(introduction...)

Amplifiers have to “amplify” signals. This means an input signal with a relatively low energy (low current and low voltage) is enlarged to an output signal of much more energy and a shape which should be (ideally) the same like the input-signal. Fig. 138 shows the general symbol of an amplifier.

fig. 138

An amplifier works on the following PRINCIPLE:

Beside the input-signal the amplifier is supplied with a supply voltage and a supply current, which means a supply power. This supply power is many times higher than the input signal power. Control-components (like for example transistors or triode valves) convert a more or less high percentage of the power into an output-signal. The part of the supply-power converted into the output signal is called the useful power Pout of the amplifer, while the rest of the supply power is dissipated as losses.

fig. 139

The main characteristics, which make an amplifer a good or a less good one, are the following:

A) a big difference between signal-power and NOISE-POWER. Every electrical device is producing electrical oscillations while operated.

These oscillations should be kept extremly small in comparison with the inputsignal.

B) As less DISTORTION of the shape of the input-signal as possible. (Low distortion or “Klirr-factor”.)

C) A good matching (see chapter - MATCHING) between:

- the input of signal source and the in put resistance and
- the output resistance and the load.

D) The fitting BANDWIDTH and a good FREQUENCY RESPONSE which means constant amplification over the whole range of frequencies which the amplifier is due to amplify.

E) A good EFFICIENCY which means as less losses as possible.

F) A good WORKING CONSTANCY which means all characteristics should be as constant as possible even under conditions when the temperature of the amplifier and the supply voltage is varying.

Amplifiers can be differentiated either by the special tasks they are built for, like for example:

- Preamplifiers (low level amplifier)
- Power-amplifiers
- NF-HF amplifiers
- selective amplifiers, or
- wide-band amplifiers....

Or by the principles they are working on, like for example:

- class A amplifiers
- class B amplifiers, or
- class C amplifiers.