Common Emitter Transistor Amplifier Circuit

- essential facts, characteristics and design details for the common emitter transistor amplifier circuit.

The common emitter transistor amplifier is possibly the most widely used transistor configuration.

The common emitter transistor amplifier circuit is often seen as the standard format for a transistor circuit where voltage gain is required.

It is often used in small class A linear amplifier stages as well as logic outputs and many other areas. Here, its characteristics lend themselves to this form of application.

Common emitter transistor amplifier basics

The common emitter transistor amplifier characteristics are what could be termed a good all round level of performance.

basic concept of the common emitter amplifier configuration showing how the emitter is common to both input and output circuits.
Common emitter transistor amplifier circuit configuration

For both NPN and PNP circuits, it can be seen that for the common emitter amplifier circuit, the input is applied to the base, and the output is taken from the collector. The common terminal for both circuits is the emitter.

Common emitter transistor amplifier characteristics summary

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


Common emitter transistor amplifier characteristics
Parameter Characteristics
Voltage gain Medium
Current gain Medium
Power gain High
Input / output phase relationship 180°
Input resistance Medium
Output resistance Medium

Common emitter amplifier impedance levels

The input impedance is typically around 1kΩ, although this can vary considerably according to the circuit values and conditions. The low input impedance results from the fact that the input is applied across the base and emitter where there is a forward-biased junction,

Also the output impedance can be relatively high. Again this varies considerably upon the circuit values chosen and current levels permitted. This may be as high as 10kΩ or possibly more. However if current drain allows higher current levels to be drawn, the output impedance can be reduced considerably. The level of resistance or impedance comes from the fact that the output is taken from the collector where there is a reverse-biased junction.

Common emitter transistor amplifier gain

Another important factor is the gain level that can be achieved. There are two forms of gain that can be determined: current gain and voltage gain.

The current gain for the common emitter amplifier circuit is denoted by the Greek symbol β. This is the ratio of collector current to base current. This may be thought of as the ratio of output current to input current. To gain an accurate figure of the gain for a signal, the current gain for small input changes in current is often used. Using this the current gain, β, and the changes in input and output current are related in the following way:

The gain formula or equation for basic transistor current gain

Where
    β = current gain
    ΔIc = change in collector current
    ΔIb = change in base current

In order to look at the voltage gain of the common emitter amplifier circuit, it is necessary to look at the resistances or impedances for the input and output.

The basic formula or equation expressing the voltage gain for a transistor stage

Where
    Av = voltage gain
    Rc = collector circuit output resistance
    Rb = base circuit input resistance

Common emitter input output phase relationship

The common emitter transistor amplifier is the only configuration that gives an inversion, 180°, between the input and output signals.

The reason for this can be seen from the fact that as the input voltage rises, so the current increases through the base circuit. In turn this increases the current thought the collector circuit, i.e. it tends to turn the transistor on. This results in the voltage between the collector and emitter terminals falling.

In this way an increase in voltage between the base and emitter has resulted in a fall in voltage between the collector and emitter terminals, in other words the phase of the two signals has been inverted.

Practical common emitter amplifier circuit

While the basic theoretical circuits shown above are able to describe the basic operation of the common emitter amplifier in concept.

However, for the circuit to be able to operate in a real system, other elements such as bias, decoupling and the like need to be added. As a result, the overall circuit for a common emitter amplifier utilises several components to ensure that it is able to operate in the fashion required.

Diagram of a practical common emitter amplifier circuit
Common emitter amplifier complete with bias resistors, decoupling, etc

Within the circuit there are a number of components that provide different functions to enable the overall circuit to operate in the fashion required:

  C1, C2 These capacitors provide AC coupling between stages. They need to be chosen to provide negligible reactance at the frequencies of operation.
 
  R1, R2 These resistors provide the bias for the base of the transistor.
 
  R3 This is the collector load resistor within the common emitter amplifier.
 
  R4 This resistor in the common emitter amplifier provides a measure of DC feedback to ensure that the DC conditions within the circuit are maintained.
 
  C3 This is a bypass capacitor. The effect of R4 is to reduce the gain of the circuit. Bypassing the resistor enables greater levels of AC gain to be achieved.
 

This circuit provides the basis for a simple AC common emitter transistor amplifier stage. It is commonly used and the circuit values can easily be calculated as described on the next page.

By Ian Poole


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