Examples of feedback systems abound in daily life. The thermostat that senses the temperature of a room and turns the air-conditioning system on and off is one example. Another is the remote control that we use to select a channel on the television or set the volume at an acceptable level. The heating and cooling system uses a simple temperature transducer to compare the temperature with a fixed set point. However, we are part of the TV remote control feedback system; we operate the control until our senses tell us that the audio and optical information is what we want.

The theory of negative feedback in electronic systems was first developed by Harold Black of the Bell Telephone System. In 1928, he invented the feedback amplifier to stabilize the gain of early telephone repeaters. Today, some form of feedback is used in virtually every electronic system. This chapter formally develops the concept of feedback, which is an invaluable tool in the design of electronic systems. Valuable insight into the operation of many common electronic circuits can be gained by recasting the circuits as feedback amplifiers.

We already encountered negative (or degenerative) feedback in several forms. The four-resistor bias network uses negative feedback to achieve an operating point that is independent of variations in device characteristics. We also found that a source or emitter resistor can be used in an inverting amplifier to control the gain and bandwidth of the stage. Many of the advantages of negative feedback were actually uncovered during the discussion of operational amplifier circuit design. Generally, feedback can be used to achieve a trade-off between gain and many of the other properties of amplifiers:

• Gain stability: Feedback reduces the sensitivity of gain to variations in the values of transistor parameters and circuit elements.
• Input and output impedances: Feedback can increase or decrease the input and output resistances of an amplifier.
• Bandwidth: The bandwidth of an amplifier can be extended using feedback.
• Nonlinear distortion: Feedback reduces the effects of nonlinear distortion. (For example, feedback can be used to minimize the effects of the dead zone in a class-B amplifier stage.)

Feedback may also be positive (or regenerative), and we explore the use of positive feedback in sinusoidal oscillator circuits in this chapter. We encountered the use of a combination of negative and positive feedback in the discussion of RC active filters and multivibrator circuits in Chapter 12. Sinusoidal oscillators use positive feedback to generate signals at specific desired frequencies; they use negative feedback to stabilize the amplitude of the oscillations. Positive feedback in amplifiers is usually undesirable. Excess phase shift in a feedback amplifier may cause the feedback to become regenerative and cause the feedback amplifier to break into oscillation. Remember that positive feedback was identified in Chapter 17 as a potential source of oscillation problems in tuned amplifiers.