Soumali Bhattacharya

C O N T E N T S

• What is a Varactor diode?
• Definition of a Varactor Diode
• Working principle of Varactor diode
• Symbol of Varactor diode
• I-V Characteristic of Variable diode
• Structure of Varactor Diode
• Ohmic loss in Variable diode
• Advantages of Varactor diode
• Important applications of Variable diode

What is Varactor Diode?

Definition of a Varactor Diode:

“The term varactor is the shortened form of a variable reactor, which refers to the voltage-variable capacitance of a reverse-biased p-n junction.”

Varactor diode is also known as a variable diode, vericap diode, tuning diode, variable reactance diode, or variable capacitance diode.

Symbol of Varactor Diode:

How does a varactor diode work?

Working principle of varactor diode:

At this point the junction capacitance be influenced by on the instigated voltage and the design parameter of the junction. A junction with constant reverse biasing can be utilized as a capacitance. Usually, the variable diode is designed to employ the voltage-variable characteristics of junction capacitance. For instance, a varactor may be hand-me-down in the radio receiver’s tuning stage to supplementary of the large variable plate capacitor. The measurement of the resulting circuit can be reduced, and its dependability is increased. All the uses of varactor diode comprise with harmonic generation, microwave frequency amplification, and active filter applications. In an abrupt P-N junction, the capacitance changes as the reverse bias Vr‘s square root.

 In a graded junction, the capacitance can regularly be written as, 

                                           C_j∝ V_r^-n   for the condition Vr >>V₀ 

In a linearly categorized junction, the exponent n is always one-third. That’s why, varactor diodes are prepared by ‘ the epitaxial growth methodology’ or by ‘the ion implantation technique’. The epitaxial layer can be devised to get junctions for which the exponent n is more significant than one-half. Such junctions are called hyper abrupt junctions.

Structure of Varactor Diode

I-V Characteristic of Varactor Diode:

Doping Profiles of Varactor Diode

Three different doping profiles has been explained above with the junction is devalued as p+ -n so that the depletion layer width W is extended originally into the n side. We can observe that the exponent n is 1/(m + 2) for the p+-n junction.

 The hyper abrupt junction¹⁶ with m = -3/2 is particularly interesting for specific varactor applications for this case, n = 2, and the capacitance is equivalent to V_r^-2. If a capacitor is connected with an inductor L in a resonant circuit, the resonant frequency varies linearly with diode’s applied voltage.

For the reason that of the wide variability of C_j vs. V_r reliance on doping profiles selection, variable diodes can be utilized in various specific uses. In one of these case, varactors can be designed to exploit the forward-bias charge storage capacitance for high-frequency applications.

Ohmic Loss of Varactor Diode:

While deriving the diode equation, we assumed that the device’s voltage appears solely over the junction. For most of these diode, the voltage drop in the neutral regions are negligible and the doping require is comparatively higher. The resistivity of each of the neutral region is small, and a characteristic diode area is outsized compared to the length.

Sometimes the ohmic loss is explained in a diode by inclusion of a simple resistance in series with the junction. The effects of voltage drop exterior to the development region are important because the voltage decline be influenced on the current, explained by the voltage across the junction. For instance, if we expressed the series resistance of a p and n constituencies by R_p and R_n, correspondingly, the junction voltage V is

                                         V = V_a – I[R_p(I) + R_n(I)]

Where V_a is a voltage applied externally to the device. There is a on the rise voltage drop in the resistance region R_p and R_n correspondingly when the current become higher and the junction voltage V is declined. An additional complication of loss calculation may occur if conductivity has been increased in neutral region with cumulative carrier injection. Though, at high injection levels, with the injected surplus carriers’ conductivity inflection can reduce R_p and R_n considerably. The Ohmic losses are often avoided in properly-outlined devices. For that reason, deviations of the current in general give the impression only for very high currents operating beyond regular region.

The forward and reverses current-voltage characteristics in a semi-log scale

The forward and reverses current-voltage characteristics of a p-n junction on a semi-log scale has been explained above. We observe a straight line on a semi-log plot for the ideal forward-biased diode, corresponding the exponential relationship of current on voltage. Considering the second-order properties, we realize different operation modes. The enhanced generation–recombination current is directed to a further distinguished diode with ‘ideality factor’ (n = 2). For neutral currents, we get an excellent low-level injection and diffusion-limited current (n = 1). At more currents, we can get a higher injection level and n = 2, while at even greater currents, the ohmic drops initiated and space charge-neutral regions become critical.

At reverse biasing, a constant reverse saturation current has been observed, during this current is independent to voltage change. However, in substance, we receive an increased, voltage-dependent leakage current. The avalanche or Zener effects cause break down in sufficiently high reverse biasing.

Advantages of using a Varactor Diode:

Since the varactor diode has low noise compared to the p-n junction diode, there is less power loss in this diode. variable diodes are lightweight and easily portable due to its small size.

Applications of Varactor Diode:

• variable diodes are used in a variable resistant tank, which is generally an L-C circuit.
• variable diode can be used as a frequency modulator.
• It is used as RF phase shifter.
• variable diodes are utilized in a microwave receiver.

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Contents of Photo diode detector

In this article we will discuss about photo diode detector as follows:

• Definition of a photodetector
• Different types
• Circuit Diagram
• Applications
• What is a Photodiode
• Features of a photodiode
• Working principle
• Avalanche photodiode
• Circuit Diagram
• Applications
• Advantages & disadvantages
• Phototransistor vs. photodiode

What is a Photo Detector?

Definition of a Photo Detector:

“Photodetectors are important elements, possess the ability to transform light into electrical signals.”

“Photodetectors are important elements useful in image processing, optical communication, security, and night vision and motion detection.”

Types of Photo Detectors:

Important Applications of Photo Detectors:

• Photo Detectors can be employed for properties like optical power, luminous flux measurements.
• These are used in various types of optical-sensor and microscope design.
• Photo detectors are essential for laser rangefinders.
• Fast photodetectors are commonly utilized in optical-fiber communication, frequency metrology etc.

What is a Photo Diode?

Definition of a photodiode:

“A photodiode is basically a typically p-n junction diode.“

When a photon strikes the diode, it will excite electron and generates a moveable electron and a positive charge hole. The absorption happens in the junction’s depletion area, the carrier will removed from the junction by the depletion region’s built-in potential.

How does a photodiode work?

Working Principle of photodiode:

A photodiode is a p-n junction or a P-I-N configuration. If a photon strikes the diode, it produces the electron and a positively charged hole. When there’s any absorption happens in the juncture’s depletion area, these carriers have been trapped in the juncture from the built-in area of this depletion region that created a photocurrent.

Photodiodes are widely used under inverse biasing or without biasing. The light or photon can drive a current across this circuitry, which gives to the forward bias, which subsequently causes ‘Dark current’ from the reverse direction to the photocurrent. This is referred to as the natural effect and can be the foundation of solar cell design. A solar panel is just a combination of multiple effectual photodiodes.

Reverse bias produce minor current along exactly identical direction. Apart from that, the photodiode exhibits less noise.

Avalanche photodiodes have a similar prearrangement, but it’s normally operated with a greater reverse biasing. This enables every single photo-generated provider to be multiplied with avalanche breakdown, leading to photodiode’s internal effects and improves the device’s overall responsivity.

Materials for a photo diode:

Material used in photodiode:

• Silicon
• Germanium
• Lead Sulfide

The materials used for the construction of photodiode is important to describing its properties because only photons with appropriate energy can excite electrons in bandgap and able to produce substantial photocurrents.

It is important to remember that, silicon-based photodiode have greater band-gap and because of this it is capable to produce less noise than germanium-based photodiodes.

Since transistors and ICs are prepared by semiconductor material too and comprise p-n junctions, may act like a photodiode. This is not the accepted, an opaque housing is mandatory to remove this effect. Though these is not entirely opaque towards high energy radiations, may still cause ICs to malfunction for induced photocurrent.

Applications of a Photo Diode:

• Photodiodes are used in consumer electronic i.e., CD players, fire and smoke detectors, remote controls, lighting etc.
• These are also employed in various medical applications, detector and high-energy physics etc.

Advantages and Disadvantages of a Photo Diode:

ADVANTAGES-

• Low noise
• Low cost
• Compact and lightweight.
• Long Lifetime
• No high voltage is required.
• High quantum efficiency.

DISADVANTAGES-

• Small area
• No internal gain
• Much lower sensitivity
• Response time is slower.

What are the characteristics of a Photo Diode?

There are two types of characteristics of photo diode

• Electrical Characteristics
• Optical Characteristics

Electrical Characteristics of Photo Diode:

SHUNT RESISTANCE, R_SH

Shunt Resistance (R_SH) is used to estimate the thermal noise when no reverse bias is applied. It’s the ratio of voltage to current.

It is computed from the slope of the photodiode’s V-I curve at the origin.

SERIES RESISTANCE

Series resistance is give by Rs and it comes from the resistances of silicon. The expression is given by the following equation –

JUNCTION CAPACITANCE, (C_j)

Junction Capacitance (C_j) is the capacitance of the diode at a given reverse bias.

The junction capacitance is proportionate to the diffusion area and inversely proportionate to the width of depletion area.

RISE AND FALL TIME ( t_r , t_f )

The time taken to reach Ninety percent from ten percent is known as Rise Time and the time taken to fall from Ninety percent to Ten percent is known as Fall time. This parameter commonly expressed in frequency response of 3dB decay as follows.

                                t_r=0.35/f_3dB

BREAKDOWN VOLTAGE (V_BR)

It is the max negative voltage which can be applied to the diode terminal.

NOISE EQUIVALENT POWER (N_EP)

The photon intensity is prerequisite to equivalent the noise at a specified reverse biasing. It is a measurement of N_EP.

RESPONSE TIME (t_r)

It is defined by the time required for a diode to respond a step input in light at a specified reverse biasing mode of operation.

Short circuit current (I_SC):

With the diode pins shorted, the current that flows at a given light intensity.

Optical Characteristics of Photo Diode:

QUANTUM EFFICIENCY, Q_E

Q_E is extensively recognized as the percentage of the incident photons that contribute to the photocurrent.

                               Q_E=R _obs/R _Id (100%)

RESPONSIVITY, R

The responsivity of a silicon photodiode is the measurement of the sensitivity to light. It is given by the ratio of Ip to the coming power of light (P) for the given wavelength.

                              R=I_P/P at specific wavelength

NON-UNIFORMITY

It is well-defined as the variations of responsivity witnessed over the active photodiode surface area with a trivial spots of light.

NON-LINEARITY

A silicon photodiode characteristic is linear in nature though minor change in current regulates photocurrent linearity.

Noises in a Photo Diode:

In a photodiode, two types of noises are introduced; they are

• Shot Noise
• Johnson Noise.

Shot Noise

It is expressed by the following equation.

Thermal Noise or Johnson Noise

The photodetector may produce Johnson noise because of the thermal generation of carrier. The magnitude of this generated current noise is:

Hence, the total noise is,

Explain Avalanche and Zener Mechanisms:

Avalanche breakdown happens only at the higher voltages. Assume the electric field (E) in the transition region is enormous. Then, an e^– incoming from the P side may get accelerated with kinetic energy to collide with the lattice and produce ionization. The interactions will create carrier multiplication; the original electron and generated electron are swept to the N side, and the create hole is flounced to the P. This process is Avalanche since each incoming carrier be able to initiate the large number of carriers.

Zener effect happens once tunneling of electrons take place from the P-side valance band to N-side conduction band, may causes reverse current flow from N to P terminal. Basic inevitabilities for tunneling current are a large number of electrons that are detached from a substantial quantity of unoccupied state by a tinny barrier. Since the tunneling probability be governed by the barrier width, doping must be great.

Compare between Photo Transistor and Photo Diode:

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C O N T E N T S

In this article we will learn about PN junction diode and it’s characteristics as follows:

• What is PN junction diode ?
• Definition of PN junction diode:
• Working Principle of PN junction diode
• Properties of PN junction diode
• Circuit and symbol of PN junction diode
• Equivalent Circuit of PN junction diode:
• PN junction Current flows
• Ideal current-voltage relationship
• PN junction Characteristics
• Diode quasi-fermi levels
• Applications of PN junction Diode

What is a PN junction diode?

Definition of PN junction diode:

“A pn junction diode is two-terminal or two-electrode semiconductor device.“

“A diode is called as P-N junction diode if it is formed by P-type on one side and N-type on the supplementary one or reverse direction.”

“The diode has to be in forward biased condition to permit the electric current flow. through it.“

• If a positive voltage is connected to the P terminals, the current then pass through from the P to N region as positive voltage helps to cross the depletion region. When we use a negative voltage is applied to the p-type, the depletion zone increases and prevents the current from flowing.

How does a PN junction Diode work?

Working Principle of PN junction diode:

In a PN junction Diode, we will consider the p-n junction with a forward-bias voltage employed. We can determine the current-voltage characteristics. The potential barrier of this p-n junction is reduced when a forward-bias voltage is applied to it. It will allow e- and hole to leak through the space charge region.

When holes begin passing through the p region throughout the space charge area, they get excess minority carrier namely, hole and extra minority carrier from drift, recombination and diffusion process.

Likewise, when electrons in the region initiate flowing through the space charge region to P.  They get surplus minority carrier electrons.

When semiconductor apparatus with p-n junctions are employed in linear amplifiers, as an instance, time-varying signs are overlaid on the dc currents and voltages. A tiny sinusoidal voltage apply to on a dc voltage applied across a p-n junction will initiate a small-signal current.

The proportion of the current to voltage generate the small-signal admittance of this p-n intersection. The admittance of a forward-biased p-n intersection includes both the conductance and capacitance parameters.

What is the PN junction current?

When a forward-biased voltage is applied to a p-n junction, a current gets generated in the device. That is known as P-N junction current.

Define the ideal current-voltage relationship:

Ideal PN junction current:

The ideal current at a p-n intersection relies on the important components of the fourth principle mentioned previous section. The total current at the intersection is the summation of these electrons and hole currents, that stay steady through the depletion area.

The gradients from the minority carrier concentrations create diffusion currents, and because we’re considering that the electrical field to be ‘0’ at the space-charge edge, we can ignore drift current for minority in this approach.

Equivalent Circuit of PN junction diode:

The small-signal equivalent circuit of the forward-biased p-n junction is derived from an equation.

Y =g_d+ J_ωc_d

It is required to add the junction capacitance in parallel to the diffusion resistance (r_d) and diffusion capacitance. The last element for the equivalent circuit is a series of resistance. The neutral n and p regions have a ‘C’ numbers pf resistances, so the actual p-n junction includes a series resistance which complete equivalent circuit is represented in above Figure.

The voltage through the actual junction is – Actual voltage (Va), and the total voltage applied to the p-n diode is specified by (Vapp) So the expression for the ideal condition as follows:

              V­_app = V_a+Ir_s

The above figure is V-I characteristics that reveals the impact of the series resistance. A voltage, that can be greater in general, is needed to find the exact same present value when a streak of immunity is included. In the majority of the diodes, the show resistance will probably be negligible.

In certain semiconductor apparatus with p-n junctions, but the series resistance will belong into some feedback loop.

Reverse Biased Recombination Current:

If a PN junction diode is in reverse biasing, It was learnt that mobile holes and electrons were wiped from the space-charge section. The negative signal explains a negative recombination rate; therefore, we’re actually generating electron-hole pairs inside the reverse-biased space charge region.

The recombination of excess holes and electrons at the procedure during the attempt to re-establish the thermal balance. Considering that the concentration of holes and electrons is essentially zero at the reverse-bias area, holes and electrons become generated through the trap level, which also attempts to revive the thermal balance.

Since the holes and electrons are generated, they are trapped from the space-charge area by the electrical field. The flow of charge is at the current direction of a reverse-biasing. This reverse-bias production current, which is principally a result of the creation of holes and electrons at the space-charge region, is added to the reverse-bias ideal saturation current.

Forward Biased Recombination Current:

For a reverse-biased PN junction, electrons and holes are cleared up mostly from the Space charge region. Under forward bias, however, electrons and holes are injected across the space charge region; during that some extra carrier charges may be at the space charge region.

There are certain possibility exists that some of these electrons and holes will recombine also during that time.

Diode quasi-fermi levels

What are the uses of PN junction, diode?

Important applications of PN junction diode:

The critical applications of PN junction diodes are:

• PN junction diode can be used as photodiodes.
• PN junction diode can be used as solar cells.
• The forward biased PN junction diode is used as LED.
• PN junction diode used as rectifiers in voltage-controlled device in varactors.

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C O N T E N T S

• Definition
• Diode symbol
• Important features
• Biasing technique of Diodes
• Important Types
• Applications of Diodes

What are Diodes?

Definition of Diode:

“A diode is a special electronic element with two electrodes termed as the Anode and the Cathode”.

Most of the diodes are made of semiconductors such as silicon, germanium, or selenium.

How does a Diode function?

Working Principle of diode:

The basic characteristics of a diode is to carry electric current in only one direction. If the cathode is negatively charged at a voltage greater than the anode, a certain current called ‘forward break over’ starts flowing through it.

When the cathode is +ve charged in respect of the anode, it will not conduct any current. These can be operated as a rectifiers, switches and limiters.

The forward break over voltage is approximately 0.6 Volt for silicon, 0.3 Volt for germanium and 1 Volt for selenium material respectively.

At the forward break over point, if an analog signal flow through the diode the signal waveform is inaccurate and distorting. All the signals that get generated are harmonic and integral multiples if the input frequency. These generally produce signals at microwave frequency with the correct level and polarity of voltage application.

Symbol of Diode:

Important features of Diodes:

• Diode is a two terminal electronic component
• It has lower resistance in one direction and higher in another direction
• Most of the diodes are made of silicon
• The voltage drop under a forward bias condition is 0.7 volts approximately.
• In the reverse biasing the region of depletion layer, will increase.

Different types of diode:

1.   P-N junction diode –

“A diode is a P-N junction with P-type on one side and N-type on the other side”.

2.   Light Emitting Diode (LED) –

“LED is a semiconductor light source that emits light when current flows through it”.

3.   Photo Diode –

“This is a semiconductor-based P-N junction diode, if exposed to light produce a potential difference”

4.   Schottky Diode –

 “This is designed by the junction of a semiconductor with a metal. Sometime known as hot carrier diode”.

5.   Tunnel Diode –

“ A semiconductor diode that has effectively negative resistance due to the tunnelling”.

6.   Varactor diode –

 “A diode with changing internal capacitance with the changes of the reverse voltage”.

7.   Zener Diode –

“A special type of diode, designed to allow current to carry backwards when a reverse voltage is applied”.

What are the Ideal Diodes?

In an ideal diode, when it is in forward bias the current starts flowing freely from the device. In an ideal one usually without voltage drop when forward biased. All the other voltage sources are dropped across the circuit resistors. When reverse biased, in an ideal diode there is zero current flow and will have infinite resistance.

What are Practical Diodes?

In a practical diode, some of the resistances allow the current to flow if forward bias. Due to the presence of the resistances, some power is dissipated when the current starts flowing through the forward biased. When it’s in reverse biased, due to the high resistance it can conduct.

A diode is normally is P-N junction.

1. It is a barrier potential. To overcome this problem by applying an extra voltage to the p-n junction, it can be able to conduct.
2. So current will pass through p-n junction when the barrier potential is omitted.
3. It is with two metallic conducts is known as p-n junction.
4. The process of external voltage applying is channel biasing.

Forward Bias:

• A battery connected +ve terminal to p-side of p-n junction diode & then connect -ve terminal to the n side.
• If we apply an external voltage which is greater than the potential barrier then it starts conducting the current to pass
• The diode is connected to a DC voltage source (V)
• The voltage across the diode is called forward characteristic of p-n junction diode
• No diode current flows till A is reached because external voltage V_f  is being opposed by built in voltage whose value is 0.
• However voltage increases beyond A and the diode current decreases rapidly.
• If the forward current is externally backward it cuts the voltage axis at a point from which V_k can be determined

Reverse Bias:

• If voltage is applied to p-n junction diodes –ve terminal is connected to the p-type semiconductor. Similarly, +ve terminal is connected to the n-type.
• Holes from p-side are attracted towards the -ve terminal. Whereas the free electrons from n-side are attracted towards +ve terminal.
• The reverse bias increases in steps and diode current is observed.
• When the reverse bias increases V_BR the diode reverse current increases very shortly.

Switching property of diodes:

In forward bias when a small voltage is applied the diode conducts which exceeds the cut in voltage known as on-state.

In reverse bias only small voltage current sources with reverse applied voltage that is less than the breakdown value is known as off-state

In switching property of a diode is switched from forward bias on-state to reverse bias off-state or vice versa.

Applications of a diodes

Rectification:

A diode is usually acting as a rectifier, flattening an AC power source into a constant power supply. This can achieve this task by obstructive the flow in one direction and pass through the other direction.

Light emission:  

LED provides a much more efficient source of light. The bulbs are cost more than their incandescent counterparts, in part because they require additional control circuitry to work with AC power.

Inductive Load Dissipation:

Diodes are used in this application, when an inductive load is switched off, the energy it has stored must go somewhere. Without the proper circuit protection, the stored energy can lead to voltage spikes that can arc across the switch and potentially overload a transistor this configuration allows the current to dissipate across the inductor and it feeds back into the power supply and protects the circuit.

Sensing and Control:

Semiconductors can easily generate electrical charges based on the optical effects. In general, these devices are packaged in such a way that it blocks out light to avoid unintended electrical activity. Photodiodes are built to optimize this effect. These photodiodes are often used in the infrared spectrum, such as inside consumer remote controls.

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• What is a Voltage regulator
• Types of a voltage regulator
• Voltage regulator Circuit
• Zener Diode as a voltage regulator
• Difference between series regulator & shunt regulator
• Series regulator
• Shunt regulator
• Regulated power supply
• The function of the voltage regulator
• Percentage regulation
• Applications of a voltage regulator

Definition of voltage regulator:

“A voltage regulator is a DC regulator that offers a constant DC output voltage that is fundamentally independent of applied input voltage, output load current, and temperature.”

Also, the regulator output can be changed as per the requirement. Hence, the function of a voltage regulator is two-fold 1. The output voltage can be regulated at the desired level. 2. The regulated voltage at the output can be maintained constant despite disturbances in supply voltage or change in load.

Voltage Regulator Types:

• The Zener Diode Based Shunt Regulators
• The Transistor Based Shunt Regulators
• The Transistor Series Regulators
• The Transistor Current Regulators
• The Transistor Controlled Series Regulators
• The Op-amp based Shunt Regulators
• The Op-amp based Series Regulators
• The Switching Voltage Integrated Circuit Regulators
• The Monolithic Regulators

Voltage Regulator Circuit:

The following figure refer to the Zener diode of a regulator.

The input current, I_S=V_S-V_Z/R_S

Where V_S= d.c input voltage to the regulator circuit V_Z= Zener voltage

The voltage across Zener diode terminals,

V_L=V_Z + I_Z r_z

V_L=V_Z (I_tr_z is negligible)

I_L=V_L/R_L

Input current, I_S=I_Z + I_L or I_Z= I_S – I_L

Zener Diode as Voltage Regulator:

In this circuit, a Zener diode is joined in reverse biased parallel with a variable voltage source supply. The Zener diode in this circuit will operate when the voltage at the reverse breakdown voltage. Then, the diode’s relatively low impedance retains the voltage.

This is a typical voltage regulation circuit with an input voltage, V_IN. This voltage is regulated down to a stable output voltage, namely V_OUT. The breakdown diode voltage is stable over a wide current range and maintains V_OUT at relatively constant even though the relative voltage may fluctuate during this operation.

As per Ohm’s law diode current, flowing through the diode, a load is placed across the diode, and as long as the Zener diode operates in a reverse breakdown, the diode will deliver a stable voltage to the load. Zener diodes in this stage are often used as a stable regulator for more advanced circuitry.

Series Regulator Circuit:

The basic block diagram of the series regulator circuit is shown below. The control element is connected in series with the load in between i/p and o/p terminal. The sampling circuit detects the variation in the output voltage. The comparator circuit will compare sample voltage with a reference one. The control element will compensate during that period and will retain a constant output. The control element conducts more when V₀ reduces and conducts less when V₀ increases.

Here a simple series regulator is presented. Transistor Q is the controlling element that is in series. The Zener diode provides the reference voltage.

Shunt Regulator Circuit:

In the linear voltage regulator category, in the shunt regulator circuit, the output is monitored, and the feedback signal initiates changes in input signals to maintain the desired output. However, in series regulators, the control unit or regulating unit is in series, and in shunt regulators, the control unit is in the shunt. The basic block diagram is shown below,

In the case of shunt regulators, as the control element is in shunt, it conducts more to provide regulation by shunting current away from the load.

What is Regulated Power Supply?

A regulated power supply is a stand-alone unit. It is able to supply a stable voltage to a circuit. This has to be operated within specific power supply limits. The regulated power supply output might be alternating or unidirectional but it is nearly a DC in standard operation.

The type of stabilization be limited to confirm that the output remains within absolute limits under a number of load condition.

The specification parameters are:

• The Input Voltage parameter
• The Output Voltage parameter
• The Output Current parameter
• Stability factor
• Ripple factor
• The Stored Energy
• The Pulsed operations
• The Load Regulation
• The Line regulation
• The Dynamic Regulation
• The Efficiency.

Comparison between Shunt and Series Regulator

What is the function of a Voltage Regulator?

A voltage regulator is to provide a constant DC output which is independent of the input voltage, output load current, and the temperature. It is an important component of a power supply circuitry. Its input voltage supplied from the rectifier circuit. The low capacity (500VA) regulators are in general used for domestic applications, for television, refrigerator, air-conditioner, etc. and for necessary equipment like computers. In these medical instruments, the sudden changes in voltages can affect the equipment leading to erroneous results and may get damaged ultimately.

What is the Percentage Regulation?

The basic performance measures for a regulator are line regulation and load regulation parameter. The line regulation is defined as the change in percentage of the output voltage for a given change in the input voltage as explains follows:

Uses of Voltage Regulators:

• Voltage Regulators is used in low output voltage switching power supply circuits.
• It is used in error amplifiers design.
• In design of the current source and  the sink circuits
• These are used for voltage monitoring and maintenances.
• It is used to design the Precision current limiter circuitry. It is applied in Analog and Digital Circuits for precision reference.
• It is used in adjustable voltage or current linear circuitry etc.

• Definition of BJT
• Types of BJT
• Configurations
• Applications
• Advantages & disadvantages
• Different Modes and Characteristics.

Definition of a Bipolar Junction Transistor:

A Bipolar Junction Transistor (also well-known as BJT) is a special type of semiconductor device with three terminals made of p-n junctions. They are able to amplify a signal as well they control current i.e., they are called as current controlled device. The three terminals are Base, Collector & Emitter.

Types of BJT:

There are two types of BJT –

• P-N-P Transistor.
• N-P-N Transistor.

The BJT has three parts named emitter, collector and base. Here, the emitter-based junctions are forward biased and the collector-based junctions are reverse biased.

PNP Bipolar Junction Transistor:

These types of transistors have two p-region and one n region. The n region is sandwiched between two p region.

NPN Bipolar Junction Transistor:

“NPN transistor is a type of Bipolar Junction Transistor (BJT) that consists of three terminals and three layers and function as either amplifiers or electronic switches.”

NPN BJT with forward-biased E–B junction and reverse-biased B–C junction

What is punch through breakdown in BJT?

In the reverse biasing configuration, the collector junction is increased, the effective base region decreases. At a certain reverse bias of the collector junction, the depletion region covers the base reducing the effective base width to zero. As the collector voltage penetrates the base, and potential barrier at the emitter junction is reduced. As a result, an excessively large emitter current flows. This phenomenon is known as Punch Through.

Applications of Bipolar Junction Transistor:

There are so many applications of a BJT, some of them are-

• In logic circuits BJT is used.
• Bipolar junction transistor is used as an amplifier.
• This type of transistor utilized as switches.
• To design a clipping circuits, Bipolar Junction Transistor is preferred for wave shaping circuits.
• In demodulation circuits, BJTs are also being used.

Advantages and Disadvantages of a Bipolar Junction Transistor:

A BJT is a one type of a power transistor. It is used in the amplifiers, multi-vibrators, oscillators etc. A BJT also has few disadvantages besides its advantages, they are:

Advantages –

1. BJT has a better voltage gain.
2. BJT has a high current density.
3. Higher Bandwidth
4. BJT gives stable performances in higher frequencies.

Disadvantages-

1. Bipolar Junction Transistor has a low thermal stability.
2. It usually produces more noise. So noise prone circuit.
3. It has a small switching frequency.
4. The switching time of BJT is not very fast.

Bipolar Junction Transistor Characteristics:

Transistor Characteristics-

Transistor Modes:

The three modes of a transistor are

• CB (Common -Base)
• CE (Common -Emitter)
• CC (Common Collector)

CB-Common Base, CE-Common Emitter and CC- Common Collector Mode of PNP and NPN Transistor has been discussed as follows:

Input Characteristics:

Input characteristics of a transistor is drawn between the Emitter current and Emitter-base Voltage with collector base voltage as a constant.

Output Characteristics:

Out characteristics of a transistor is drawn between the Collector current and Collector-base Voltage with emitter current as a constant.

The output characteristics is distributed into different sections:

The Active Region –

In this Active mode, all together the junctions are in reverse biased and no current pass through the circuitry. Therefore, the transistor stays in the OFF mode; operate as an open switch.

The Saturation Region –

In this Saturation mode, both junctions are forward biased and current pass through the circuitry. Hence, the transistor stays in the ON mode; operate as a closed switch.

Cut-off Region –

In this Cut-off mode, one of the junction is in forward biased and other one is connected in reverse biasing. This Cut-off mode is utilized for current amplification purpose.

CB (Common Base)

In the Common Base mode operation the base is grounded. The E-B junction is connected at forward biased during the standard operation; the input characteristics are analogous to p-n diode. I_E get increased with the increase of |V_CB|. If the functional voltage at |V_CB| increases, the size of the depletion region at the C-B junction enlarged, thereby reducing the effective base region. The “variation of the effective base width” by the voltage applied in collector terminal is termed as early effect.

From nodal analysis we know,

I_E=I_B+I_C

Now, α= the ratio of I_C & I_E

So, α=I_C/I_E

       I_C= αI_E

       I_E=I_B+ αI_E

      I_B=I_E (1- α)

Input characteristic common-base silicon transistor:

Output characteristic common-base silicon transistor:

CE (Common Emitter)

In CE mode, the emitter is grounded and the input voltage is applied in between emitter and base and output is measured from collector and emitter.

β=ratio between I_C & I_B

β=I_C/I_B

I_C= βI_B

I_E=I_B+ βI_B

I_E=I_B (1+ β)

The Common Emitter mode, the emitter is common to the input and output of the circuitry. The input current I_B  is plotted to the voltage V_BE with output voltage V_CE in the time being. This is because width of depletion region at collector emitter junction increases. This is called Early Effect.

Input characteristic common-emitter silicon transistor

Output characteristic common-emitter silicon transistor

CC (Common Collector)

In CC or Common Collector mode collector has to be grounded and the input is applied from base collector and output is taken from collector to emitter.

The ratio, I_E/I_B = I_E/I_C.I_C/I_B

Or, I_E/I_B = β/α

We know α= β (1- α)

                 β = α β+ α

               I_E=I_B (1+ β)

Relationship between α & β:-

We know,

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• What is photo transistor ?
• Working principle of photo transistor
• Photo transistor uses
• Photo transistor symbol
• Characteristics of photo transistor
• Advantages and disadvantages of photo transistor

The photo transistor is an transducer which is capable of converting the light energy into the electrical energy. The parameters like wavelengths, alignments, interfaces, etc. should be considered with higher importance while designing the circuit.

Definition of Photo Transistor:

“The phototransistor is a semiconductor device that is able to sense light levels and alter the current flowing between emitter and collector according to the level of light it receives.”

As the name suggests, the phototransistor is a transistor which can sense the light and vary the flow of currents in-between the terminals of the transistor.

In general transistors are sensible to lights. This property of transistors are used in phototransistors. NPN type photo transistor is one of the types.

Here, in a phototransistor light striking the base supplants the voltage, actually applied to the base, so a phototransistor amplifies disparities as per the light signal. The phototransistors may or may not be have a base terminal in it. If it is present, the base region permits it to biasing the phototransistor’s light impacts.

• This type of transistor is controlled by exposure of light. It is like a photodiode controlling a BJT.
• Photo Transistor can be any one type such as BJT or FET.
• These types of transistors are typically covered with the plastic materials and one of the part is kept open or transparent for light.

Symbol of Photo Transistor:

Examples Photo Transistor:

• KDT00030TR
• PS5042
• OP506A, OP550A, OP506B
• TEKT5400S, TEMT1030
• SFH314-2/3, SFH 325 FA-Z
• QSE113E3R0
• BPW17N, BPV11F, BPW85C etc.

Working Principle of Photo Transistor

Output of a photo transistor is taken from its emitter terminal; hence the light rays are allowed to the base region.

A photo-transistor can be three or a two terminal device as per our requirement. The base of the photo-transistor is used for only biasing purpose. For NPN transistor, the base is made +ve in respect to the emitter terminal, and in a PNP transistor the collector terminal is made –ve in respect to the emitter terminal.

At first, the light ray enters the base region of a photo transistor and generates an electron hole pairs. This process mainly occurs under reverse biasing. The active region of this type of transistor is used for generating current. The cut-off and saturation region are used to operate the particular transistor as a switch.

A photo-transistor and its working depends on so many internal and external factors, such as:

• The intensity of the photocurrent will be more with higher DC current gain.
• Luminous sensitivity is given by the ratio of the photo electronic currents to the incoming luminous fluxes.
• If the wavelength gets increased, the frequency will be decreased.
• If the area of the collector-base junction gets wider, the Amplitude of the photo current engendered by the photo-transistor will be higher.

Characteristics of Photo Transistor:

Here the X axis is V_CE– denotes the voltage applied to the collector-emitter lead and the Y axis is I_C – denotes the collector current that carrying through the circuit in mA.

As we can see, the curve is clearly indicating that current is increasing with the intensity of the radiation which is at the base region.                  

Advantages of Photo Transistor:

• The efficiency of this type of transistor is greater than a photodiode. The transistor’s current gain is also more compare to photodiode; even if incident light is same, the photo transistor will produce more photo current.
• In compared to a photo diode, the response time of a photo transistor is more. So, it means this type of transistor has a faster response time.
• The photo-transistors are immune to any noise interference.
• Photo-transistors are less costly.
• The circuitry of a this type of transistor is less complicated.

Disadvantages of Photo Transistor:

• The efficiency of the phototransistor decreases with the electromagnetic field interferes.
• At higher frequencies, photo transistors do not function properly. Due to this problem it fails to convert the photo current effectively at high frequency.
• Electric Spikes occur frequently.

Applications of Photo Transistor:

• Photo-transistors are used in counting systems.
• This type of transistors are utilized in the computing system.
• This type of transistor can be used to generate variable voltage.
• These types of transistors are used in.
• Due to high light to current conversion efficiency these are widely used in remote, printing machine.
• The most important application of this type of transistor is to use it as a light detector. It can also detect very less light also.
• They also play important role in making punch cards.
• This type of transistors are crucial optoelectronics device which are also used in optical fibers

Why is phototransistor reverse biased?

Photodiodes are connected in reverse bias to decrease the charges area and narrow the capacitance at the junctions. This permits higher bandwidth. The light acts as I_B, so in an NPN phototransistor the collector have +ve voltage by a resistive load, whereas the emitter will be grounded one.

Difference between photo resistor and phototransistor

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