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Common emitter amplifier

Common Emitter Amplifier, BJT Transistor Common Emitter Amplifier

In this article, you will be able to learn and understand the Common Emitter Amplifier its Working, characteristics, and their applications.

In our previous discussions, we learned that a transistor can be configured in three different modes, the most widely used transistor configuration is Common-Emitter Configuration, and this is due to the reason, that a common emitter configuration provides good voltage and current gain.

What is the common emitter amplifier?

Common Emitter Amplifier has Emitter terminal as common for both input and output. Input is applied to the Base-Emitter terminal and output is taken from Emitter-Collector terminal.

 common emitter amplifier

The base-emitter junction is forward bias and emitter-collector junction is reverse bias, it is because a transistor must remain in an active region in order to perform amplification.

In order to understand the working of Common Emitter amplifier let’s first understand how does a transistor work as an amplifier?

How Transistor Amplifies?

When a weak AC signal is given to the base of the transistor, a small base current IB starts flowing. Due to transistor action, a much larger (β times the base current) a.c. current flows through the collector load RC. As the value of RC is quite high (usually 4-10 kΩ), therefore, a large voltage appears across RC.

Thus, an applied weak signal at the base circuit appears in amplified form in the output of collector terminal. It is in this way that a transistor acts as an amplifier.

Common Emitter Amplifier Working:

 As shown below a Common Emitter amplifier is made up of voltage divider bias, the input is Base-Emitter terminal and output is Emitter-Collector collector. During Positive cycle of input, a sinusoidal AC signal is applied at the input terminals of a circuit that cause the forward bias of base-emitter junction hence VBE is increased resulting in an increase in IB.

The collector current Ic is increased by β times with the increase in IB, hence VCE is correspondingly decreased.

common emitter amplifier working

common emitter amplifier working


formula of vo=vcc-icrc

Thus in a Common-Emitter amplifier, a positive going signal is converted into a negative going output signal i.e..180° phase shift is introduced between output and input signal and it is an amplified version of an input signal.

Practical Common Emitter Amplifier Circuit

In order to perform amplification with a common emitter amplifier, we must consider the basing, capacitor and different resistors values.

Figure down below shows the circuit of practical common emitter amplifier.


common emitter amplifier circuit

common emitter amplifier circuit


  • C1 and C2: These two capacitors are placed in the input and output of an amplifier; they are used to couple one circuit with another hence they are called as coupling capacitors.
  • CB: This capacitor is known as bypass capacitor which is used to bypass the AC signal to ground. It is very helpful because any noise signal that may be presented in AC signal will be passed out from bypass capacitor.
  • R1 and R2: These are places in between of collector to the base terminal they provide biasing to the transistor hence they are known as biasing resistors.
  • RC: RC is placed in the collector terminal in order produce faithful amplification. It places an important role in the operation of amplification (VC-VCC-ICRC)
  • RE: This RE resistor is placed in the emitter terminal of a transistor, and it is useful to control the gain of an amplifier.

Common Emitter Transistor Characteristics:

  • It has Large Voltage and Current Gain.
  • It has hence large power gain.
  • It has input to output phase shift of 180°.
  • It has moderate input and output impedance.

The voltage gain of Common Emitter Amplifier:

  • The voltage gain of Common Emitter amplifier is the ratio of output voltage to the input voltage.
  • Here output voltage is referred to as ΔVC and input voltage is referred to as ΔVB.
  • Av=β Vc/vb.

The current gain of Common Emitter Amplifier:

  • Current gain in CE amplifier is the ratio of output current to the input current. In CE configuration the current gain is denoted by greek symbol beta.
  • the output current is referred to as Ic and input current is referred to as Ib.
  • β=Ic/Ib

Input Impedance of CE Amplifier: 

The input impedance is also an important parameter in CE amplifier, because when a one CE amplifier drive another amplifier circuit then the output of one amplifier is input for another amplifier.

In CE amplifier input impedance is around 1kΩ-2k, input impedance values changes with respect to circuit configuration usually CE amplifier has input impended in between mentioned values.

Zin = R1 || R2 | | Zin (base)

Output Impedance of CE amplifier:

The output impedance of the CE amplifier is the resistance looking in at the collector and is approximately equal to the collector resistor. The output impedance of CE amplifier is around 50k-70k.

Rout = RC

Input characteristics Of Common Emitter 

Input characteristics curve of a common-Emitter amplifier is the curve between IB and VBE whereas VCE is constant.

 input characteristcis curve of common emitter amplifier

Output characteristics of Common Emitter

Output characteristics curve of a common-Emitter amplifier is the curve between IC and VCE whereas IB is constant.

output characteristcis curve of common emitter amplifier

Applications of Common Emitter Amplifier:

why common emitter amplifier is widely used?

Common Emitter amplifier configuration is widely used due to its advantage of moderate current and voltage gain.

  • It is used in Audio Amplifiers
  • It is used in Microphones, RADIO, and Music Players
  • It is used in the Frequency generation circuit to increase the strength of the input signal.
  • It is used to increase the speed of Fans, Motors, and Timer circuits.

Advantages of Common Emitter Amplifier:

Common Emitter Amplifiers is most widely used amplifier than the Common Base amplifier and Common Collector amplifier because:

  1. An Ideal amplifier must have very low input impendence, and CE amplifier has very low input impendence.
  2. An Ideal amplifier must have very high output impendence, and CE amplifier has very high output impendence.
  3. It provides 180° phase shift. Or we can it is inverting amplifier.
  4. The current gain and voltage are moderate.

Disadvantages of Common Emitter Amplifier:

  • In the CE amplifier, there is high thermal instability.

Also, read:

  1. Common Base Amplifier, BJT Transistor Common Base Amplifier
  2. Common Collector Amplifier, BJT Transistor Common Collector Amplifier
  3. What is the Difference Between NPN and PNP Transistor
  4. Transistor configurations, Common Emitter, Common Base, Common Collector, and Applications
  5. Transistor Biasing, Self Bias, Emitter Bias, Voltage Divider Bias, and applications
  6. Introduction to BJT Transistor.


In Common Emitter Amplifier, Input is applied to B-E Junction and Output is taken from E-C terminal, here emitter terminal is common for both input and output.

It is a widely used amplifier circuit because it provides good current gain and good voltage gain and it is also known as inverting amplifier because it gives 180° phase shift from input to output. It is widely used in audio amplification and signal amplification circuits.

Full Wave Rectifier

FULL WAVE RECTIFIER- Center-Tapped and Bridge Rectifier

welcome to this article, In this article, we are going to learn about FULL WAVE RECTIFIER- Center-Tapped and Bridge Rectifier as we Learned about Half-wave rectifiers previously.

Today we are going to study about Full Wave rectifier and their types. so before going to start let me tell you if you have not learned my article on Half Wave rectifier so you must check out that you will understand the basics of rectifiers circuits. so let’s get started.

What is Rectifier Circuit?

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A rectifier circuit is that circuit which performs conversion of AC voltages to DC Pulsating Voltages. So how this Rectifier Circuit Works? I mean how the Full Wave Rectifier circuits work? and why we use Full Wave Rectifier circuits? let’s start our discussion.

A Half-Wave rectifier has very few applications, But the full wave rectifier is the most commonly used rectifier. why is it so? it is because it is mostly used in every type of dc power supplies.

In this section, you will use the same concepts that you learned previously in half-wave rectification article, we will take that concept and expand it to full-wave rectifier working. You will learn about two types of full wave rectifiers.

  1. Center-Tapped Full Wave Rectifier.
  2. Bridge Full Wave Rectifier.

Working of Full Wave Rectifier:

A Full Wave rectifier allows current in unidirectional (one-way) through the load during the entire of the input cycle, whereas a half-wave rectifier allows current through the load only during one-half of the cycle.

The result of full-wave rectification is an output voltage with a frequency twice the input frequency and that pulsates every half-cycle of the input, as shown in given picture below.
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Full wave rectifier block diagram

The number of +ve alternations that make up the full wave rectified voltage is twice that of the half-wave voltage for the same time interval. The average value, which is the value measured on a dc voltmeter, for a full-wave rectified sinusoidal voltage is twice that of the half-wave, as shown in the following formula:
Full Wave rectifier output voltage

Working of Center-Tapped Full-Wave Rectifier

A Center-Tapped rectifier is a type of full wave rectifier that uses two diodes connected to the secondary of a center tapped transformer, as shown in Figure given below. A Centre Tapped Transformer is one whose secondary number of turns are grounded to provide two isolate circuits in secondary of Transformer.

Mostly the Word Centre Tapped is used whenever the circuit is grounded in its center. The input voltage of  Centre Tapped Full Wave Rectifier is coupled through the transformer to the center-tapped secondary. Half of the total secondary voltage appears between the center tap and each end of the secondary number of turns as shown in given figure.

Center Tapped Full wave rectifier

For a positive half-cycle of the input voltage:

The polarities of the secondary voltages are as shown in Figure (a). This makes forward-biases diode D1 and reverse-biases diode D2. The path for current is through Diode 1 and the load resistor, as indicated.

For a negative half-cycle of the input voltage:

The voltage polarities on the secondary are as shown in Figure (b). This makes reverse-biases D1 and forward-biases D2. The current path is through D2 and RL, as indicated. Because the output current during both the positive and negative portions of the input cycle is in the same direction through the load, the output voltage developed across the load resistor is a full-wave rectified dc voltage, as shown below.

Center Tapped Full wave rectifier

Read More: What is the Duty Cycle?

Effect of the Turns Ratio on the Output Voltage

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The output voltage of a center-tapped full-wave rectifier is always one-half of the total secondary voltage less the diode drop, no matter what the turns ratio.output voltage of Center Tapped Full wave rectifier

Peak Inverse Voltage

PIV for Center Tapped Full wave rectifier

Each diode in the full-wave rectifier is continuously changing from forward-biased and then reverse-biased. The maximum reverse voltage that a diode can handle is the peak secondary voltage Vp(sec). The peak inverse voltage across either diode in a full-wave center tapped rectifier is:

PIV for Center Tapped Full wave rectifier

Working of Bridge Full Wave Rectifier

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The bridge rectifier is a best full wave rectifier which uses four diodes that connected as shown in Figure below. When the input cycle is in going for positive alternation as shown in part (a), the diodes D1 and D2 are in forward-biased and they conduct current in the direction as shown.

A voltage is generated across Load Resistor that looks like the +ve half of the I/P cycle. During this period of duration, diodes D3 and D4 are reverse-biased.

When the input cycle of bridge full wave rectifier is going in the negative cycle as in (b), the diodes D3 and D4 are also going in forward bias and they conduct current in the same direction through Load Resistor as during the +ve half-cycle.

when the negative half-cycle is coming for the diode, D1 and D2 are going in reverse-biased. A full-wave rectified output voltage appears across RL as a result of this action.

Bridge Full Wave Rectifier operation

Bridge full wave rectifier Output Voltage

A bridge rectifier with a transformer-coupled input is shown in (a). During the +ve half-cycle of the secondary voltage, diodes D1 and D2 are forward biased. we are neglecting diode drop here.

The same is true when D3 and D4 are forward-biased during the negative half-cycle.

output voltage of Bridge Full Wave Rectifier

As you can see in Figure (b), two diodes are always in series with the RL during +ve and -ve half-cycles. If these diode drops are taken into account, the output voltage is.

output voltage of Bridge Full Wave Rectifier

Bridge Full Wave Rectifier circuit

Peak Inverse Voltage for Bridge Full Wave Rectifier

Since the output voltage is ideally equal to the secondary voltage, If the diode drops of the forward-biased diodes are included as shown in Figure 2–40(b), the peak inverse voltage across each reverse-biased diode in terms of Vp(out) is

PIV of Bridge Full Wave Rectifier

The Peak Inverse Voltage rating for Bridge Full wave rectifier’s diodes is less than that required for the center-tapped configuration.

If we neglect the diode drop, the bridge rectifier requires diodes with half the PIV rating of those in a center-tapped rectifier for the same output voltage.

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Bridge Full Wave Rectifier

This was all about for FULL WAVE RECTIFIER Center-Tapped and Bridge Rectifier if you like this article then appreciate our efforts by doing comment thanks for visiting. check daily for more best articles relative studies and technology.

Also Read:

  1. A Detailed article on What is PN Junction Diode, Characteristics and Applications.
  2. Well Explained Zener Diode Operations and Characteristics.
  3. What is LED- How Light Emitting Diode Works?
  4. What is Photo-diode- How it works?
  5. Game Changer Tunnel Diode, Working and its Operations.
  6. What is Varactor Diode, How it works?
  7. What is Diode Clipper and How they work?
  8. What is Diode Clamper Circuits and How they Work?
PN Junction Diode

PN Junction Diode, its Characteristics and Applications

PN junction Diode plays a vital role in our electronic fields, because of their unique property (current flows in only one direction) they are used in many electronic or electrical circuits like rectifiers, switches, clippers, clampers, voltage multipliers.

In this article, we will learn about what is a PN Junction Diode and how it Works and also effect on PN Junction diode with different modes and I am sure this article will help you a lot to understand about Diode.

After completing this article you will be able to:

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  • Understand the PN Junction Diode.
  • Understand the Working of PN Junction Diode.
  • Understand the Effect of Forward Bias and Reverse Bias on PN Junction Diode.
  • Understand the V-I Characteristics of PN Junction Diode.
  • Understand the Practical Applications of PN Junction Diode.

What is PN Junction (Diode):

PN Junction Diode is a two-terminal semiconductor device. It’s made up from a small piece of semiconductor material (usually Silicon), it allows the electric current to flow in one direction while opposes the current in other direction. In the Forward Bias, the diode allows the current to flow in uni-direction. On the other hand, when the diode is reverse biased it opposes the electric current to flow. A PN Junction Diode is a semiconductor device with two opposite region such as (P-type region and N-type region).

  • The P-region is called as the anode and is connected to a positive terminal of a battery and it has Holes in majority carrier and electrons in minority carrier.
  • The N-region is called as the cathode and is connected to the negative terminal of a battery and it has Electrons as a Majority carrier while holes as Minority carrier.

When the P-type semiconductor material is joined with the N-type semiconductor material, a P-N Junction is formed, hence resulting P-N Junction is also called as a P-N Junction Diode.

The basic diode structure and symbol of PN Junction Diode is shown in the figure below.

PN Junction Diode

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Biasing of Diode:

Biasing means applying external voltages to the device, biasing of a diode is of two types: Forward Biasing and other one is Reverse Biasing.

Forward Biasing of Diode: We connect positive terminal of the battery to the P-type Material and Negative terminal of the battery to the N-type, hence this configuration is called as Forward Bias Configuration of Diode. In this configuration Diode allows the current to flow in uni-direction.

Reverse Biasing of Diode: We connect Negative Terminal Battery to the P-type Material and Positive terminal of Battery to the N-type Material, hence this configuration is called as Reverse Bias configuration of Diode. In this configuration, diode does not allow the flow of current.

Biasing of PN Junction Diode

For More Read: Biasing of Diode [in Detail]

Forward Bias of Diode:

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Forward bias is the condition that allows current through the PN junction Diode. The voltage source is connected in such a way that it produces a Forward Bias. This external bias voltage is designated as V(bias). The resistor limits the forward current to a value that will not damage the diode.

Note that the -ve side of VBIAS is connected to the n-region of the diode and the +ve side is connected to the p-region. This is one requirement for forward bias.

A second requirement is that the bias voltage, V(bias), must be greater than the barrier potential.

Forward bias of diode

What is Barrier Potential of PN Junction Diode?

A Barrier Potential is an internal potential a semiconductor material, in case of Silicon-based PN Junction diode it is 0.7v and in case of Germanium, it is 0.3v. It means in order to forward bias the PN junction diode V(bias) should be greater than 0.7 for silicon and 0.3V for germanium.

As we know the N-type material is consist of Electrons and the P-type material is consist of Holes.

A fundamental picture of what happens when a PN junction diode is forward-biased is shown below. When the P-type material is connected with a positive terminal of battery it transfers the holes (positive charge carrier), which travels from p-type material to the N-type material through (Junction).

When the N-type material is connected with a negative terminal of battery it transfers the free electrons (negatively charged carriers), which travels from n-type material to the P-type material through (junction).

These free electrons are attracted towards the positive terminal of the diode while the holes are attracted towards the negative terminal of a diode.

Working of diode
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For More Read: Forward Bias of PN Junction [in Detail]

Reverse Bias of Diode:

Reverse bias is the condition that essentially prevents current through the PN junction diode. As mentioned above if we connect -ve terminal of the battery to P-type material and +ve Terminal of Battery to N-type material this lead to the diode in Reverse Bias. note that the depletion region is shown much wider than in forward bias.

A diode connected for reverse bias. A limiting resistor is shown although it is not important in reverse bias because there is essentially no current.

Reverse Bias of PN junction Diode
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An illustration of what happens when a PN junction diode is reverse-biased is shown below. When the P-type material is connected with a negative terminal of a battery, the holes are attracted away from the junction and attracted to the negative electrodes of batter.

Similary when the N-type material is connected with a positive terminal of a battery, the free electrons are attracted away from the junction and attracted towards the positive electrodes.

This results in an increase in the depletion region. As the depletion region widens, the availability of majority carriers decreases. As more of the n- region and p-regions become depleted of majority carriers, the high potential barrier is created thus opposing electric current to flow in reverse bias.

Effect of deplation region on PN junction Diode

Note: if the reverse bias voltage is increased up to a high value, it will damage the PN junction diode.


As you have learned, forward bias produces the current through a PN junction diode and reverse bias essentially prevents current, except for a negligible reverse current. Reverse bias prevents current as long as the reverse-bias voltage does not exceed the breakdown voltage limit of the junction. Now we will examine the relationship between the voltage and the current in a diode on a graphical basis.

Read More: What is the Duty Cycle?

Effect of Forward Bias on V-I Characteristics of PN Junction Diode:

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When a forward-bias voltage is applied across a diode, there is current. This current is called the forward current.

When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward current begins to increase rapidly, as illustrated in Figure given below. As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode is constant till 0.7v for silicon and 0.3v for germinium.

PN junction diode on forward bias

Effect of Reverse Bias on V-I Characteristics of PN Junction Diode

When a reverse bias is applied across a PN junction diode, there is an extremely small reverse current (IR) through the PN junction due to minority carriers.

Once the applied bias voltage is increased to a value where the reverse voltage across the diode reaches the breakdown value of the diode which is (VBR), the reverse current begins to increase rapidly. As you further increase the bias voltage, the voltage across the diode increases above Breakdown, and diode become damaged, thus it’s not a normal mode of operation for most PN junction devices.

PN junction diode on Reverse bias

Complete V-I Characteristics on PN Junction Diode

Characteristics curve of PN junction diode

Combine the curve for both forward bias and reverse bias, and you have the complete V-I characteristic curve for a PN junction diode, as shown in Figure give below.

Applications of PN Junction Diode:

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  1. PN Junction Diodes are mostly used for rectification (Alternative Current to Pulsating DC).
  2. They are used as clipper to clip the portion of AC.
  3. They are used as clamper to change the reference voltage.
  4. They are used as switches in many electronic circuitry.
  5. They are used in Voltage Multipliers to increase the output voltage.
  6. They are used in power supplies.

There are Many different types of PN Junction Diode, and we have covered all of them check out the  working of different types of diodes:

  1. What is a Zener Diode?
  2. What is LED?
  3. What is PhotoDiode? 
  4. What is Tunnel_Diode?
  5. What is Varactor_Diode?

This is all about PN Junction Diode Working, Operations, and its V-I Characteristics if you like our article or you think you have learned from this PN Junction Diode, its V-I Characteristics please share and comment below. Thanks and Stay connected with

Difference between diode and zener diode

Difference Between Diode and Zener Diode (Updated)

A major difference between Diode and Zener Diode is that a PN junction diode can operate in forward bias only whereas a Zener Diode can operate in Forward bias as well as in reverse bias. There are a number of differences between a normal diode and a Zener diode and in this article, we will cover them one by one.

We will cover differences between a Diode and Zener diode with respect to their Symbols, Constructions, Operations, and Applications and after reading this article you will be able to understand all major differences between a Diode and Zener diode.


Difference between Diode and Zener Diode

Chart down below shows the difference between Diode and Zener Diode.
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Definition Diode is a semiconductor device which allow current in only one direction.Zener diode is special purpose diode which allows current in forward and reverse bias both.
SymbolDiode symbol (2019)Zener diode
OperationDiode is always operated in forward bias it get damages when operated in reverse bias.Zener diode is special diode it can work in forward bias as well as in reverse bias.
DopingDiode is less doped semiconductor device.Zener diode is 1000 times more doped compared to a diode.
ConductivityDiode is uni-directional device (only allow current in one direction).Zener diode is bi-directional device (can allow current in forward and reverse direction).
Breakdown VoltagesDiode has very low breakdown voltage it can not sustain reverse voltages.Zener diode has high breakdown voltages it can sustain large breakdown voltages.
It ObeyDiode obeys Ohm's LawZener diode does not obey Ohm's Law.
ApplicationsDiode is used in rectification, Clipping, Clamping, Voltage Multipliers, Power supplies, Protection Circuits .etc.Zener diode is mostly used in voltage regulators circuits.


What is Diode?

A diode is a semiconductor device which is formed when two alternative semiconductors are joined together i.e. when P-layer of Semiconductor and an N-layer of a semiconductor is joined together a junction is formed called a PN junction also called a Diode.

A diode is a current controlling device which is used to control the current in one direction. It is used for applications like Switching, Rectification, Clipper circuits, Clamper circuits, Voltage multipliers, etc.

The P-layer can be considered as a positive layer because it has holes in the majority whereas N-layer is considered as a Negative layer which has electrons in the majority.

Diode symbol (2018)

A diode is similar to a switch it has two modes of operation. When the diode is forward bias it behaves like a closed switch (ON Switch), and when the diode is reverse bias it behaves like an open switch (Off switch).

When the P-type material is connected with a positive terminal of the battery and N-type material is connected with a negative terminal of the battery then the diode is said to be as Forward bias.

When the N-type material is connected with a positive terminal of a battery and P-type material is connected with a negative terminal of battery then the diode is said to be as Reverse Bias.

During forward bias of the diode, the diode does not conduct immediately, but after a unique forward voltage, it starts to conduct. That forward voltage is commonly known as Diode knee voltage. If the diode is made up of silicon material than the knee voltage is 0.7V and if the diode is made up of germanium then the knee voltage is 0.5v.

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Pn junction diode

During reverse bias of diode, the depletion layer starts to widen. Hence a wide depletion region has more resistance for the movement of majority carries thereby electric conductivity is very low. Therefore; no electric current flows in reverse bias of diode.


But the minority carriers can flow during reverse bias condition, constituting a very small current in the diode, that small current is temperature dependent. If the reverse bias increase beyond the value of temperature also increases and the minority carriers also increases, which can cause the diode to damage hence reverse bias condition is not used in diode operation.

Therefore, a diode is always operated in the forward bias mode.

What is Zener Diode?

The Zener diode is a special purpose diode which is always operated in the breakdown region. It has the special ability to allow the current to flow in forward direction as well as in the reverse direction, The Zener diode is highly doped as compared to the normal diode.

Zener diode

In terms of the construction, the Zener diode is constructed similar way as a normal diode is constructed; the only difference in terms of their construction is that a diode is less doped as compared to a Zener diode.
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The Zener diode has two modes of operation, When the Zener diode is forward bias It behaves same like a normal diode after the knee voltage of 0.7V it conducts the current in one direction just a like a normal diode, but when the Zener diode is reverse biased, it operates in breakdown region which means it does not get damaged in reverse bias but it works in reverse bias region which makes this diode bidirectional semiconductor device.

zener diode construction

During the reverse bias of the Zener diode, the depletion layer starts to reduce, because a Zener diode is a highly doped diode hence it has very thin depletion region, hence the electric conductivity is high. When the reverse voltage reached at breakdown region the current start to increase in reverse direction and Thus, a Zener diode behaves as a voltage regulator.

Read More: Download any E-Book for Free from Z-Library

Key differences Between Diode and Zener Diode

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  1. Normal diode is operated in forward bias, whereas a Zener diode is a special purpose diode which is operated in forward bias and reverse bias.
  2. In terms of their electric current conductivity, a Diode is a uni-directional device which conducts current in only one direction whereas Zener diode is a bi-directional device which can conduct in forward bias as well as in reverse bias.
  3. In terms of their doping, A normal PN junction diode is less doped as compared to a Zener diode whereas Zener diode is highly doped semiconductor diode.
  4. In terms of their breakdown voltage, A normal diode has very low breakdown voltage it can sustain less amount of reverse voltage, whereas A Zener diode has a very high breakdown voltage which means it can sustain greater reverse voltage compared to the normal diode.
  5. In terms of their operation, A normal diode can operate only in forward bias whereas a Zener diode can operate in reverse bias as well in forward bias.
  6. A normal diode is a primary device and a Zener diode is a secondary device by just applying more doping to a normal diode a Zener diode can be obtained.
  7. A normal diode obeys Ohm’s law whereas a Zener diode does not obey Ohm’s law. Hence, a diode is PTC device whereas Zener diode is NTC device
  8. In terms of their applications, A normal diode is used as rectification operation, clipping operations, voltage multipliers, etc. whereas a Zener diode is used as a voltage regulator.


The Diode and Zener diode are different from each other with respect to their symbols, construction, operations, and applications, a major difference between diode and Zener diode is the electric current conduction normal diode can conduct in one direction whereas Zener diode is able to conduct in both forward and reverse direction.
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Another major difference is that normal diode is less doped whereas Zener diode is a highly doped semiconductor.

A normal diode can get damaged in reverse direction but Zener diode is designed to operate in reverse direction.

A normal diode is used for rectification purpose, clipping, clamping and voltage multiplication operations whereas the Zener diode is commonly used for voltage regulation operations.

What is Varactor Diode | Construction, Working, Characteristics, And Applications

In this article, We will cover Varactor Diode also known as varicap diode, we will cover their VI Characteristics, Construction, and their practical applications step by step.

What is a Varactor Diode?

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A P-N junction diode which acts as a variable capacitor by changing reverse bias is known as a Varactor Diode.

  • Word Varactor is given to it due to its property of varying capacitance.
  • Varactor diode can only be operated in reverse bias. It acts like variable capacitance in reverse bias operation.
  • Varactor Diode is also known as Tuning Diode, Variable Reactance Diode, Varicap Diode or variable capacitance diode.
  • The property of varying capacitance is utilized to achieve a change in the phase of an electrical circuit or in the frequency.

Symbol of Tunnel Diode:

The Varactor diode is a two terminal device one terminal is Cathode and another Anode. Its cathode has an extra capacitor, parallel two lines show the construction of capacitor and space between them shows a dielectric medium.

symbol of varactor diode

Working of Varactor Diode

this is important

  • When a P-N junction is formed, the depletion layer is created in the junction area. Since in the depletion region there are no charge carriers, hence the zone acts as an insulator.
  • The P-type material with holes (considered as positive) as majority carriers and N-type material with electrons (considered as negative charge) as majority carriers act as charged plates in the PN junctions. Thus the diode may be considered as a capacitor, whose n and p-region are forming opposites charge plates and with depletion region between them is acting as a dielectric, as happens in the capacitor. This is illustrated in Fig. given below.
  • A varactor diode is specially constructed to have high capacitance under reverse bias, it varies its capacitance when it is applied reverse biased voltages.
  • As the reverse potential increases the width of the depletion region increases, which in turn reduces the capacitance.

V-I Characteristics of Varactor Diode.

By applying a reverse voltage across varactor diode, the width Wd of the depletion layer increases. Therefore, the capacitance of the junction decreases.

On the other hand, if we reduce the reverse voltage across the varactor diode, the width Wd of the depletion layer decreases. Consequently, the total capacitance of the junction is increased.

Given fig: below shows the curve between reverse bias voltage VR across varactor diode and total junction capacitance CT.

characteristics of varactor diode

Relation Between Voltage and Capacitance.


  • Note that capacitance can be changed simply by varying the voltage VR. For this reason, a varactor diode is also called the voltage-controlled capacitor.

Let’s see how it is constructed 😉

Construction of Varactor Diode:

Consider the PN junction, The P-type is consisting of free carriers which are called holes, and N-type is consisting of free carriers which are called electrons. Since they are free carriers, when they are brought together, a junction is formed. Because they have a tendency to merge with each other; the hole has a tendency to merge with the electron and vanish.
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When this happens, the region at the junction becomes free of carriers. Thereby, it creates a potential barrier; this is called the junction potential, and this potential barrier of a junction, happens to be around 0.5V to point 0.7 volts. It starts conducting only when a junction potential is about 0.5V to 0.7 volts is applied, Until that time, current is negligible. So, this region, where it is depleted of carriers, is called the depletion region. So, every P-N junction has a depletion region in between, and this depletion region width is dependent upon the voltage.

When the junction is forward biased, the barrier potential for these carriers decreases; so there is the notable amount of free carriers moving from this (electrons and holes), due to that moment it constitutes a current; hence large current can flow.

When the junction is reversed biased; the potential barrier is increasing; the reverse bias will help this potential barrier to get increased. That means, more area in this gets depleted of carriers and this region widens. So, we can conclude that the depletion layer width is dependent on the applied voltage.

When it is forward biased, the depletion layer width is decreased, and when it is reversed biased, the depletion layer width increases. Now, since the depletion region has depleted all the carriers, we can consider this region as equivalent to two parallel plates of a capacitor within which we have sandwiched an insulator (Di-electric), No carriers exist there; so, it is an insulator. So, we can say its equivalent to a capacitor.

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Application of Varactor Diode:

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Voltage-Controlled Tuning Circuit:

We have learned that we can decrease or increase the junction capacitance of varactor- diode by changing the reverse bias voltages on the diode. This makes a varactor diode ideal for being used in circuits that require voltage-controlled tuning. Given Fig: below shows the use of varactor diode in a tuned circuit.

voltage controlling tuning circuit

the voltage controlling tuning circuit

Note that the inductor is parallel with the capacitance of varactor diode. The inductor and the varactor diode form a parallel LC circuit. For normal operation, a varactor diode is always operated in reverse bias as we studied.

In fact, this condition is shown in the picture above. The resistance RW in the circuit is the resistance of inductor’s winding. This winding resistance of the inductor is in series with the potentiometer (R1).

Thus RW and R1 form a voltage divider which is used to measure the amount of reverse bias voltage across the varactor diode D1 and therefore its capacitance. By adjusting the setting of a potentiometer (R1), we can vary the capacitance of diode easily. This, in turn, changes the resonant frequency of the Inductor Capacitor LC circuit.

The resonant frequency Fr of the Inductor Capacitor LC circuit is given by:

resonant frequency formula

If the amount of reverse bias on varactor diode is increased, the value of Capacitance (C) of the varactor increases. The increase in Capacitance will cause the resonant frequency Fr of the circuit to decrease.

Thus, by decreasing reverse bias will decrease the resonant frequency and vice-versa.

Advantages of Varactor Diode:

The following are the few advantages of the varactor diode.

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    • It has small size and portable you can use it in circuits easily.
    • It is reliable you can use it to vary capacitance.
    • It is very low cost, thus it can be used in various electronic and electrical applications.
    • It generates very less noise as compared to others P-N junction diodes.
    • It has very low power loss because varactor diode produces very low noise.

    Disadvantages of Varactor Diode

    The following are the few Disadvantages of the varactor diodes.

    [su_note note_color=”#f9fcd2″]These type of diodes are specially designed to work in the reverse biased mode, Hence they are not useful when operating them in forward bias mode.[/su_note]

    Applications of Varactor Diode

      1. Varactor diodes are mostly used as tuned capacitors and they have significantly replaced mechanically tuned capacitors in various electronic applications. Hence they can be used in televisions in the resonant LC tank circuit.
      2. A varactor is used in Radio Receivers for tuning the circuit.
      3. In terms of electronic application, it is also used as a frequency multiplier.
      4. A varactor is helpful in frequency modulation.
      5. A varactor is always operated in reverse bias and it produces capacitance effect, this is because its anode and cathode terminals act as the plate of the capacitor and the free space between them acts as a dielectric material. And because of a large range of capacitance variation these diodes are used in high-frequency areas like TV tunning and FM radio operations and frequency modulation.
      6. They are also used in frequency control circuits.
      7. They are used in bandpass filters.
      8. Voltage control oscillators are widely used in receiving and transmitting circuits in the field of communication. And a varactor diode plays a very significant role in the construction of voltage controlled oscillator.

      This is all about Varactor Diode Working, Construction, and Practical Applications, and if you like our post give a thumbs up and comment below to appreciate the work and stay connected with us.

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      6. Transistor Configurations | Common Emitter, Base and Collector Circuits

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