Common Base Transistor Amplifier

The common base amplifier configuration is not widely used except in high frequency amplifiers where it has some distinct advantages.

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The common base amplifier is the least widely used of the three transistor amplifier configurations. The common emitter and common collector (emitter follower) configurations are far more widely used because their characteristics are generally more useful.

The common base amplifier configuration comes into its own at high frequencies where stability can be an issue.

Common base transistor amplifier basics

The emitter follower transistor amplifier characteristics enable the circuit to be used as a buffer amplifier.

Common base transistor configuration showing the base connection common to both input and output circuits
Transistor common base circuit configuration

For both NPN and PNP circuits, it can be seen that for the common base amplifier circuit, the input is applied to the emitter, and the output is taken from the collector. The common terminal for both circuits is the base. The base is grounded for the signal and for this reason the circuit may sometimes be called a grounded base circuit.

The common base amplifier configuration is not used as widely as transistor amplifier configurations. However it does find uses with amplifiers that require low input impedance levels. One application is for moving-coil microphones preamplifiers - these microphones have very low impedance levels.

Another application is within VHF and UHF RF amplifiers where the low input impedance allows accurate matching to the feeder impedance which is typically 50Ω or 75Ω. The configuration also improves stability which is a key issue.

It is worth noting that the current gain of a common-base amplifier is always less than unity.

However the voltage gain is more, but it is a function of input and output resistances (and also the internal resistance of the emitter-base junction). As a result, the voltage gain of a common-base amplifier can be very high.

Common base transistor amplifier characteristics summary

The table below gives a summary of the major characteristics of the common base transistor amplifier.

Common base characteristics
Parameter Characteristics
Voltage gain High
Current gain Low
Power gain Low
Input / output phase relationship
Input resistance Low
Output resistance High

The common base circuit does not find many applications for low frequency circuits - normally a high input impedance and low output impedance are desirable. However, it finds use in some high-frequency amplifiers, for example for VHF and UHF. In the common base configuration the input capacitance does not suffer from the Miller effect, which degrades the bandwidth of the common-emitter configuration. Also there is a relatively high isolation between the input and output and this means that there is little feedback from the output back to the input, leading to high stability.

Common base transistor amplifier circuit

The diagram below shows how a common base amplifier circuit can be implemented.

The same bias constraints apply to the common base circuit, but the application of the signals is different, allowing the base to be earthed and hence common to both input and output circuits.

Circuit of a basic common base transistor amplifier
Circuit of a common base transistor amplifier

In this circuit, the same biasing conditions apply. However care has to be taken in the choice of the emitter resistor to ensure that the correct impedance match is provided for the input signal.

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