Feedback Amplifiers part1

Feedback Amplifiers

Introduction

Most physical systems incorporate some form of feedback. It is interesting to note, though, that the theory of negative feedback has been developed by electronics engineers. In this search for methods for the design of amplifiers with stable gain for use in telephone repeaters, Harold Black, an electronics engineer with the Western Electric Company, invented the feedback amplifier in 1928. Since then the technique has been so widely used that it is almost impossible to think of electronic circuits without some form of feedback, either implicit or explicit. Furthermore, the concept of feedback and its associated theory are currently used in areas other than engineering, such as in the modeling of biological systems.

Feedback can be either negative (degenerative) or positive (regenerative). In amplifier design, negative feedback is applied to effect one or more of the following properties:

l. Desensitize the gain: that is, make the value of the gain less sensitive to variations in the value of circuit components, such as might he caused by changes in temperature

2. Reduce nonlinear distortion: that is, make the output proportional to the input (in other words, make the gain constant, independent of signal level),

3. Reduce the effect of noise: that is, minimize the contribution to the output of unwanted electric signals generated, either by the circuit components themselves, or by extraneous interference.

4. Control the input and output impedances: that is, raise or lower the input and output impedances by the selection of an appropriate feedback topology

5. Extend the bandwidth of the amplifier.

All of the desirable properties are obtained at the expense of a reduction in gain. In short, the basic idea of negative feedback is trade off of gain for other desirable properties.

THE GENERAL FEEDBACK STRUCTURE

Figure 5.1 shows the basic structure of a feedback amplifier. Rather than showing voltages and currents, Fig. 5.1 is a signal-flow diagram, where each of the quantities can represent either a voltage or a current signal. The open-loop amplifier has a gain A; thus its output is related to the input , by

 

The feedback signal su e source al hich is the input to the complete feedback a ier, signal s the input to the basic amplifier,

Here we note that it is this subtraction that makes the feedback negative; In essence, negative feedback reduces the signal that appears at the input of the basic amplifier.

Figure 5.1 General structure of the feedback amplifier. This is a signal-flow diagram, and the quantities x represent either voltage or current signals.

Implicit in the description above is that the source, the load, and the feedback network do not load the basic amplifier. That is, the gain A does not depend on any of these three networks. Figure 5.1 also implies that the forward transmission occurs entirely through the basic amplifier and the reverse transmission occurs entirely through die feedback network.

The quantity is called the loop gain, a name that follows from Fig. 5.1. For the feedback to be negative, the loop gain should be positive; that is, the feedback signal should have the same sign as thus resulting in a smaller difference signal . Equation (5.4) indicates that for positive , the gain-with-feedback will be smaller than the open-loop gain A by the quantity 1 + , which is called the amount of feedback. If, as is the case in many circuits, the loop gain is large, ≫ 1, then from Eq. (5.4) it follows that ≅ 1⁄ , which is a very interesting result: The gain of the feedback amplifier is almost entirely determined by the feedback network. Since the feedback network usually consists of passive components, which usually can be chosen to be as accurate as one wishes, the advantage of negative feedback in obtaining accurate, predictable, and stable gain should be apparent. In other words, the overall gain will have very little dependence on the gain of the basic amplifier, A, a desirable property because the gain A is usually a function of many manufacturing and application parameters, some of which might have wide tolerances.

Equations (5.1) through (5.3) can be combined to obtain the following expression for the feedback signal,

Thus for ≫ 1, we see that = , which implies that the signal 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 becomes an almost identical replica of the input signal . An outcome of this property is the tracking of the two input terminals of an op amp. The difference between and , which is is sometimes referred to as the "error signal," Accordingly, the input differencing circuit is often also called a comparison circuit. (It is also known as a mixer.) An expression for can be easily determined as

from which we can verify that for ≫ 1, becomes very small. Observe that negative feedback reduces the signal that appears at the input terminals of the basic amplifier by the amount of feedback, (1 + ).