Negative Feedback

Negative Feedback

Feedback means using a fraction of the output voltage of a circuit to add to the input. When the signals at the input and the output are oppositely phased (the output is a mirror image of the input), the feedback signal is said to be negative. Negative feedback has the effect of subtracting the fed-back signal from the input signal so that it reduces the overall gain of the amplifier. The effect on the gain is as follows:

Let A _o = gain of amplifier with no feedback (also known as the open-loop gain)

? = feedback fraction (or loop gain), so that V _out/ ? is fed back

Then the gain of the amplifier when negative feedback is applied is:

For example, if the open-loop gain is 100 and ? = 20 (so 1/20 of the output voltage is fed back in opposite phase), the closed-loop gain is:

A very useful approximation is that if the open-loop gain A _o is very much larger than the feedback fraction (loop gain), the closed-loop gain is simply equal to ?. This is because A/ ? is large, much larger than unity, so the 1 in the equation can be neglected. This makes the expression become:

Negative feedback, in addition to reducing gain, also reduces noise signals that originate in the components of the amplifier if these components are within the feedback loop. It will also...