Basic Electronics for Scientists and Engineers

Introduction

The op-amp astable oscillator covered in Section 6.5 was our first example of an oscillator – a circuit that produces a periodic output signal without an input signal. These types of circuits have some kind of feedback mechanism that allows them to oscillate spontaneously. We can categorize oscillators into two broad groups: relaxation oscillators and sinusoidal oscillators. Each of these groups has common features. The relaxation oscillators are characterized by non-sinusoidal output waveforms, timing that is set by capacitor charging and discharging, and the non-linear operation of its active components. The analysis of relaxation oscillator circuits is done in the time domain (i.e., by determining the circuit response as a function of time). For example, our op-amp astable oscillator has a square wave output with a period set by the charging/discharging of capacitor C through resistor R f, and the op-amp is operating non-linearly, switching back and forth between its saturation voltages. On the other hand, sinusoidal oscillators, as the name implies, have sinusoidal output waveforms and linear operation of the active components, and the analysis is done in the frequency domain (i.e., by considering how the circuit responds to different frequencies). We will now examine examples of each type of oscillator.

Relaxation oscillators

SCR sawtooth oscillator

Our first relaxation oscillator is shown in Fig. 7.1. It uses two components we have studied previously: the SCR and the bipolar transistor.