13.1. LIMITS OF CLASS A AMPLIFIERS

In very cheap and small transistorradios we find sometimes - as an exception - class A amplifier even for the power stage, but operating only an earphone or a very tiny loudspeaker. The reason why is easy to see if we have a closer look to the quiescent point.

fig. 153

If you have a short look back to fig. 144a you will easily see, that in this case in a class A amplifier the output-voltage must be half of the supply voltage. The collector-current is therefore half of the maximum current and this means: AT QUIESCENCE (NO INPUT SIGNAL AND NO OUTPUT SIGNAL) THE POWER CONSUMPTION OF A CLASS A AMPLIFIER IS ALREADY 25 % OF THE MAXIMUM POWER CONSUMPTION OF THIS STAGE.

This makes clear: for high amplification of considerable power another kind of amplifier is necessary otherwise the energy losses are too much.

The next amplifier class which we will come to know now is called class B amplifier and it is designed for power-amplification.

A difference between class A and class B amplifiers easily to be realized is:

- Class A amplifiers amplify with a single transistor a whole sinewave of the input signal.
- Class B need at least two transistors to do the same.

GENERALLY:

If we want to avoid the losses - caused in a class A amplifier at quiescence - we have to shift the quiescence working point to lower powers.

This can be achieved by reducing the base-current to about “0”.

But if the base-current is kept “0” at quiescence at the output of a COMMON-EMITTER-CONNECTION (as we call the transistor circuit used up to here) can only be produced half of a sine-wave as shown in fig. 154.

fig. 154

So we have to use two transistors in order to amplify both parts of the sinewave and to find a way how to “couple” both parts together again.