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

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.

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.

PNP Transistor Working Principle

The Emitter-Base intersection is linked to forwarding bias. Along with also the +ve terminal of a voltage supply (V_CB) 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 (I_E). So I_C, the Collector current will pass through the Collector-Base layers because of holes.

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 V_B the base voltage isn’t -ve in comparison to the V_E 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 R_L and R_B are connected in the circuit. That to restricts the current to pass through the transistor’s maximum possible height.

The Emitter’s voltage is V_EB as input side. Here the emitter current (I_E) flows from the input side, and it flows in two directions; one is I_B and other is I_C.

I_E= I_B+ I_C

But only 2 to 5 % of the total current flows in the I_B, so I_B 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.
• It’s applied in the amplifying circuits.
• They are used in Darlington pair circuits.
• 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 V_E, the current will not able to pass through the circuit. Thus, V_B is at least 0.72 Volt in reverse bias connection to operate the transistor.

So, if the V_B is 0 or > 0.72Volt, the current will not pass and operate as an open switch.

PNP vs NPN Transistor

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.

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Contents

In this article we will discuss about the basic concepts related to transistor and its characteristics. 

• Definition of Transistor
• Characteristics of a Transistor
• Diagram of PNP and NPN transistor
• Configuration
• What is a P-type Semiconductor?
• What is N-type Semiconductor?
• What is Band Gap ?
• Forbidden Gap
• Valance Band
• Conduction Band
• Intrinsic semiconductor
• Extrinsic semiconductor
• Direct and indirect bandgap
• Moss–Burstein Effect

Definition of a Transistor:

“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.

A transistor consists of three terminals

• Emitter
• Base
• Collector

Transistor has divided into two key categories

• Bipolar Junction Transistor (BJT)
• Field Effect Transistor (FET)

There also exist three modes in a Transistor

• Common Emitter or C-E Mode
• Common Base or C-B Mode
• Common Collector or C-C Mode

Diagram of PNP and NPN transistor

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:-

Image Credit: Tem5psu, N and p doping, CC BY-SA 4.0

Band Gap

“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.

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.

Valance Band and Conduction Band

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.

To build up a transistor, we need two types of semiconductors, which are:

1. 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 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.

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

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.

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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 Amplifier Class: 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 Amplifier Class: as per the input signal

As per the input signal’s level the categorization are as follows:

Small signal Amplifier

− 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 Amplifier Class: 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 Amplifier Class: 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 Amplifier Class: 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 Amplifier Class: 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.

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.

I_E=I_B+I_C

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.

I_E=I_B+I_C

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.

Read more about Differential Amplifier.

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What is an NPN Transistor ?

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

1. B-Base
2. C- Collector
3. 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.

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 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 V_BE 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 (I_E) flows from output side and it flows in two directions; one is I_B and other is I_C. So we can write,

            I_E=I_B+I_C

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 (I_C). The I_C is comparatively high when compared with the base (I_B).

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 (V_CC) using a load resistance (R_L). 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 (V_B) with resistance R_B. The base resistance is used to restrict the maximum base current (I_B).

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.

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 V_CE 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 V_CC 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.

Active Mode

• In this time, the transistor functions as a current amplifier circuit.
• In the transistor’s active mode, the B-E junction is at forward bias, and the C -B junction is at reverse biased.
• The current passes in between emitter and collector and the quantity of current are proportional to the applied base present.

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• Definition of Active Band Pass Filter
• Passband & Stopband
• How does an active bandpass filter work
• Types of the active bandpass filter
• Frequency response & time response
• The transfer function of active BPF
• Applications of active BPF
• Advantages
• Comparison between Active Band Pass Filter & Active Band Stop Filter
• Short note on All-Pass Filter

Definition of bandpass filter:

“A Band Pass Filter (BPF) is an electronic filter or device which passes frequencies within a certain range and rejects or attenuates frequency outside the particular range.”

Now an Active Band Pass Filter is a filter, consists of active components and has a passband between two cut-off frequencies, f_ce (lower cut-off frequency), and f_cu (upper cut-off frequency) such that f_cu>f_ce. All the other frequencies outside the passband are attenuated.

Passband – “Pass-band is the particular range of frequencies which a filter pass through inside it.”

Stopband – “A filter always carries filters within a given band, and rejects the frequencies which are below the given range. This particular range is known as a Stopband”.

Working principle of an Active Band Pass Filter:

Bandwidth:

               In an active bandpass filter, the range of frequency between two cut-off frequencies, f_ce, and f_cu, is called the bandwidth.

                                          BW=(f_cu-f_cl)

The bandwidth of this filter is not mainly centered on the resonant frequency, i.e., f_r.

We can easily calculate the resonant frequency(f_r) if we know the value of f_cu and f_cl

If the bandwidth and ‘f_r‘ are known, the cut-off frequencies can be obtained from,

                                     f_cu = (f_cl+BW)

There are two types of Band Pass Filter exist, they are –

Wide Band Pass Filter:

A Wide Bandpass filter has a bandwidth, double or fourth, of its resonant frequency.

This filter is made by cascading a low-pass and a high-pass filter circuit.

A wide bandpass filter provides a cut-off frequency of the low pass section, which is greater than that of the high-pass area.

Characteristics of a wide bandpass filter-

• In a wide bandpass filter, a low pass filter’s cut-off frequency should be ten or more times than the high pass filter’s cut-off frequency present in the circuit.
• Each section of the filter(LPF & HPF) present in wide BPF should have the same passband gain.
• The high pass filter determines the lower cut-off frequency f_cl.
• The low pass filter determines the higher cut-off frequency fcu.
• The gain is always maximum at the resonant frequency, f_r, and equal to the passband gain for both filters.

Frequency Response of an Active Band Pass Filter:

Here,

The voltage gain magnitude of the bandpass filter equals the voltage gain magnitudes of the high pass and the low pass filter.

Where,

                     A_FL,A_FH= pass band gain of the low pass and high pass filter,

f= frequency of the input signal(Hz);

f_CL= lower cut-off frequency(Hz);

f_CU= higher cut-off frequency(Hz);

Center Frequency =

• NARROW BAND PASS FILTER: In general, a narrow bandpass filter is made of multiple feedback circuit with a single op-amp.

Characteristics of a narrow bandpass filter:

• A narrow bandpass filter consists of two different blocks, i.e., two feedback paths; hence, it is known as ‘Multiple Feedback Filter.’
• An inverted op-amp is used here.
• We can change the center frequency without changing the gain or the bandwidth of this filter.

The gain of the filter-

Bandwidth-

Transfer function of Active Band Pass Filter:

What is a Transfer Function?

“Transfer function is a complex number that has both magnitude and phase. In the case of filters, the transfer function helps to introduce a phase difference between input and output.”

A bandpass filter need is made of at least two energy-saving elements, which are capacitor and inductor. So a first-order bandpass filter is not possible. The transfer function of a second- bandpass filter can be derived as;

Where T₁=R₁C₁, T₂=R₂C₂  T₃=R₃C₃

Applications of an Active Band Pass Filter:

1. An active bandpass filter is used in optics like LASER.
2. Bandpass filters are widely used in the audio amplifier circuits.
3. Bandpass filters are used to choose signals with particular bandwidth in the communication system.
4. In audio signal processing, this filter is used.
5. BPF is used to detect signal to noise ratio and sensitivity of a receiver.

Advantage of using a bandpass filter:

An active bandpass mainly controls the narrowband and passbands. It also removes distortion and has a sharp selectivity. Due to excellent electrical performance and mechanical reliability, BPF is used widely is the communication field.

Difference between Band Pass Filter & Band Stop Filter:

A bandpass filter carries frequencies within a given band and attenuates all the other frequencies below the range. In contrast, a band-stop filter does precisely the opposite and attenuates all the frequencies above the given frequency range.

Apart from that, a bandpass filter removes the energies outside of the passband, but a band-stop filter does not remove all the powers outside the passband at all.

What is an All-Pass Filter?

An active all-pass filter passes all frequency components of the input signal without attenuation and provides some phase shifts between the input and output signal.

All pass filter is generally used in digital reverberators. When signals are transmitted over transmission lines from one end to another, they undergo some phase changes. To avoid such phase changes and loss, the all-pass filters are used.

The capacitor creates an inverting amplifier at high frequencies, which is in a short circuit.

The capacitor is an open circuit when the frequency is low, and it creates a unity gain voltage buffer, i.e., there will be no phase shift.

At the corner frequency ω=1/RC, the circuit generates a 90˚ shift. That implies the output appears to be delayed by a quarter from the input.

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• Definition of an active LPF
• What does an active LPF do?
• Components of an active LPF
• Frequency Response
• Design active LPF
• Frequency scaling
• Transfer Function
• What is a second-order LPF
• The transfer function of second-order active LPF
• Design a second-order active LPF
• Comparison between active low-pass and passive low-pass filter
• Why do we use active LPF
• Advantages of an active LPF
• FAQs

What is an Active Low Pass Filter?

First things first, let’s discuss what a simple Low Pass Filter is-

“Low Pass Filter is a type of filter which mainly passes signals with a frequency lower than the particular cut-off frequency and attenuates all the frequency higher than the cut-off range ”.

Now, an Active Low Pass Filter is made of active components like Op-amp, resistors, and it also carries lower frequency signals with less resistance and has a constant output gain from zero to a cut-off frequency.

Components of Low Pass Active Filter: 

Active filters consist of active components as the name implies such as Operational amplifier, transistors or FET within the circuitry.

An active filter typically consists of amplifiers, capacitors and resistances.

So generally, Low Pass Active Filter is any filter using an Op-amp to improve the performance and predictability in such a low cost.

How does an Active Low Pass Filter work

In the above figure, it’s a commonly used low pass active filter.

Frequency Response of Low Pass Filter:

Active Low Pass Filter Design:

Resistance R =

              F_c = cut-off frequency

              Ω_c = cut-off frequency

              C = capacitance

A cut-off frequency can be varied by multiplying it with RC or C.

Transfer Function of a First Order Active Low Pass Filter:

Differential Equation for the filter –

Second-Order Active LPF:

What is a second-order LPFs?

To build a second-order filter, we usually use an op-amp, and therefore the second-order filter can also be called as a VCVS filter; where VCVS is referred to ‘Voltage Control Voltage Source’ amplifier. We design a second-order filter along with a first-order active RC filter.

As it is a low pass filter, it only allows the lower frequency signals to pass, and it attenuates all the higher frequencies above the specified frequency range.

A second-order low pass filter attenuates the higher frequency signals more precisely. The gain reduces at the rate of 12 dB per octave. In other way it is 40 dB/decade.

In a second order filter,

When the resistor and capacitor values are different,

When the resistor and capacitor values are same,

Transfer Function of a Second Order Active Low Pass filter:

The Transfer Function is denoted as,

The magnitude of the Transfer function –

Where ω_c is the cut-off frequency.

Frequency-responses of second-order low-pass active filters is given.

Design of a second-order active low pass filter

First, we choose a value of the cut-off frequency, ω_c (or f_c).

Find R,

• Rf comes as –

                              R_f = K(2R) = 3.172 R.

• Find R1 while K = 1.586

Differences between Active Low Pass Filter & Passive Low Pass Filter:

• Active components are effectively costlier, that’s why the active filters are expensive as well, whereas the cost of passive filters is lower due to the presence of the passive components.
• Active Low Pass filter circuit is a complex one, while a passive low pass filter circuit has less complexity.
• To operate an active LPF, we need an external power supply for operating it. But passive filters do not require external power because it drives the energy for its operation from the applied input signal.
• Passive filters contain more components than an active low pass filter; that’s why they are heavier in weight.
• Active LPF is more sensitive during temperature change, but the passive ones show less sensitivity with the growth of temperature.

Why to use Active LPF?

Due to the less complex circuitry and lower price than the other active filters, we use Active LPF in many fields.

Check out them here – Low Pass Filter Applications.

• Low pass filter is used in ‘hiss’ filters.
• These filters are also used in ADCs. They act as anti-aliasing filter in that circuits.
• LPFs are also used to prevent the harmonic  emissions from the RF transmitters.
• These filters find applications in the music systems also. There these filters omits the high-frequency components.

Advantages of an Active Low Pass filter:

• For a transfer function with inductive characteristics, it can achieve satisfactory output with an acceptable range of frequencies.
• The high input impedance and low output impedance of the op-amp make the circuit excellent while cascading.
• Due to better amplification, it provides more gain.

What is 3db frequency in an active low pass filter?

3db is the power level, where the cut-off frequency is at 3dB below than the maximum value, and 3dB is usually half of the maximum power.

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In this article, we will discuss about few basic concept related to active high pass filter and try to answer few questions in following sections and we will try to learn about some important application of active high pass filters with advantage.

• What is an active high pass filter?
• Working Principle of an active HPF
• Time Response & Frequency Response
• Cut-off Frequency of an active HPF
• What is a transfer function for an active HPF
• Design a first order active order HPF
• Second order active HPF
• Transfer Function for second order HPF
• Advantages of active High Pass Filter
• Applications of a HPF
• FAQS

Active high pass filter definition:

An active high pass filter is nothing but a circuit contains an active component such as a transistor, an operational amplifier(op-amp), etc. These components are mainly used for better performance or better amplification.

What are the components of an active high pass filter?

We can make an active high pass filter by adding an op-amp across a passive high pass filter.

To imply simplicity, time effectiveness and due to growing technologies an op-amp designing, generally, an op-amp is used for an Active High Pass Filter design.

In an active high pass filter, the limitation we have is the op-amp bandwidth. It means that the op-amp will pass the frequency according to its gain and the open-loop characteristics of the op-amp.

Circuit Diagram of active high pass filter:

In the above figure, the CR network does the filtering, and the op-amp is connected as a unity-gain follower. The feedback resistor, R_f, is included to minimize the dc off-set.

Here,

The voltage across the resistor R,

Since op-amp gain is infinite, we can therefore derive.

Where

= Passband gain of the high pass filter,

f = Frequency of the input signal (Hz),

= cut-off frequency of the high pass filter (Hz)

    The Gain Magnitude,

And phase angle (in degree),

Working Principle of an active high pass filter:

First-order filters are the simplest form of any filters that contain only one reactive component, i.e., capacitor, as it is also used in passive filters. To transform it into an active filter, an op-amp is used to the output of a passive filter.

Now, the op-amp is used for different configurations. Each configuration has additional attributes to the performance of the filter.

The main thing to be remembered is a first-order filter’s roll-off rate. The roll-off rate is the rate of change in the gain of a filter in its desired stopband. It shows us the steepness in the curve and how fast the growth tends to increase with frequency.

First-order filters have a roll-off rate of 20dB/decade or 6dB/octave.

        Roll Off Rate = -20n dB/decade = -6n dB/octave

Time Response & Frequency Response of an active HPF

To operate a high pass filter, the verification can be done from the gain-magnitude equation as follows:

At very low frequency, i.e., f<f_c,

At f=f_c,

At f>>f_c,

The bandwidth of the active high pass filter shows the value of frequency from which signals are allowed to pass. As an example, if the bandwidth of that high pass filter is given as 50 kHz, that means the only frequencies from 50 kHz to infinity are allowed to pass the range of bandwidth.

The phase angle of the output signal is +450 at the cut-off frequency. The formula to calculate the phase shift of an active high pass filter is

                     Ø= arctan (1/2πfRC)

Active High Pass Filter Transfer Function

The impedance of the capacitor keeps frequently changing, so electronic filters have a frequency-dependent response.

The complex impedance of a capacitor is given as,

Where, s= σ +jω, ω is the angular frequency in radians per second.

The Transfer Function of a circuit can be found using standard circuit analysis techniques such as Ohm’s Law, Kirchoff’s Law, Superposition Theorem, etc.

The form of a T.F is derived from the ratio of Output Voltage to Input Voltage

The standard form of the transfer function is :

Where,

a₁ = Amplitude of signal

ω₀ = Angular cut-off frequency

Cut-off Frequency:

What do we mean by cut-off frequency ?

By cut-off frequency, we define the useful or essential part of a spectrum. It is simply a frequency level above or below a device or filter cannot response or can be operated properly.

The Cut-off frequency for an active high pass filter is the particular frequency at which the load(output) voltage equals 70.7% of the source(input) voltage. The origin or output voltage is more significant than 70.7% of the input or load voltage and vice versa.

The cut-off frequency also indicates the frequencies at which the power of the output path falls to half its maximum value. These half-power points correspond to a fall in the gain of 3dB(0.7071) relative to the maximum dB value.

Filter Designing of Active High Pass Filter:

To construct an active high pass filter, we need to implement the following steps-

A value of the cut-off frequency,

is chosen.

A value of the capacitance C, usually between 0.001 and 0.1µF, is selected.

The value of the resistance R is calculated using the relation,

Now, the values of R₁ and R_f are selected depending on the desired pass-band gain, using the relation,

Second-Order Active High Pass Filter:

What is a second-order filter?

The maximum delay in each sample used in generating each output sample is called the order of that particular filter.

Second-order filters mostly consist of two RC filter, which is connected together to provide a –40dB/decade roll-off rate.

Where DC gain of the amplifier =

The Transfer Function of a second-order active high pass filter can be obtained from the transfer function of the low pass filter by the transformation,

• Substituting s=jω, the transfer function is,

In the above equation, when ωà0, |H(jω)|=0. Thus the low-frequency gain of the filter is zero.

If we compare it with Butterworth filter transfer function, we get

The Frequency response of a second-order active high pass filter is shown in the above diagram. It is noted that the filter has a very sharp roll-off response.

The design procedure for a high pass will be as same as low pass.

The frequency response will be a maximally flat one, i.e., having a very sharp roll-off response.

Advantages of using Active High Pass Filter:

There are so many vital benefits of an active High Pass Filter, some of them are:

• Whenever there is a small signal is present, an active High pass Filter is used to increase the amplification factor, which also increases the amplitude of those small signals.
• Due to very high input impedance, active high pass filters can transfer efficient signals without any loss in any preceding circuit.
• Active filters usually have very low output impedance, which is perfect for transferring efficient signals to its next stage, mostly when they are used in different multistage filters.
• This type of filters gives us smooth frequencies.
• They have a sharp roll-off response.
• Strong broadcasting power to receivers to select desired channel frequency.
• Best for audio processing in any electrical or electronic device.
• Active HPF prevents amplification from DC etc.

Application of Active High Pass Filter:

• To transmit higher frequency in case of video related filters.
• We use HPF as a treble equalizer.
• We often use HPF as a treble boost filter.
• We are changing the frequency depending on different waveforms.
• Active High Pass filters are also used in oscilloscopes.
• In the generator, these filters are used.

Frequently Asked Questions

Where are high pass filters used?

      The high pass filters are used in all audio sources to remove unwanted noise that lurks below the important frequencies.

Many unwanted sounds can be hidden by some louder core of a high pitch signal and can be overlooked. We don’t get to hear the rumble due to the limits of hearing as the lowest parts of the spectrums are around 20-40 Hz. High pass filters also eliminate those noises or reduce it that makes them nearly silent.

Can I get the output of a high pass filter as a power source?

A high pass filter is an electronic filter that passes signals with higher frequencies which are above the cut-off frequency range and also attenuates the frequencies which are below the cut-off range.

Now, the output of the specific high pass filter has no DC(0Hz) voltage due to its specified cut-off frequency(f_c). The lower cut-off frequency of an active high pass filter is 70.7% or -3dB(dB= -20log V_out/V_in) of the voltage gain which it allows to pass can be used as a power supply as well.

What does corner frequency mean in regards to high pass filter?

The corner frequency, which is also called as cut-off frequency, defines a specific frequency at which the transfer attenuation reaches -3dB below(50%) the magnitude from the 0dB or pass-band level.

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• Definition of Low Pass Filter
• Circuit Diagram
• Active and Passive Low Pass filter
• What does a LPF do? How does it work?
• Operation
• Frequency Response
• Transfer Function of a LPF
• Designing a LPF
• Corner frequency of a low pass filter
• Ideal & Real Filter
• Low Pass Filter vs High Pass Filter
• Advantages of a LPF
• What are the uses of a low pass filter
• FAQs

Definition of LPF:

 “Low pass filter carries lower frequency signals with less resistance and has a constant output gain from zero to a cut-off frequency.”

Generally, a low pass filter does attenuation of frequencies above cut-off points.

Circuit Diagram of a Low Pass Filter:

There are two types of active filter exist, they are-

• Active Low Pass Filter – consists of mostly active components like op-amp, a transistor.

• Passive Low Pass Filter – consists of mostly passive components like capacitors, resistors, etc.

How does a Low Pass Filter Work?

What does a low pass filter do?

In figure 1.1, it’s a commonly used low-pass active filter.

The filtering is commonly done by the RC network, and the op-amp is used as a unity gain amplifier. The resistor R_F(= R) is included for Dc offset.

At DC, the capacitive reactance is infinite, and the dc resistive path to ground for both terminals should be equal.

Here, all the voltages V_i, V_x, V_y, V₀ are measured concerning ground.

The input impedance of the op-amp is always infinite; no current will flow into the input terminals.

According to the voltage divider-rule, the voltage across the capacitor,

Since the op-amp gain is infinite,

Where,

= pass-band gain of the filter

                 f = frequency of the input signal

= cut-off frequency of the signal

A_cL

 = closed-loop gain of the filter as a function of the frequency.

The Gain Magnitude,

And Phase Angle (in degree),

Operation of a Low Pass Filter:

The operation of the low-pass filter can be verified from the gain magnitude equation as follows-

At very low frequencies, i.e., f>>f_c,

At f=f_c,

              |A_cL|<A_F

Thus the filter has a constant gain of A_F from 0 Hz to the cut-off frequency f_c. At f_c, the growth is 0.707A_F, and after f_c, it decreases at a steady rate with an increase in frequency.

Here, the actual response deviates from the linear dashed-line approximation at the vicinity of ‘f_c.’

Frequency Response of Low Pass Filter:

How to make a Low Pass Filter?

Low pass filter design:

A value of the cut-off frequency ω_c is chosen.

Capacitance C is selected with a certain value; usually, the value is between 0.001 and 0.1µF. Mylar or tantalum capacitors are recommended for better performance.

The value of R is calculated from the relation,

              F_c = cut-off frequency in hertz

              Ω_c = cut-off frequency is in radian second.

              C = in Farad

Finally, the values of R₁ and R_F are selected depending on the desired pass-band gain by using the relation,

Frequency Scaling:- Once a filter is designed, there may be a need to change its cut-off frequency. The method of converting an original cut-off frequency f_c to a new cut-off frequency is called ‘frequency scaling.’

To change a cut-off frequency, multiply R or C, but not both by the ratio:-

Corner frequency & Cut-Off Frequency of a Low Pass Filter:

The transition of a low pass filter is always swift and smooth from the pass-band to stopband. Also, a cut-off frequency is not any parameter to measure the goodness or badness in a range of frequency. The cut-off frequency is more accurately referred to as the -3dB frequency, i.e., it is the frequency at which the magnitude response is 3dB lower than the value at 0 Hz.

What is Pass-band?

“Pass-band is the particular range of frequencies which a filter pass through inside it.”

For low pass filters, the frequencies that move towards the end of the pass-band cannot have any significant gain or attention.

What is Stopband?

“A filter always carries filters within a given band, and rejects the frequencies which are below the given range. This particular range is known as a Stopband”.

As the limitations are there for low pass filters, the stopband attenuates at a particular frequency, which moves near the cut-off frequency closer to 0 Hz.

The transfer function of a Low pass Filter:

What is a Transfer Function?

“Transfer function is a complex number that has both magnitude and phase. In the case of filters, transfer function helps to introduce a phase difference between input and output.”

Since low pass filter allows low-frequency AC signals to pass through it, the output gets attenuated. We use different active and passive components to make a filter, which eventually has other characteristics. The transfer function tells us how one input is related to an output depending on the component’s characteristics. The transfer function can easily be determined from a graph of the output signal at various frequencies. We can also calculate the transfer function using Kirchoff’s Laws to derive the filter’s differential equation.

As more signal passes through it, the filter will apply a phase shift to the output signal for the input signal. Hence, the transfer function of a filter is a complex function of frequency. It also contains all the vital information we need to determine the magnitude of the output signal and its phase.

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 behavior, i.e., as a ‘Real filter.’

The main key terms of an ideal filter are

• A gain unit
• Complete degradation of the input signal across the bands.
• The transition of response from one zone to another is quite abrupt.
• It does not create any distortion when the signal passes through the transit zone.

What are the differences between the Low pass filter & High pass filter?

What are the advantages of a Low Pass Filter?

• Low-Pass filters can easily remove aliasing effects from a circuit, which makes the circuit working smoothly.
• Low-Pass Filters are cost-effective so that it can be used easily.
• Low-Pass Filters have low output impedance; thus, it prevents the filters cut-off frequency from being affected because of the load.

Applications of a Low Pass Filter:

• A low-pass filter is used in ‘hiss’ filters.
• LPF is used in audio speakers to reduce high frequency.
• LPF can be used as an audio amplifier and an equalizer.
• In Analog to Digital converter, LPF is used as anti-aliasing filters to control signals.
• LPF is used in image smoothening, image blurring.
• LPF is also used in radio transmitters to block harmonic emissions.
• These filters are used in music systems to filter the high-frequency sounds, causing echo at higher sounds.

What is a passive low pass filter?

A passive low pass filter is a filter made of all passive components like capacitors, resistors, etc. It causes a lesser output level compared to the input level.

What is an RC low pass circuit?

An RC low pass circuit is made of only Resistors and Capacitors, as the name implies. It is an essential passive filter, as well. In this filter, the reactance of a capacitor varies inversely with frequency, and the value of the resistor remains constant as the frequency changes.

What is a Butterworth Low pass Filter?

A Butterworth filter is that type of filter where the frequency response is flat over the pass-band region. A Low-Pass Butterworth filter provides a constant output from DC source to a particular cut-off frequency and rejects the higher level frequencies.

How can a second order low pass filter be constructed?

We know that a first-order low-pass filter can be made by connecting a single resistor and capacitor whose single pole can give us a roll-off slope -20dB/decade. To make a second-order passive low pass filter, we connect or cascade two passive filters (first-order). It is also a two-pole network.

Write down the corner frequency of a second-order filter.

In a second-order low pass filter, we observe a -3dB corner frequency point and therefore, the pass-band frequency changes from its original value as calculated in the equation:

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Content

• In this article, we will discuss about few basic concept related to filter and few Frequently asked questions

• What is a filter ? filter definition
• How does a filter work?
• What are the types available ?
• What are the Active Filters? Explain with definition
• What are the Passive Filters? Explain with definition
• What are the points of conflict between active and passive types.
• What is the filter symbol?
• Examples
• What are the Applications of filter in optics and electronics industry ?
• What is the Response curves for most common types?
• What is Time response of a filter ?
• What is Frequency response of a filter?
• What is Order of a filter?
• How first-order output differs with A second-order filter ?
• What is Corner frequency, Cut off frequency, break frequency
• What is Bandwidth (BW) ?
• What is Resonant Frequency?
• What is Resonant Filters
• What is ideal and real filters ?

What is a Filter?

“A filter is a frequency selective network consisting of four terminals with reactive elements to transmit a specified range frequency.“

• The band of frequency transmitted through it is called Pass-band.
• The band of frequency, which gets attenuated by it, is called attenuated on Stop-band.

Filters are of two types- analogue digital. Now, based on the components used, They are of two types – Active types and Passive Types.

The below image represents a diagram of active types(one of the very popular and important types).

Know about Active High Pass Types. Click Here!

Characteristics of an active filter

As the titles suggest, these types are made using active components. Some of the active transistors are – transistors (BJT, FETs), any other electronics devices which are capable of amplifying a signal or can produce powers.

If there is a need to increase the characteristics, various stages are joined in a certain or specific ways.

How to design an active low pass filter?

To design an active types, we may use IC741, an Op-amp, configured with 8 pins. The op-amp is to be supplied with DC power along with resistors and capacitors of different values.

Passive Filters

Passive types are designed using passive devices.

Know about Applications of Active High Pass Types. Click Here!

Comparison of Active and passive types

An active type may have an advantage, increasing the signal power available in comparison with the input. Whereas a passive one dissipates energy from the signal. For various ranges of frequencies, such as at sound frequencies and under, an active type may realize a specified transfer function with no use inductors, that are comparatively big and costly components in contrast to resistors and capacitors, and that are more costly to create with the essential high quality and precise values.

Numerous stages may be cascaded when wanted to enhance attributes. By comparison, the layout of multiple-stage passive blockers needs to take into consideration every phase’s frequency-dependent loading of the previous stage. Since inductors aren’t utilized, they can be reached in really compact dimensions.

• Passive type suffers from attenuation of signals. There are various ways; one popular method of control or restoration is by using amplification through the Active type applications. The major point of conflict between the active and passive types is the ‘amplification’.

• Compared to a passive one, active type are composed of active components in operational amplifiers, transistors, or FET’s within their circuit design, as described in earlier sections. These components draw power from the external power source, use it for amplification of output. That’s an added advantage compared to a passive one.

Know about Active Low Pass Types. Click Here!

Why is an active filter needed at low frequencies?

• An active type is needed at lower frequencies because it helps achieve low output impedance while providing high input impedance. It also stabilizes different frequency ranges as multiple stages can be cascaded with it.

Difference between Active & Passive types

Know about Applications of Active Low Pass Types. Click Here!

These can be categorized and sub-categorized from several points of view. The most common divisions and sub-divisions are- active or passive type; high-pass type low-pass type, bandpass type, band-reject/notch type or all-pass type; digital or analog type discrete-time or continuous-time type; linear or non-linear type; infinite impulse response (IIR) or finite impulse response (FIR) and so on.

Examples:

Active types and Passive types are designed to modify certain band of frequency in a desired way. They have different types according to their needs. The categories are given below.

• Low pass types (LPF)
• High pass types (HPF)
• A bandpass types (BPF)
• Band reject/stop types (BSF)

Applications:

They are nowadays used in many purposes of the electronic circuit, and its applications are immense. Moreover, it’s possible to improve the circuit gain by using different filters in different ways, either active or passive types, especially in active types. Active types use amplifiers, and we know that it helps increase gain. This article will discuss two type, such as Low pass type, High pass type with appropriate diagrams and simulated wave shapes for both active and passive condition in the following sections with the importance of using higher-order in the HPF and LPF.

In electronics, some applications are as follows:

• In Radio communication System for Radio tuning to a specific frequency: They are used to enable radio receivers to only “see” or “detect” the desired signal and reject all the other signals by assuming their different signal frequency. So noise-free signals can be received. The high-frequency bandpass types are used for channel selection in central telephone offices.
• Power Supply Design: They are used to remove noise or high frequencies usually present on AC input lines. These are also applied to decrease the ripple.
• Analog-to-digital conversion (ADC): They are utilized in most of the ADC input to minimize aliasing.
• Modify digital images: It can be used to modify digital images also.
• Data analysis: They are also very helpful to remove specific frequencies in data analysis.

Frequency Response & Time Response:

Time-domain refers to the change of signal’s amplitude with respect to time. In contrast, in the frequency domain, Frequency refers to the occurrence of an event in a given period.

What is Bandwidth (BW)?

For filters, bandwidth is the difference between the upper and low -3dB points.

For example, if a bandpass filter has -3dB cut-off points and set to 200Hz and 600Hz, then the filter bandwidth will be = (BW) = 600-200 = 400Hz.

What do you mean by the Q Factor?

Q factor is given by the ratio of resonant frequency to the BW.

Q = 2 * π * (Maximum Amount of Energy Stored) / (Energy Dissipated per Cycle)

A greater Q value represents the filter is more selective as Q factor is a parameter which judges the selectivity.

Resonant Frequency

Resonant frequency is simply given by the frequency of the given resonant circuitry. A Resonant circuit is also popularly known as the tank circuit or the LC circuit. A resonant circuit is designed using parallelly placed inductors and capacitors and resistors.

 Oscillation of a system is given by the following equation –

Where,

f= frequency in Hertz

L= Inductance in Henry

C= Capacitance in Farads

Orders of Filters

Higher-order filters provided more excellent roll-off rates between passband and stopband. Higher-order filters are also required to achieve required levels of attenuation or sharpness of cut-offs.

Active type and Passive type also have variation in different types of orders, such as:

• First Order Low pass active types, First Order High pass active types, First-order Bandpass active types, First-order Band stops active types.
• Second order Low pass active types, Second order High pass active types, Second-order Bandpass active types, Second-order Band stops active types.

The frequency response of second-order are shown below –

Ideal type & Real type:

Sometimes, for the reason of simplification, we often use the active filters to approximate ways. Later they are modified and termed as ‘ideal filter’. Filters, which operate in reality considering all the possible factors, are real ones.

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