The output of a system is measured and a signal representing this measurement is fed back to the input.
Negative feedback acts to correct errors in the output ...
Normally negative feedback is wanted. This acts to correct errors in the output.
- a proportion of the output is fed back and subtracted from the input signal
- it reduces distortion in audio amplifier circuits and also reduces the gain
- it increases the bandwidth of the amplifier
- it controls the gain of operational amplifiers and makes the gain predictable and easy to calculate
- it controls the gain of radio frequency amplifiers to give a constant audio output while the radio signal gains or loses strength (fading)
- it acts to correct errors in control systems (too cold so turn on the heating/too hot so turn off the heating)
Positive feedback exaggerates errors in the output ...
Positive feedback is needed for circuits that are intentionally unstable (oscillators, bistable, monostable, astable circuits and the Schmitt trigger).
- a proportion of the output is added to the input signal
- it increases the gain of amplifiers (but may cause instability or unwanted oscillations)
- it allows bistable circuits to settle in the fully-on or fully-off state (never half way)
- it causes sudden switching between states in astable circuits
- it causes the Schmitt Trigger on voltage to differ from the off voltage
- it causes loud howling noises with public address systems if the microphone is too near the loudspeakers
Systems which include feedback can suffer from hunting. This is an unwanted oscillation of the output caused by temporary positive feedback. A lack of damping may cause hunting that dies away. Certain types of audio feedback are examples of hunting. Control systems that respond too slowly to input changes may suffer from hunting. (Drunk people ???)
Another common example found in everyday life is engines (eg farm tractors) going vroom, vroom, vroom in a characteristic way. The engine revs are too low so the throttle is opened. The engine speeds up but overshoots and is now running too fast. The fuel is cut off and the revs drop. This happens repeatedly. The hunting is caused by the control system responding too slowly to the engine revs measurement. The negative feedback temporarily becomes positive due to the time delay.
Hunting can be reduced by altering the frequency response of the control system elements. Filtering electronic signals and damping mechanical systems can both be useful.
Hunting can be reduced by using "Fuzzy Logic". Instead of using binary on/off circuitry, analogue systems can be used. These gradually change the output in response to gradual input changes. Imagine a car cruise control system which used only full or zero throttle settings. It would control the speed of the car but also be most uncomfortable.
