Distortion occurs when the output differs from
what would be expected from an ideal circuit.
Amplifiers should be linear. That means the output should be exactly proportional
to the input.
Only the best designed circuits come close to this ideal.
Negative feedback reduces distortion and most audio amplifiers use this technique.
Types of distortion ...
- Crossover distortion - Occurs
when the output signal is crossing the zero volt level. This is most noticeable
with small signals.
- Limiting - Occurs when the
output ought to exceed the power supply voltage. As this is not possible,
limiting occurs. This happens with large signals.
- Non linearity - Most transistors (bipolar or FET) are not exactly linear.
Doubling the input will not exactly double the output. This causes distortion.
The effect can be minimised by using negative feedback.
- Hum - This is not strictly distortion. There are three common causes of
hum. It can be caused by ...
poor power supply smoothing. The DC voltage includes a
residual AC signal.
Poor input cable screening causing mains hum to be
picked up from nearby powered devices.
Failure to use single
point earthing.
- Noise - This is not strictly distortion. Random electron movements (because they are warm) can be amplified and these appear as a hissing noise at the output.
