9.1 INTRODUCTION

In the preceding chapter, we intensively discussed the concept of negative feedback and its effects on amplifiers in detail. In the present chapter, the effect of feedback will be discussed in the context of working and analysis of oscillator circuits to generate sinusoidal and non-sinusoidal pulses and waveforms. The basic concept of oscillator differs from that of an amplifier. An amplifier amplifies a given AC input signal by using the DC power supply VCC and resistive networks to provide proper bias and stability with the aid of negative/degenerative feedback so as to stabilize gain and enhance bandwidth of operation. As completely opposed to this, an oscillator circuit needs no input AC signal. The power supply DC input VCC helps in picking up noise signals from its wide frequency spectrum, which due to regenerative/positive feedback networks and amplifier, gets sustained in the oscillatory circuit. Hence DC input gets converted into AC output. Thus, an oscillator can also be referred to as AC-DC converter. The working of an oscillator is exactly opposite to rectifier; hence it can be called an ‘inverter’. But it should be well understood that an oscillator does not create energy, instead it converts unidirectional current drawn from DC source into alternating current of desired frequency. Hence, the oscillator can be broadly defined as an amplifier with positive feedback. The difference between amplifier and oscillator is explained in the block diagram of Figure 9.1(a).

9.2 CLASSIFICATION OF OSCILLATORS

Oscillators can be broadly classified using following two aspects,

• a) Classification based on design principle

• b) Classification based on frequency of oscillation

a) Figure 9.2 shows the detailed family tree of various types of oscillators based on the design principle.

From Figure 9.2, based on the design principle, commonly used oscillators can be broadly classified as harmonic and relaxation oscillators. Harmonic oscillators generate sinusoidal waveforms, whereas relaxation oscillators generate pulse/rectangular/sawtooth waveforms. The basic tuning circuit for harmonic oscillators can be made of combination of L-C networks, which are suitable for generating RF (radio frequency) oscillations from 720 KHz up to hundreds of MHz, whereas R-C networks are suitable for generating AF (Audio Frequency) oscillations in the range of 20 Hz–20 KHz. The detailed description of each of the oscillator is performed in subsection 9.5 and 9.6.