Analog Electronics, Electronics, Transistor

Common Base Amplifier, BJT Transistor Common-Base Amplifier

In our previous discussion, we learned that a common-emitter amplifier is most widely used amplifier circuit due to the fact that it provides good current gain and voltage gain, and Common Base Amplifier is least widely used amplifier from all three types of transistor amplifier configurations.

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

 

What is a Common Base Amplifier?

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

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

common base amplifier

The common Base amplifier provides a high voltage gain and unity current gain and it has low input impendence and high output impendence. 

Common Base Amplifier is least widely used amplifier configuration compared to other two amplifier configurations. However, it has very useful use in high-frequency applications because its terminal characteristics at high frequencies are better than those of a common-emitter configuration.

If you don’t know how transistor amplifies? Click here.

Working of Common Base Amplifier:

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

The Base-Emitter junction is forward bias by VEE and Base-Collector Junction is reverse biased by VCC.  Q-point parameters of common base Amplifier are calculated through the values of DC bias and resistor values.

common base amplifier working
common base amplifier working

When no signal at the input is applied, there is no effect at the output. And let’s see when we apply AC signal to the input of a common base amplifier what happens.

During Positive cycle of input, the forward bias of base-emitter junction is decreased because VBE is negative w.r.t ground resulting in a decrease in IB.

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

V0=Vc-ICRC

Practical Common Base Amplifier Circuit:

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

common base amplifier circuit
common base amplifier circuit

 

  • Here:
  • Cb:  Cb is bypass capacitor connected in the Base terminal of Common base amplifier, it is used for removing ac noise signal.
  • Ce: This is coupling capacitor in case of Common base amplifier, it is used to couple input voltages to the circuit.
  • R1 and R2: They are biasing resistors, they are used for the providing stable biasing to the amplifier circuit.
  • RC: It is collector resistor, the output is taken after this Rc resistor.
  • RE: This RE resistor is placed in the emitter terminal of a transistor, and it is useful to control the gain of the amplifier.

Characteristics of Common Emitter Amplifier

  • It has very low input impendence almost (30-200ohm).
  • It has very high output impendence almost ( 400-500K).
  • It has a current gain of unity or (< 1).
  • It has a large voltage gain of (approx.: 1500 to 2000).
  • It has no phase reversal between output and input.

The voltage gain of Common Base Amplifier:

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

Here output voltage is referred to as ΔVc and the input voltage is referred to as ΔVe.

Av=Rc/re’.

Current Gain of Common Base Amplifier:

Current gain in CB amplifier is unity. In CB transistor configuration the current gain is denoted by greek symbol alpha (α).

The output current is referred to as IC and input current is referred to as IE. Hence IC≅IE.

α=IC/IE1

Power Gain of CB Amplifier:

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

Ap=AvAi=Av*1

∴ Ap=Av

Input characteristics

It is the curve between IE and VBE whereas VCB is constant.

 Input characteristics curve of common base amplifier

Output characteristics

It is the curve between IC and VCB whereas IE is constant.

 Output Characteristics Curve of Common Base amplifier

Applications of Common Base Amplifier:

Common Base amplifier configuration is least widely used. However, it has a few good applications as listed below.

  • It is used as Pre-amplifiers in the microphone circuit.
  • It is used in Used for providing high output impendence in UHF and VHF amplifiers.
  • The CB amplifier is useful at high frequencies when impedance matching is required because input impedance can be controlled.
  • It is used as a current buffer amplifier because it provides constant current gain. 
Disadvantages of Common Base Amplifier:
  • In the CB amplifier, there is no current gain.

Also, read:

  1. Common Emitter Amplifier, BJT Transistor Common Emitter 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.

Conclusion


In Common Base Amplifier, Input is applied to B-E Junction and Output is taken from B-C terminal, here base terminal is common for both input and output. It is least widely used amplifier circuit because it only provides good voltage gain whereas current is unity. And it has very low input impendence hence it is used in pre-amplifiers in the microphone base circuit and also for impendence matching application of frequency amplifier.

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