The effects of frequency and the model

The voltage and current gains of an amplifier are frequency dependent. The

variation of gain with frequency is due to capacitive effects. The source of

such effects is twofold:

(i) coupling capacitors

(ii) internal capacitances of the amplifier.

Let us briefly examine each of these sources in turn.

(i) The effects of coupling capacitors placed in the circuit

note the coupling capacitors C 1 and C 2.

The purpose of these capacitors is to couple, from an a.c. point of view,

the input and output of the amplifier to other stages, without upsetting the

d.c. conditions on the amplifier.

Within the designed frequency range of operation, the coupling capacitors

should behave as short circuits to signals.

Within this range the amplifier can be represented by the simple 'black

box' model as

If we are working outside the designed frequency range of the amplifier,

however, we may have to include the coupling capacitors in the model as

Remember, that a capacitor's reactance falls with increasing frequency. Thus, if a coupling capacitor can be regarded as a short circuit within the designed operating

range, then this assumption will be even more valid at frequencies above this

range. At frequencies below this range, however, the effects of the coupling

capacitors cannot be ignored.

(ii) Internal capacitances of the device within the box

There can be many causes of 'internal' capacitance. For example, in the

case of a transistor amplifier there are the interelectrode capacitances of

the transistor itself. The diagram below shows the three interelectrode

capacitances of a transistor.

It can be possible to show that two of these capacitance effects can be represented by a single equivalent capacitor C ¢i. Our black box model can be amended, if required, to include this equivalent input capacitance.

Remember, that the capacitance C ¢i appears in parallel with the input resistance of the amplifier. If, within the normal frequency range of the amplifier the effects of

C ¢i can be ignored, then this will be even more true at frequencies below this

range. At higher frequencies, however, the reactance of C ¢i will fall and its

effects cannot be ignored. It will tend to shunt the input resistance.