Common Collector Amplifier, BJT Transistor Common-Collector Amplifier

In our previous discussion, we learned that a common-emitter amplifier is most widely used amplifier circuit and Common Base Amplifier is least widely used amplifier whereas a Common Collector amplifier is moderately used amplifier.

In this article, you will be able to learn and understand the working of Common Collector Amplifier, their characteristics, and their applications.

 

What is a Common Collector Amplifier?

Common Collector Amplifier has Collector terminal as a common for both input and output.

Input is applied to the Base-Collector junction and output is taken from Emitter-Collector junction.

common collector amplifier 

The common collector amplifier is also known as Emitter Follower, and this is due to the fact that the signal applied to the input of this amplifier follows the output which is taken from the emitter terminal. Or in simple words, we can say that the Output follows the input hence it is known as Emitter Follower.

 

The common Collector amplifier can provide a good amount of current gain and very less amount of voltage gain approx. unity (<1), and it has high input impendence and low output impendence. 

The common-collector amplifier is considered a voltage-buffer since the voltage gain is unity

Working of Common Collector Amplifier:

 As shown below a Common Collector amplifier is made up of voltage divider bias, the input is Base-Collector junction and output is Emitter-Collector junction.

common collector amplifier working

During Positive cycle of input, the forward bias of base-collector junction is increased since Vbe is positive with respect to ground resulting increase in IB.

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

V0=Vc-ICRC

Consequently, we get positive half-cycle of the output. It means that a positive-going input signal results in a positive going output signal and, consequently, the input and output signals are in phase with each other

Practical common Collector amplifier circuit

In order to perform amplification with a common collector amplifier, we must consider the DC basing, capacitor and different resistors values. Figure down below shows the circuit of practical common Collector amplifier.

 common collector amplifier circuit

Here:

  • C1: C1 is input capacitor commonly known as coupling capacitors because of they couple input and the output circuit with the amplifier circuit.
  • R1 and R2: They are biasing resistors, they are used for the providing stable biasing to the amplifier circuit
  • RE: This RE resistor is placed in the emitter terminal of a transistor, and it is useful to control the gain of an amplifier.

Characteristics of Common Collector Amplifier

  • It has very high input impendence almost (20-500Kohm).
  • It has very low output impendence almost ( 30-1000).
  • It has a high current gain of  β+1 (50-500).
  • It has a very low voltage gain of (approx.: <1).
  • It has no phase reversal between the output to the input circuit.

 

The Voltage gain of Common Collector Amplifier:

The voltage gain of Common collector amplifier is the ratio of output voltage to the input voltage.

Here output voltage is referred to as ΔVo and the input voltage is referred to as ΔVi.

Av=1.

Current Gain of Common Collector Amplifier:

In CB configuration the current gain is denoted by greek symbol gamma (γ). And it is the ratio of output current to the input current.

The Current gain could be calculated using the equation

Ai = – Av * Zin / RL

Power Gain of CC Amplifier:

The Power gain of CC amplifier is the product of current gain and voltage gain, and in CC amplifier the voltage gain is unity hence power gain will be:

Ap=AvAi=Ai*1

Ap=Ai

Input Impendence of Common Collector Amplifier:

It is the ratio of the Input voltage (Vin) to input current (Ini)

Zin = Vin / Ini

Input impendence of CC amplifier could be calculated using the equation

Zin = Rs / (Av/Av`- l)

Output Impendence of Common Collector Amplifier:

It is the ratio of the Output voltage (Vout) to Output Current (Io). Zout = Vout / Io

the output impendence could be calculated using the equation:

Zout = (Av / Av`-l) * Rs 

Input characteristics

It is the curve between IB and VCB whereas VCE is constant.

 Input Characteristics Curve of Common Collector amplifier

Output characteristics

It is the curve between IC and VCE whereas IB is constant.

 Output Characteristics Curve of Common Collector amplifier

Applications of Common Collector Amplifier:

Common Collector amplifier is a very useful amplifier and it has few good applications which are listed below.

  • It is used for impendence matching.
  • It is used for driving heavy loads because it provides high current gain.
  • It is also used as voltage translation stage.
  • It is used as a voltage buffer because voltage gain in cc amplifier is constant.

 

Disadvantages of Common Collector Amplifier:
  • In the CC amplifier, there is no voltage gain.

Conclusion


In Common Collector Amplifier, Input is applied to B-C Junction and Output is taken from E-C terminal, here Collector terminal is common for both input and output. It is also known as Voltage Follower and it is moderately used amplifier circuit because it has a good current gain but the voltage gain is unity. And it has very high input impendence very low output impendence making it ideal for being used as voltage buffer amplifier. It is used for driving heavy loads because the current gain is very high and it can drive any high resistive circuit.

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