Feedback and Oscillators Amplifiers
FEEDBACK AMPLIFIERS
An ideal linear amplifier in the mid frequency region, provides an output signal that is an exact replica of the applied input signal. But, the practical amplifiers depart from the ideal one for several reasons like non linearity of transistor characteristics, parameter variation, temperature effects etc. As a matter of fact, some of these factors can be minimized by improving the involved basic device. The best approach is to use the principle of feedback to achieved a desired degree of improvement. The feedback may be defined as a process of injecting some energy from the output and then return it back to the input. Thus, the process of combining a fraction of the output energy (i.e. voltage or current) back to the input is called the feedback. The amplifiers, which use the feedback principle, are known as feedback amplifiers.
Feedback can either be negative or positive. In negative feedback, a portion of output signal is subtracted from input signal; in positive feedback, a portion of output signal is added to input signal. Negative feedback, for example, tends to maintain a constant value of amplifier voltage gain against variations in transistor parameters, supply voltages, and temperature. Positive feedback is used in the design of oscillators and in number of other applications. In this chapter, we will concentrate on negative feedback.
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