Oscillator Theory, Lecture notes of Electronics

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OSCILLATOR THEORY

OSCILLATOR THEORY 9.1 Introduction Feedback plays an important role in almost all electronic circuits. It is almost invariably used in the amplifier to improve its performance and to n.ake it more ideal. In the process of feedback, a part of output is sampled and fed back to the input of the amplifier. Therefore, at input we have two signals : Input signal, and part of the output which is fed back to the input. Both these signals may be in phase or out of phase. When input signal and part of output signal are in phase, the feedback is called positive feedback. On the other hand, when they are out of phase, the feedback is called negative feedback. The positive feedback results into oscillations and hence used in electronic circuits to generate the oscillations of desired frequency. Such circuits are called oscillators. 9.2 Concept of Positive Feedback The feedback is a property which allows to feedback the part of the output, to the same circuit as its input. Such a feedback is said to be positive whenever the part of the output that is fed back to the amplifier as its input, is in phase with the original input signal applied to the amplifier. Consider a non-inverting amplifier with the voltage gain A as shown in the Fig. 9.1. Output signal Feedback signal eben $2255 0° phase shift Fig. 9.1 Concept of positive feedback Assume that a sinusoidal input signal (voltage) V, is applied to the circuit. As amplifier is non-inverting, the output voltage V, is in phase with the input signal V,. The part of the output is fed back to the input with the help of a feedback network. How much part of the output is to be fed back, gets decided by the feedback network gain B. No phase change is introduced by the feedback network. Hence the feedback voltage V, is in phase with the input signal V,. Key Point: As the phase of the feedback signal is same as that of the input applied, the feedback is called positive feedback.