Common Base Transistor Amplifier

- details of the common base transistor amplifier configuration.

The common base transistor amplifier configuration is not as widely used as the other two formats.

Nevertheless the common base transistor amplifier finds applications where a low input impedance is required.

The common base format is probably most commonly used for RF applications, where its input impedance can be used for matching to the low source impedances often found in this arena.

Common base transistor amplifier basics

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

Diagram showingt he common base transistor circuit configuration - see the base connection is common to both input and output
Common base transistor 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 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.

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

However the voltage gain may be 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 transistor amplifier characteristics
Parameter Characteristics
Voltage gain High
Current gain Low
Power gain Low
Input / output phase relationship
Input resistance Low
Output resistance High

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.

Diagram showing a practical realisation of a common base transistor circuit
Common base amplifier circuit

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.

By Ian Poole


<< Previous   |   Next >>


Share this page


Want more like this? Register for our newsletter








The Art of Electronics 3rd Edition
The Art of Electronics 3rd Edition

Paul Horowitz & Winfield Hill
The Art of Electronics was the best book for electronic circuit designers and...
Read more . .

USA bookstore UK bookstore
Guidebooks for electronics engineers
Training
Online - Effective Spectrum Analyzer Measurements
Learn how to make spectrum analyzer measurements at RF and microwave frequencies

More training courses

Books for electronics engineers
Whitepapers
New External Power Supply Regulations Are Coming in 2016
In this whitepaper, power supply experts CUI look at the standards & requirements for power supplies around the globe that will hit the industry by 2016.

More whitepapers










Radio-Electronics.com is operated and owned by Adrio Communications Ltd and edited by Ian Poole. All information is © Adrio Communications Ltd and may not be copied except for individual personal use. This includes copying material in whatever form into website pages. While every effort is made to ensure the accuracy of the information on Radio-Electronics.com, no liability is accepted for any consequences of using it. This site uses cookies. By using this site, these terms including the use of cookies are accepted. More explanation can be found in our Privacy Policy