The General Feedback Structure

Shows the basic structure of a feedback amplifier, it is a signal-flow diagram, where each of the x can represent

either a voltage or a current signal.

 

The open-loop amplifier has a gain A; thus its output xo is related to the input xi by xo = Axi

 

The output xo is fed to the load as well as to a feedback network, which produces a sample of the output. This sample xf if related to xo by the feedback factor β, xf = βxo

 

The feedback signal xf is subtracted from the source signal xs, which is the input to the complete feedback amplifier, to produce the signal xi, which is the input to the basic amplifier, xi = xs - xf

(This substraction that makes the feedback negative, negative feedback reduces the signal that appears at the input of the basic amplifier)

 

The gain of the feedback amplifier can be obtained, Af = xo / xs = A/(1+Aβ), aka closed-loop gain.

 

The quantity Aβ is called the loop gain. For the feedback to be negative, the loop gain Aβ should be positive; that is, the feedback signal xf should have the same sign as xs, thus resulting in a smaller differene signal xi. For positive Aβ the gain-with-feedback Af will be smaller than the open-loop gain A by the quantity 1+ Aβ, which is called the amount of feedback.

 

Therefore, the feedback signal xf is xf = [Aβ/ (1+Aβ)]xs

 

Thus for Aβ>>1 we see that xf = xs, which implies that the signal xi at the input of the basic amplifier is reduced to almost zero. Thus if a large amount of negative feedback is employed, the feedback signal xf beomes an almost identical replica of the input signal xs. An outcome of this property is the tracking of the two input terminals of an op-amp. The difference between xs and xf, which is xi, is sometimes referred to as the error signal.' Accordingly, the input differencing circuit is often called a comparison circuit. (mixer). xi = xs/(1+Aβ)

 

The negative feedback reduces the signal that appears at the input terminals of the basic amplifier by the amount of feedback, (1+Aβ)