In this chapter we shall be concerned with circuits which generate waveforms (oscillators) and circuits which change the shape of waveforms. Oscillators form a basic part of a large number of electronic systems ranging from radio and television transmitting and receiving systems to industrial control and measuring systems. The basic principle of most oscillators is that positive feedback is applied to an amplifier so that it provides its own input. Such an oscillator consists of three parts (shown in figure 6.1a): the amplifier; a network providing feedback and selecting the fraction of output signal fed to the input; and, finally, a frequency-sensitive circuit that controls the oscillator output frequency. Many feedback oscillators use the frequency-selective properties of a tuned circuit; others use resistor-capacitor phase shifting networks.
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