Oscillator operation-working-feedback circuit block diagram


Posted on Feb 6, 2014

A feedback amplifier having closed-loop gain, Af greater than unity can be obtained by the use of a positive feedback. This result also satisfies the phase condition and thus results in the operation of an oscillator circuit. An oscillator circuit then provides a constantly varying output signal. If the out ­put signal varies sinusoidally, the cir


Oscillator operation-working-feedback circuit block diagram
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cuit can be called as a sinusoidal oscillator. But, if the output voltage rises and drops from one voltage level to another quickly, the circuit can be called a pulse or square-wave genera ­tor. To understand how an oscil ­lator produces an output signal without an external input signal, let us consider the feedback circuit shown in fig (a). In the figure Vin is the voltage of ac input driving the input terminals B-C of an amplifier having voltage gain A. This voltage drives a feedback circuit that is usually a resonant circuit, as we get maximum feedback at one frequency. The feedback voltage returning to point a is given by equationVf = A ² Vin where ² is the gain of feedback network If the phase shift through the amplifier and feedback circuit is zero, then A ² Vin is in phase with the input signal Vin that drives the input terminals of the amplifier. Now we connect point a` to point b` and simultaneously remove voltage source Vin, then feedback voltage A ² Vin drives the input terminals b c of the amplifier, as shown in fig. (b). In case A ² is less than unity, A ² Vin is less than Vin and the output signal will die out, as illustrated in second fig. (a). On the other hand if A P is greater than unity, the output signal will build up, as illustrated in second fig. (b). If A ² is equal to unity, A ² Vin equals Vin and the output signal is a steady sine wave, as illustrated in fig. (c). In this case the circuit supplies...




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