Feedback

What is feedback and what's it good for?

- By Stuart Ball

When most people hear the word “feedback” they probably think of the ear-piercing squeal that comes from a PA (public address) system when the microphone is too close to the speaker. But feedback is an essential part of electrical, software, and system design — as long as it’s intentional and working how you want it.

WHAT IS FEEDBACK?

Feedback is what occurs when the output of a signal or system is fed back to an input, thereby affecting the output. Feedback comes in two basic varieties, positive and negative. Positive feedback is what hurts your ears in the aforementioned PA system squeal. With the exception of some rock bands, PA system feedback is usually something you want to avoid. Positive feedback occurs when the output is fed back to the input in-phase, reinforcing the signal.

POSITIVE FEEDBACK

Figure 1 is a schematic of a simple single-transistor phase-shift oscillator. This oscillator produces a sine wave at about 1.4KHz (depending on component tolerances) at the collector.

A phase-shift oscillator works because the resistor/capacitor ladder (R3, R4, R5, C2, C3, C4) produces 180° phase shift at one frequency and the inverting transistor produces another 180° phase shift, for a total of 360° phase shift. Provided there is enough gain in the entire loop, the circuit will oscillate. This circuit could also be built with an opamp. I won't go into oscillator theory here. I want to use this example to make a few points.