Electronic feedback loops
- Electronic feedback loops
Electronic feedback loops are used to control the output of electronic devices, such as amplifiers. A feedback loop is created when all or some portion of the output from an electronic device is fed-back to the input. A device is said to be operating open loop if no output feedback is being employed and closed loop if feedback is being used.
Electronic feedback loops take two forms: negative feedback loops and positive feedback loops.
Negative feedback loops exist when the fed-back output signal is out of phase with the input signal. This occurs when the fed-back signal is anywhere from 0° to < -360° with respect to the input signal. Negative feedback is generally used to correct output errors or to lower device output gain to a pre-determined level. In feedback amplifiers, this correction is generally for waveform distortion reduction or to establish a specified gain level. A general expression for the gain of a negative feedback amplifier is the asymptotic gain model.
Positive feedback loops occur when the fed-back signal is in phase with the input signal. Under certain gain conditions, positive feedback reinforces the input signal to the point where the output of the device oscillates between its maximum and minimum possible states. Under some circumstances, positive feedback may cause a device to latch, i.e., to reach a condition in which the output is locked to its maximum or minimum state.
References
P. Horowitz & W. Hill "The Art of Electronics" Cambridge University Press (1980) Chapter 3, relating to operational amplifiers.
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