|Radio and Electronics (DED Philippinen, 66 p.)|
PROBLEMS, IN A SIMPLE CLASS A AMPLIFIER.
If amplifiers would be constructed like shown in fig. 147 they would only work correctly for a very short time.
After a short while of operation they would get instabil and would produce a lot of distortion.
DISTORTION means: the shape of the output-signal is very different from the shape of the input-signal.
DISTORTION DUE TO A WRONG QUIESCENT-VOLTAGE
The output-voltage V2 can only reach levels between full supply-voltage and almost 0 Volts.
EFFECT OF TOO LOW BIASING VOLTAGE
Let us imagine the amplifier is supplied with maximum input-voltage V1, so that the sinewave - produced at the output - has a peak-to-peak-voltage of almost full supply-voltage as shown in fig. 147a.
If the quiescent voltage is now shifted because of any reason towards lower values the output voltage never can be higher than the supply-voltage. This means: during the negative halfwave of the input signal the output signal would be distored, as shown in fig. 148a.
EFFECT OF TOO HIGH BIASING VOLTAGE
the same problem arises if the quiescent-voltage at the input increases. Now the output signal would be destorted during the positive halfwave of the input signal as shown fig. 148b.
EFFECT OF A TOO HIGH AMPLITUDE OF THE INPUT SIGNAL THE
In case, the quiescent-voltage is constant but the input-signal is bigger than allowed for this amplifier-stage. This would cause a distortion on both peaks of the output-signal as shown in fig. 148c.
The method how to limit the input-signal will be shown later. At this stage of our course it is necessary to understand how the quiescent-voltage can be hold constant.