12.2. FUNCTION OF A SIMPLE CLASS A AMPLIFIER

We know now the basical components of a class A amplifier and it is possible to have a closer look to the effects in this circuit when a signal is injected to the input of the circuit of fig. 147 (page 43).

- Suppose the signalsource has a positive halfwave of a signal between time 1 (t1) and time 2 (t2). This will cause a charging current for C1.

- This charge-current is flowing via the loop: signalsource, C1 and R2. At R2 this current will flow additional to the current flowing at quiescence. This means too an increase of the voltage at R2.

- The voltage at R2 is equal to the base-emitter-voltage. An increase of that voltage causes (according to the input-characteristics of the transistor) an increase of the base-current.

- The output-characteristics of the transistor tells us, that an increase of base-current causes an increase of collector-current.

An increase of collector-current lets the voltage at the collector-resistor increase, which again causes the collecter-emitter-voltage to decrease.

-Now there will flow a discharge current through C2 and this will cause a negative voltage drop at the load-resistance.

- Between t2 and t3 the negative halfwave will be injected, this will cause the same effects in our circuit, but with opposite direction.

fig. 147

REPRESENTATION OF THE FUNCTION OF A CLASS A AMPLIFIER IN THE FOUR-QUADRANT - CHARACTERISTICS.

Supposed the circuit is the same as described for fig. 147, and supposed that the input-signal has an amplitude of 50 mV we would find an output signal as derived in fig. 147a.

fig. 147a