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In an ideal world, electronic circuits would process signals perfectly.
In real life, circuits introduce errors in the output called DISTORTION.
Well designed circuits minimise distortion.

Clipping, Limiting or Saturation

• Amplifiers can not produce an output voltage greater than the power supply voltage.
• When this happens, the top and/or bottom of the waves is clipped off and the output signal is damaged.
• Bad audio quality at high volume is one example.
• In the diagram, the black wave is the ideal output. The red and blue show the clipped output.
• The left image is an oscilloscope X/Y plot. The right hand image is a normal oscilloscope display with time on the X Axis.

Cross-Over Distortion

Cross over distortion happens when the input signal is so small that neither device in a push-pull amplifier is turned on. Low input voltages are missing from the output. This can be reduced by BIASING the amplifier MOSFETS and by adding NEGATIVE FEEDBACK to the circuit.

Non-Linearity

The red wave is what the amplifier should produce. The blue wave is distorted from the wanted shape. There is no clipping. The low input levels are not damaged. NEGATIVE FEEDBACK greatly reduces this type of distortion.

Output Slew Rate Problems

Amplifiers have an upper limit to the rate at which the output can rise or fall. This is called the slew rate.

A sudden input voltage change will cause a less sudden output change. Even sine wave inputs can be damaged. The output tries but fails to keep up with the input changes.

This is somewhat like the 0 to 60 performance figure for a car. If you "floor" the accelerator, it still takes time for the car to build up speed.

Biasing and negative feedback don't help.

The only fix is to use devices with a higher slew rate. Both signals in the diagram below are distorted due to the slow output slew rate.

Don't confuse Distortion with Noise

Distortion occurs when circuits don't reproduce input signals perfectly.

Noise comes from external sources or sometimes from the circuit itself because it's warm. This warmth causes random atom vibrations along with the electrons. This random electron movement, if amplified, appears as a hissing noise in the output.