836. In the meantime Thomas Alva Edison came up with the idea of Electric Bulb in the year 1882.
Basic Structure of Electrical transformers
A single phase electrical transformer consists of three main components. They are – Primary Windings, Secondary Windings & the Magnetic Core.
Primary Windings – It is the part that is connected with the source. It is made up of coils of wire. Magnetic flux initially produces here.
Secondary Windings – It is the part that is connected with the load. It is also made up of coils. There is a turn’s ratio that defines the number of turns of the wire to make both the windings’ coils. There is no metallic connection between primary windings and secondary windings, as mentioned before.
Magnetic Core – It is the iron structure that wraps up both the primary and secondary windings. It is a soft iron core, made up of small elements to reduce the core’s losses.
The construction of Electrical transformers depends on how the primary and secondary windings are wrapped around the iron core structure.
There are two categories of transformers. One is Closed-core type and another is Shell-core type.
A. Closed Core Transformer –
Here, both the windings are wrapped from outside of the core. (Both windings means – Primary Windings and Secondary Windings). In this construction, windings wrap-up every legs of the core. Half of the primary windings and half of the secondary windings are kept on over other densely on each leg. Magnetic flux passes by this process and increases magnetic coupling. This type of transformer has a drawback, known as – ‘leakage flux.’
Structures of of core type transformers
B. Shell Core Transformer –
In this type, both the primary and secondary windings are inside the iron core. Here, the iron core forms a shell-like Structure for the windings, that’s why it is known as Shell Core Transformer. The windings share the same center leg, which has a cross-sectional area twice as the outer legs. This type of transformers overcome the issue of ‘leakage flux.’
Windings: Windings are the current-carrying part of the transformer. Mainly copper or aluminium wire is used to make the coil of the windings. Transformer coils and windings can be classified into two main categories. They are – Concentric Coils and Sandwich Coils. Sandwich Coils are generally used in Shell Type Transformer. Alternate discs are made to spiral form.
There are also Helical Windings, which are used in low voltage, high power applications. There are some insulators inside every type of windings. Insulators are one of the important elements for electrical transformers.
Cooling: Cooling of a device helps the machine to operate more years flawlessly. Some electrical transformers need forced cooling, and some are self-cooling types. Forced cooling includes cooling by oil, water, or both. Large transformers with high power ratings are filled with transformer oils, which cool and insulate windings. Some transformers are filled with gases for cooling.
Insulation: Insulation is necessary between turns of windings, between two windings, between core and windings. Layers of papers and polymer films are used as insulators. Large insulators use transformer oil as insulation purposes.
Bushing: Bushing is a hollow electrical insulator that allows a conductor to pass through a barrier. Large, high rated transformers has bashings made up of porcelain or polymers.
The polarity check of Electrical Transformers
An Electrical transformer’s polarity is defined as the direction of induced emf in both the primary and secondary windings. It is of two types –
A. Additive Polarity
B. Subtractive Polarity
A. Additive Polarity
-In this type of polarity, the same polarity terminals are connected in both the windings.
B. Subtractive Polarity
– In this type of polarity, different polarity terminals are connected in both the windings.
What does the transformer do?
Electric transformers increases or decreases the supplied voltage and current. It does not change the frequency or the power of the supplied electrical signal. The need for using a transformer is that electrical appliances need a certain amount of voltage, which is lower or higher than the supplied power. For example, a LED which works on 1.5 volts – 2 volts will blow out if we connect it to a normal household rated power supply. So we need to use a step down transformer to use the LED.
Click hereto know about Working Principles, Efficiency and Losses of a transformer.
Application of a Transformer
Transformers has a lot of applications in today’s world. Some of them are –
i) Power Distribution:
A large amount of voltage is produced in the power stations. But we cannot use that voltage directly for our household applications. In this time, a transformer comes into action. Transformers stepped down the voltage to our required voltage. This type of transformer is known as power transformers. There are also transformers which steps up the voltage. Because of this type of transformer it is possible to provide electricity to houses.
This type of transformers allows telephonic circuits to allow a two-way conversation over a single pair of wire. They also interconnections between audio systems. It can be used to match impedance like low impedance loudspeaker can be matched with high impedance amplifiers.
Three phase transformers have wide use in industrial purposes where single phase transformers can not serve the purposes.
Instrument transformers can isolate two device or system using its properties.
Radio frequency transformers or RF transformers are used in Radar like devices and has application in radio frequency domain.
Pulse transformers are used for transferring electric pulses in electronic circuits, digital circuits and in power distribution and controlling system.
Advantages & disadvantages of using a Transformer
Advantages of Electrical transformers
Transformers are used for various purposes because of its advantages. Some of the advantages are –
Transmits Power: Transformers allow transmitting of electrical signal over long distance. The resistance of the transmission line get reduced after increasing the voltage and that is possible only by transformers. Thus, the power loss is less and electricity can be supplied to every household. Otherwise the resistance would be so high that it is quite impossible to supply.
Continuous workings: Transformers can work continuously over long times. It does not need to switch off in a day or give rest.
Low Maintenance: Transformers not only works continuously but also they need not high maintenance. Checking oil, cleaning the parts are the only maintenance a transformer needs. Also, the maintenance does not cost much and also not time consuming.
No delay: Transformers has no delay while starting. It starts operation immediately. Once a transformer is implemented, it starts immediately.
Efficient: Though transformers suffer losses but they are efficient enough for distribution economically. Almost 95% efficiency is achievable.
Disadvantages of using Electrical Transformers
Few disadvantages are –
Larger in size : Though there are transistors that are small in size but as the voltage rating increases the transformer size get increased too. Not only the basic structure increases, the cooling system size get increased too. So it takes a lot of space to accommodate.
Requires a cooling system: Transformers operates continuously and it produces a lot of heat. So to operate in a efficient way, a transformer need a cooling system attached with it.
AC working only: Transformer works only for alternating current or AC voltages as it need time varying current to produce magnetic flux. Connecting with a DC voltage will burn out the transformer.
Analog Filter related selected MCQ Questions has been discussed in this article particularly for Core Technical Round Electronics domain interview. This is useful for different competitive and Semester Exam.
Q. The input-terminals of an op-amp are termed as
High & low terminals
Differential & non-differential terminals
Inverting and non-inverting terminals
Positive & negative terminals
Ans-(3)
Q. In a series resonance circuit, to obtain a LPF character, across which, output voltage should be measured?
Inductive element
Resistive element
Capacitive element
All of these
Ans-(3)
To learn about low pass filter and it’s characteristics click here
Q.In a series resonance circuit, to get a high-pass filter character, across which, output voltage should be measured?
BJT or Bipolar Junction Transistor has two main types. N-P-Nis one of the classifications of BJT. It is a three terminal device and used for amplification and switching.
This transistor also consists three sections, they are
B-Base
C- Collector
E- Emitter
The NPN emitter is used to supply charge carriers to the collector through the base.
The Collector area gathers charge carriers from the emitter region.
The base of the transistor does the job of triggering and it works as the controller to limit the amount of current that will be allowed to go across this region.
Note:
Unlike a MOSFET where only one carrier is present, the BJT has two types of charge carrier – Majority and Minority. In case of NPN transistor, the electrons are the majority charge carrier.
Conversely, in P-type semiconductors, electrons aren’t available much, and the hole acts as a majority charge carrier and current will be carried through because of them.
n-p-n transistor construction:
The diagrammatic representations of n-p-n transistors are given below.
NPN Transistor as diode connection
NPN Transistor
The NPN transistor’s equivalent circuit.
We can say that the working of a n-p-n transistor is similar to the working of 2 p-n junction diode connected one after another. These PN junction diodes are termed as the collector-base C-B junction and base-emitter B-E junction.
Consideration as per Doping:
The emitter section is heavily doping section. The general rule is to keep the base’s width minimum among all the three terminals. As emitter is heavily doped, it can shoot up charge carriers to the base regions.
As mentioned earlier, the base has the minimum width and it also has the minimum doping. The base passes numerous charge carriers to the collector, which is carried from the emitter.
The collector regions is in comparison moderately doped and used for collecting charges from the base region.
NPN Transistor Symbol
NPN Transistor Symbol
NPN Transistor Pinout
As mentioned earlier, a transistor has three terminals. They are – Base, collector, and emitter.
How to identify NPN Pin?
In the majority of the configurations, the center part is for the base terminal.
The pin that is below this is a collector, and also, the rest of one is the emitter pin.
When the dot isn’t marked, all terminals has to be identified using there orientation or uneven terminal space between pin. Here the center pin is the base. The nearest pin is the emitter, and the rest pin is a collector terminal.
Applications of NPN transistors:
Usually, the NPN transistor is used as bipolar transistor because of electrons’ mobility, as it is higher than the mobility of holes.
These are also used in amplifying and switching the signals. These are used in amplifier circuits i.e., push-pull amplifier circuits.
The NPN transistor is used Darlington pair circuits to amplify weak signals to significantly scaling up signal.
If there is a need to sink current, then also NPN transistors could be used.
Other than these, NPN transistor has many applications in temperature sensors, circuits like logarithmic converters, etc.
How Does an NPN Transistor Work?
NPN transistor needs both the reverse and forward bias for working. The forward bias is established between the Emitter voltage and the emitter. The reverse bias is connected between the collector voltage and the collector.
Now, as the n side of a diode has electrons as majority and p side has holes as majority, all the voltage connections get arranged as forward and reverse bias accordingly. The base emitter junction is set as the reverse bias and the collector base junction works as forward bias. The depletion region of this emitter-base area is narrower compared to the depletion area of the collector-base intersection.
As the junction is reverse biased (emitter), the holes flow from the supply to the N junction. Then the electron moves towards the p side. Here, neutralization of some electron occurs. The rest of the electrons move towards the n-side. The voltage drop in respect to the emitter and base is VBE as input side.
In N-type emitters, the charge carrier is mostly electrons. Hence, electrons carried through N-type emitters to a P-type base. A current will be carried through the emitter-base or E-B junction. This current is known as the emitter current (Ie). Here the emitter current (IE) flows from output side and it flows in two directions; one is IB and other is IC. So we can write,
IE=IB+IC
However, the base area is relatively thin and lightly doped. Hence, mostly electrons will pass the base area, and only few will recombine with available holes. The base current is minimum in comparison with emitter current. Usually, it’s up to 5% of the entire emitter current.
The current flowing from the rest of the electrons is referred to as the collector current (IC). The IC is comparatively high when compared with the base (IB).
N-P-N Transistor Circuit
The voltage source is connected to the NPN transistor. The collector terminal is joined to the +ve terminal of supply voltage (VCC) using a load resistance (RL). The load resistance can also be utilized to decrease the most current flowing through the circuit.
The base terminal is joined to the +ve terminal of the base provide voltage (VB) with resistance RB. The base resistance is used to restrict the maximum base current (IB).
When the transistor is ON operation, large collector current passing through the circuit between the collector and from the emitter. However, for that little quantity of base current must flowing to the bottom terminal of the transistor.
NPN Transistor Circuit
The markings represents the typical currents of Collector, bas and emitter.
Advantages and Disadvantages of using a NPN Transistor:
Advantages:
Small in size.
Can work in low voltage.
Very cheap.
Low output impedance.
Long lasting.
Spontaneous actions.
Disadvantages:
High Temperature sensitivity.
Produce low energy and power.
Can get damaged during a thermal runaway.
Cannot be operated in high frequencies.
NPN Transistor Switch
The transistor operates
Switched ON in the saturation mode
Switched OFF in the cut-off mode.
Switched ON in the saturation mode
When both junctions are in the forward bias condition, sufficiently high voltage is applied to input voltage. Hence, the transistor functions as a short circuit as VCE is approximately zero.
At that time two junctions are in the forward bias state, adequate voltage is in the input.
In this state, the current will pass between collector and emitter. The current is flowing within circuit.
Switched OFF in the cut-off mode.
If the two junctions of the transistors are in reverse bias, the transistor goes into OFF state.
During this mode of operation, the input signal voltage or the base voltage is zero.
Consequently, the total VCC voltage acts across the collector.
Operating Mode of Transistor
It has three modes of operation as per biasing, are as follows:
Active mode
Cut-off mode
Saturation mode
Cut-off Mode
The transistor acts as an open circuit.
In cut-off, the two junctions are in reverse bias.
The current won’t be allowed to flow through.
Saturation Mode
The transistor perform as a close circuitry.
Both junctions are configured in forward bias only.
As the base-emitter voltage is comparatively high a current pass from collector to emitter.
To define a rheostat, we need to know what is Resistor or Resistance. Resistors are electrical devices that are to controls the flow of current. A formal rheostat’s definition will be –
“Rheostat is an element of an electrical circuit whose resistance value can be changed whenever needed, that means a variable resistor.”
It is a three-terminal device, of which two are usable. There is a slider as a moving terminal, and only one is usable out of two fixed terminals. A typical rheostat also consists of a resistive material and a slider.
What does a rheostat do
The basic principle of this device is simple. In electrical circuits, whenever we need to change the resistance value, a rheostat comes into action. If we need to increase the flow of current – we will increase the resistance of the device. When we need to decrease the current flow in the circuit, we will raise the resistance value.
How does a rheostat work
Rheostat works on the property of resistance. The resistance of a material (let’s say wire) depends linearly with the length and inversely with the cross-section area.
R∝L/A
R=?L/A,
?is the resistivity of the material
Thus, if we keep the cross-section area constant, increasing the length will increase the resistance. As shown in the figure, the slider- is moved through the resistive element for linear rheostats. It moves either from input to output or vice-versa. The effective length changes accordingly. While moving the wiper towards the output port, the effective length decreases, causing a fall in resistance, increasing the current.
Working of a Rheostat
Rheostat symbol
Institute of Electrical and Electronics Engineers(IEEE) and the International Electrotechnical Commission(IEC), has defined two different rheostat’s symbols.
The IEEE standard Rheostat symbol
The IEC standard Rheostat symbol
Rheostat Switch
Rheostats control current of a circuit by controlling the resistance of the circuit. A rheostat, thus, can be used as as switch to vary the resistance as well as the current of the circuit. That is why a rheostat is used as switch.
RheostatsApplications
A rheostat has its application in an electrical circuit. When there’s a need to control the flow of current with a change of time. Based on the property of current controlling, some of the purpose of rheostat is given below.
Why is a rheostat connected in series?
To connect rheostat in the circuit, we must place it in series, not in parallel. The flow of current is much in a lesser resistive path. Thus, when it finds an option between a less resistive path and a more resistive path, it always chooses the lesser one.
Now, a rheostat is a device that has some variable resistance value. If we connect it to the parallel path, that path gains some more resistance than the other way available. When current flows in the circuit, electrons will never choose the parallel-path instead- they will flow straight through the series path. So, the rheostat will not function at all. It needs the current flow to work as a rheostat.
Series Connection
Type of Rheostat
Though there are several types of rheostat available, three main types are –
Linear Rheostats
Rotary Rheostats
Preset Rheostats
A. Linear Rheostats:
This type of rheostat consists of a cylindrical resistive element. The slider is moved linearly along the resistive element. It has two fixed terminals; one- is used, and another connects the slider. This type of rheostats is mostly used – in laboratories and experiment purposes.
This type of rheostat has a resistive element, which is circular. To use it, one needs to move the slider in a rotary way. They find applications in power electronics and also, they are used widely because of their smaller size than linear types. As the wiper needs to rotate to change the value, that’s why it is called a rotary rheostat.
When it is needed to implement a rheostat in a PCB (Printed Circuit Board), one should use preset rheostats or trimmers. It provides fine-tuning, that’s why they have found application in calibration circuits. This types of rheostats are suitable for industrial uses.
Differences between rheostat and potentiometer
There is a misconception that a rheostats and potentiometer are the same things, but there are some differences. Let us discuss some of them –
Rheostats are rated – in Amperes and Watts. There is also a resistance value. For example – 50W – 0.15 A, 100k Ohm. It means the maximum amount of current that can be measured is 0.15A. The resistance offered by the rheostat will be in the range of 0 to 100k Ohm.
2. How to select a rheostat based on rating?
While selecting a rheostat, the current rating is more important than the power rating.
It is the current that limits what power the device will generate provided for any resistance value. One should select rheostats with a current rating more than or equal to the actual need of current in the circuit.
3. What are the differences between resistors and rheostats?
A resistor is a passive electronic component that reduces the flow of current by providing resistance. On the other hand, rheostats are – variable resistors, which gives different values of resistances as per the need.
4. What is the function of a rheostat in the circuit?
A. It decreases the current
B. It increases the current
C. It limits the current
D. It makes the current constant in the circuit
E. All of the above
The correct option will be E. All of the above. Using Ohm’s law, we can find the answer to the question. As the ohm law states, V = IR, where V is the applied voltage, I is current, and R is the resistance. Rheostat provides variable resistances value; thus, it can increase, decrease, and limits the current. Keeping the resistance value constant will keep the current steady. So, all the options are correct.
5. Can rheostats be used as a potentiometer?
The answer is no, but there is some way to do this. A rheostat is a two-terminal device, while a potentiometer is a three-terminal device, so it seems impossible. But if a rheostat has inbuilt three terminals, the unused terminal can be joined to the circuit to use it as a potentiometer.
6. Can potentiometers be used as a rheostat?
Yes, a potentiometer is usable as a rheostat. A potentiometer controls voltage in a circuit. A potentiometer has three terminals. One terminal should connect the wiper while another should leave unconnected.
7. What are the drawbacks of using a rheostat?
There are few drawbacks of using this device. Some of them are –
A. The main drawback of this device is that it produces excessive heat causing loss of power.
B. It is bigger & does not fit into modern devices. That’s why rheostats are not used – in modern technologies. Though in rotors and various laboratory experiments, they are irreplaceable. Some of the replacements of rheostats are – triacs, SRCs, etc.
8. What type of taper a rheostat has?
Rheostat has linear types of taper. A taper is a relationship between resistance and sliding position. It is one of the most important parts of the device.
9. What is the use of a rheostat in a Wheatstone Bridge?
A Wheatstone bridge is used in labs to measure mean value resistances. Rheostats find its application in a Wheatstone Bridge to determine the value of unknown resistance in imbalanced conditions. The maximum resistance a rheostat can offer is the maximum resistance that a mounted Wheatstone Bridge can measure.
10. Why is choke of a coil preferred over a rheostat in AC circuits?
A rheostat is a resistive element. It provides resistance and produces an excessive amount of heat. So it causes a loss of electricity. On the other hand, a choke coil is an inductive element by nature. It maintains the same power but changes voltage according to Faraday’s Law. That is why a choke coil is preferred more.
11. Does a Rheostat change voltage?
No, a rheostat doesn’t change the voltage of the circuit. One of the conditions for working of a Rheostat is to keep the voltage constant. As the Ohm’s law states- V= IR, where V is the voltage, I is current, R is resistance. Using a rheostat, we change the current. One of the conditions for working of a Rheostat is to keep the voltage constant. Then only it can change the current of the circuit.
12. Does a rheostat’s terminal have polarity?
A rheostat is a three-terminal device of which two are fixed, and one is a moving terminal. The terminals have no polarity. So, any terminal can be connected.
“Band reject filter is combined of low pass and high pass filter which eliminates frequencies or stop a particular band of frequencies.”
Band rejection is obtained by the parallel connection of a high pass section with a low pass section. Now, the general rule is that, the cutoff frequency should be higher than the cut-off frequency of the low-pass area.
There is another way to create it. If a multiple feedback system is incorporated with an adder, then that functions like the desired operation. It is called as notch.
The Frequency response of a bandstop filter is calculated by considering the frequency and gain.
The bandwidth is chosen through the lesser and greater cut-off frequency. Notch filter is used to remove the single frequency. From this frequency response, we can also obtain Passband ripple and stopband ripple.
Where ∂p= magnitude response of the passband filter
∂s= magnitude response of the stopband filter
The frequency response of a band-stop filter
Why It is called band stop filter ?
Bandstop filter rejects a certain band of frequency and allows another frequency component of the primary signal. If the band of the frequency is narrow, the stopband filter is known as Notch Filter. The filter attenuates the specific band. The filter has several applications.
For example, a band-stop filter is designed to reject frequencies between 2.5 GHz to 3.5 GHz. The filter will allow frequency components lower than 2.5 GHz and above 3.5 GHz. The filter will We will explore the filter in the below sections.
Passband and stopband of a filter
Before diving into a band-reject or bandpass details, let us understand what pass band and stop band means. A passband is the frequency bandwidth that is allowed by a filter. On the other hand, a stopband is the band of frequency which one filter hasn’t allowed to pass. For a bandstop filter, there are two passbands and one stopband.
What does a band-stop filter do?
As the name suggests, a band-stop filter simply ‘stops band.’ That means a band-stop filter resistor doesn’t allow a certain band of frequency to pass through the
What is a band-stop filter used for
When there is a need to attenuate a certain band of frequency and pass other frequency components, a band-stop filter is used. Bandstop filters are useful in various applications.
Band stop filter applications
Being a very important type of filter, bandstop filters has several applications. Let us find out some of the applications.
Medical Engineering: Bandstop filters are used in medical engineering. Like – in ECG machine. 60 Hz bandstop filters are used to remove the supply frequency from the output.
Audio Engineering: Bandstop filters have huge applications in audio engineering. They remove the unwanted spikes and noises from the score and provides a good quality of audio.
Telecommunication: Bandstop filters are used in telephonic connections to remove the internal noise from the lines.
Radio communication: Band rejects filters are widely used in radio stations to transmit a better audio quality.
Optical filters: Band-stop filters are used to block certain wavelengths of light in an optical communication system.
Digital Image Processing: Bandstop filters are also used in digital image processing to remove certain periodic noises.
Miscellaneous: Whenever there is a need to remove the noise of a certain frequency, a band-stop filter is used.
Band stop filter diagram
This article explains the bandstop filter with various circuit diagrams, block diagrams, and graphs. This article includes a block diagram, band reject-with op-amp, frequency response of band stop, passive circuits, bode plots.
Circuit diagram of band-stop filter
The bandstop filter can be designed in several ways. It can be active types (which has op-amp). It can be for passive kinds (without op-amp). Active types have several varieties, too as well as passive filters have different styles too. That is why there are several circuits available also. In this article, almost all possible courses are given below. Check out the needed one.
Band stop filter block diagram
The bandstop filter is a combination of both high pass filters as well as low pass filters and another amplification factor for the filter. The block diagram is given below.
Narrow band stop filter
If the freq. of the bandstop filter is narrowed than general, the filter is often known as a Notch filter(hyperlink) or narrow bandstop filter.
Simple band stop filter
Unlike notch filter or higher-order filters, the simple bandstop filter is a basic filter which attenuates certain band of frequency allowing other bands.
Band stop filter using op-amp
Active bandstop filters are designed using operational amplifiers. Op-amp is one of the most important devices in making a filter. In passive filters, as there is no op-amp, there is no amplification. Thus, using the op-amp as a circuit element gives amplification.
Bandstop filter circuit using op-amp
This filter consists of a high-pass filter, a low-pass filter, and a summing amplifier to summation the lpf and hpf’s o/p, The circuit is shown below.
Band pass filter vs Band stop filter
There are fundamental differences between bandpass and bandstop filter.
The main principle of a bandpass filter is that it allows a certain band of frequency. At the same time, the main tenet of the bandstop filter is that it blocks a certain band of frequency.
Let us take an example to demonstrate. Let us say there is a lower cutoff frequency of Flow and a higher cutoff frequency high. Now, for a bandpass filter, the frequency in between the lower cutoff and higher cutoff will only pass, and other components below the Flow and above the fhigh will not pass.
Now, for a band-stop filter, the frequency band lower Flow, and above fhigh will pass. But the band in between the frequency limit will not pass.
Band stop filter vs notch filter
A notch filter is one type of bandstop filter. The main difference between them is that a notch filter attenuates a narrower band of frequency than a bandstop filter. In other words, bandstop filters have a wider band of frequency to attenuate.
Band stop filter RLC circuit
The band stop filter can be designed using basic components like resistor, capacitor, and inductor. There are two ways of developing the filter – 1. RLC parallel band-reject filter or Parallel resonant band-reject filter, & 2. RLC series resonant band-reject filter. As we are using passive elements, so both the filters will be of passive types.
Parallel RLC band stop filter
As mentioned earlier, a bandstop filter can be designed with basic components like – resistor, capacitor, and inductor. There are two ways of developing the circuits. The methods are discussed below.
Parallel RLC band stop filter
A Parallel RLC bandstop filter is a tank circuit. It also works fine as a frequency attenuator as the tank circuit is providing a lot of impedance. The below image shows the circuit diagram of a parallel rlc bandstop filter.
The parallel resonant band stop filter
The parallel resonant bandstop filter is also known as the parallel rlc bandstop filter. The details of the circuit and filter are given previously.
Series resonant bandstop filter
The main instruments for this filter are – capacitor and inductor. As the name suggests, the inductor and capacitor are kept in series. This part is the filter. At resonance, the circuit can attenuate certain frequencies before reaching the load. The below image shows the circuit diagram of the series resonant circuit.
Passive bandstop filter circuit
The passive bandstop filter is made of passive components, such as – resistor, inductor, and capacitor etc. Previously given circuits are an example of such filters. These filters do not have any operational amplifiers. Thus, there is no amplification process. A passive band stop filter consists of both passive hpf and passive lpf.
Active band stop filter
Unlike passive bandstop filters, active band-reject filters come with active components. The most important active part is the operational amplifier which also introduces amplification. Circuit using op-amp or the functional bandstop filter diagrams are given previously in this article.
Active bandstop filter design
Let us design a band stop filter. The center frequency will be 2 KHz. The bandwidth will be -3 dB of 200 Hz. Take capacitor value as one uF.
The depth in decibel comes as: 20log (0.1) = -20 db.
Band stop filter transfer function
The transfer function of a device refers to a mathematical function that provides output for every input. The transfer function of a band-stop filter is given below.
The second-order band-stop filter transfer function
The transfer function expression for the second-order band-stop filter transfer function is given below.
Band stop filter graph
The phase response stands for the phase output of the bandstop filter, bottom one represented the phase response.
The bandwidth of the bandstop filter depends on the requirement. The band-width is the range of freq. in which the filter will attenuate. In general, the bandwidth is referred to as the specification of a filter.
The impulse response of band-stop filter
Bandstop or band-reject filter can be designed digitally. There are two types of digital band-reject filters, They are – Infinite Impulse Response(IIR) and Finite Impulse Response(FIR). FIR method is more popular.
There are two design methods of FIR filter. They are also known as non-recursive filters. The methods are – 1. Window method & 2. Weighted-Chebyshev method.
Sallen key band stop filter
Low pass filters allow the lower frequency components of a filter and reject the higher frequency components. So, for the low pass filter, the stopband is the high-frequency component.
Sallen key is another topology of designing filters. The bandstop filter can also be created using the topology. Sallen key topology is designed using operational amplifiers for creating higher-order filters. Thus, we can understand this topology is for active filters.
Basic Sallen Key topology comes with one non-inverting op-amp and two resisters. It creates a Voltage Control Voltage Source or VCVS circuit. The circuit provides high input impedance and low output impedance which useful for filter analogy.
This Sallen Key topology also provides good stability of the system, which is highly suggested. The circuit is also very simple. They are connected one after another to achieve the higher-order filters. The circuit diagram of the band-reject filter using Sallen key topology is given below.
Band stop filter formula
There are some important equations for designing a band stop filter. Using these equations, we can find out important parameters. But one of the values of the parameter should be supplied as it is needed to design the filter.
The Normal Frequency Equation:
The Lower Frequency Cut-off:
The Higher Frequency Cutoff:
Here, the RL is lower resistance, and RH is higher resistance.
The center frequency:
The bandwidth: fBW = fH – fL
The Q Factor of the filter: Q = fC/fBW
Band stop filter example
The bandstop filter is an important concept that has several applications. That is why there are several examples as well. There is a band stop filter for blocking certain frequencies. Like – 2.4 GHz Band Stop filter. There is a band-reject filter for blocking narrower frequency bands, like the Notch filter, which has several applications. Audio bandstop filters, optical band-reject filters, digital-analog filters are some of its examples.
60 Hz band stop filter
From the filter’s name, we can understand that this bandstop filter is designed for attenuate frequency bands of 60 Hz. Now, the question comes why the 60 Hz band reject filter is so popular. It is because, in the USA, their supply frequency is 60 Hz. So, in most cases, when there is an interference of the supply frequency with the working signal, a 60 Hz bandstop filter is used to remove the frequency band from the output.
Band stop filter bode plot
At first, let us understand what the abode plot means. Abode plot refers to the graph of the frequency response of a device. The freq. response of the band-reject filter is presented below.
The cutoff frequency of a band-reject filter refers to the frequency of the band to be attenuated. There are formulas for lower frequency cutoff and higher frequency cutoff.
The lower cutoff frequency: fL = 1 / 2π RL C
The higher cutoff frequency: fH = 1 / 2π RH C
Band stop filter image processing
The bandstop filter is used in image processing. There are some different kinds of noises. The noises are repetitive. They have certain frequencies. A band-stop filter omits such noises. At first, the frequency is matched with the noise frequency. Then the bandstop filter removes the noises and makes the image a better one.
Band stop filter pole-zero plot
A band-reject filter can be designed using two zeros placed at ±jω0. These types of designs don’t have a unity gain at zero frequency. A notch filter can be developed by putting two poles close to the zeros.
Bandstop filter using op-amp 741
As mentioned earlier, band-reject filters can be designed using operational amplifiers. That is known as creating active band-reject filters. The band-reject filters consist of both low pass and high pass filters. Both these filters require operational amplifiers to design. Op-amp 741 is used here. Another summing op-amp is also necessary to sum the outputs of the previous filters and provide amplification. Op-amp 741 can be used in all those cases.
Band stop notch filter
A bandstop notch filter is just a special type of band-reject filter. Bandstop notch filter has a narrower bandwidth than usual band-reject filters. To know more about the notch filter, check out my article on Notch filter.
Band stop vs. Bandpass filter
The name of both the filters explains the difference between them. Here band means the range of frequency. Bandpass filter allows the specific band to pass through the filter and attenuates other components. At the same time, band-reject filters attenuate the particular band of frequency while it will enable other parts.
Characteristics of band stop filter
The bandstop filter has several characteristics. Some of them are listed below.
It has two passbands and one stopband.
It comes with a combination of lpf and a hpf.
If the bandstop filter has a narrow bandwidth, it is a notch filter that has great depth.
Bandstop filters are also known as band-reject filters as it ‘rejects’ the specified band.
Constant k band stop filter
Constant k filter is another topology of designing a filter. It is quite a simple topology, but it has a shortcoming. Here, the ‘k’ is referred to as the impedance level of the filter. It is also known as the nominal impedance. The terminating resistance is also considered as ‘k’ ohms (Rk2 = k2). The bandstop filter using constant k topology is shown below.
Design Procedure: At first, the center frequency, the bandwidth, and the intended characteristic impedance should be specified. Then follow the steps.
Calculate C2 using wH -wL = RkC2w02/2.
Calculate L2 using L2 = 1/w02C2.
Calculate L1 using L1 = k2C2, as L1/C2= k2.
Calculate C1 using C1 = 1/w02L1.
FIR band stop filter
FIR or Finite Impulse Response Filter is a digital bandstop filter. The formula for an FIR bandstop filter is given below.
N signifies the dimension of the filter. F1 and F0 are the cut off freq and Fs is the sampling freq.
lC band stop filter
A passive band-reject filter can be designed with an LC circuit. The working of the LC filter is quite simple. Inductors come with a reactance as well as capacitors also come with capacitive reactance. Now an increase in the frequency causes the decrease in capacitive reactance and increase in inductive reactance. This is the primary principle behind LC bandstop filter.
Notch band stop filter
As mentioned earlier, the notch bandstop filter is a normal bandstop filter that has a narrower bandwidth. It has several applications as it has great depth and performance than a band-reject filter. To know more about notch band-reject filters, check here. <link>.
Optical band stop filter
Optical band-reject filters block a certain wavelength of light and allow other components to pass. Just like normal band-reject filters, an optical filter rejects a certain wavelength. For example, there is a 532nm optical bandstop filter. Now, it will block the light, which has a wavelength of 532 nanometers.
RC band stop filter
The bandstop filter can also be designed using resistance and capacitors. Such band-reject filters are known as RC band top Filter. The circuit is shown below. It is a first-order filter. The resistors and capacitors are connected in parallel at first; then, they are connected in series. The frequency components are trapped in between them.
<Image>
RF band stop filter
The bandstop filter has several applications in Radio Frequency Domain. For example, during the measurement of non-linearities of a power amplifier. Also, when radio signals are transmitted from stations, band-reject filters are used to remove interfering noises.
Twin-t band stop filter
It is another method of implementing a higher-order filter and provide great depth and accuracy in performance. That is why this method is popular for notch filters. The twin t filter is made of two T networks, there is an RCR circuit, and another is the CRC network
Mathematical Expression for a Band Stop Filter:
Band Reject Filter can also be obtained by using the multiple -feedback bandpass filter with an adder. A notch filter is created using a circuit which eliminates the output of a bandpass filter from the unmodified signal.
Characteristics of a Band Reject Filter:
A band-stop filter works a frequency remover which is not within a specific range, reason it is called a rejection filter.
A band-stop filter passes frequencies of a particular bandwidth with maximum attenuation.
Different types of band-stop filters produce a maximum rate of roll-off rate for a given order and flat frequency response in the passband.
Applications of a Band Stop Filter:
An active Band Pass filter is used in the public addressing system and speakers for enhancing the quality.
A bandstop filter is also used in telecommunication technology as a noise reducer from different channels.
BSF is used in radio signals to remove static on the radio devices for better and clear communications.
Besides radios and amplification, this filter is also used in many other electronic devices to decrease a specific range of frequencies, known as ‘noise.’
In the medical field, BSF is used in making many useful devices like ECG machines, etc.
It also plays a vital role in image processing.
What is a Notch filter?
Notch filters find applications when there is a need to attenuate the undesirable frequencies while passing the necessary frequencies.
Advantages & Disadvantages of a band stop filter:
A bandstop filter attenuates the frequencies that are below the cut off range, so key advantage of using this filter is, it eliminates the external and unwanted noise or signals as well as gives us a stable output.
On the other hand, due to some certain limitations a band stop filter does not function properly under sustainable conditions. The parallel arrangement between the high pass and low pass filter my vary about the change of frequencies.
Frequently Asked Questions :
What is the Q factor or ‘Quality Factor’?
Q is given by the ratio between the resonant frequency to the bandwidth. It is an important parameter and it helps us to calculate the selectivity.
The higher the value Q, the more selective is the filter, i.e., narrower is the bandwidth.
How do a Band stop filter work?
A band stop or band reject filter always cuts or rejects frequencies that are not within a certain range, as the name implies. Besides this, it also gives easy passage to the frequencies to pass which are not in the range. These types of filters are often termed as ‘Band Elimination Filters’.
How to design a Band Reject Filter?
To make a Band Stop/reject filter we always need a Low Pass Filter(LPF) & a High Pass Filter(HPF). Therefore we combine them and make a ‘parallel’ connection with both the filters to create a band reject filter.
What does a Notch Filter do?
Notch Filter is also band reject filter. They can be used to fix frequency noise sources which are from the line frequency within a certain limit. Notch filter is also used to remove resonances from a system. Like a Low pass filter, notch filter creates less phase lag in a control loop.
Find out the differences between between a band reject filter & a notch filter?
A band reject filter or band stop filter is a filter that carries or passes the frequencies without altering and attenuates them in a specific range to low level. This is the opposite of a band pass filter.
On other hand, a notch filter is a bandstop filter which has a narrow stop band and has good high ‘Quality factor’(Q-factor).
What is Ideal Filter & Real Filter?
Sometimes, for the reason of simplification, we often use the active filters to approximate ways. We upgrade them into an ideal and theoretical model, which is called ‘Ideal Filter.’
The use of these standards is insufficient, leading to errors; then, the filter should be treated based on accurate real behaviour, For example, the Real filters.
The characteristics of an ideal filter are:
The response transits between zones in a sudden way.
It does not create any distortion when the signal passes through the transit zone.
