Oscillator Types

Oscillators fall into two broad families according to the kind of waveform they produce. Harmonic or linear oscillators generate a smooth sine wave by sustaining oscillation in a resonant feedback network, while relaxation oscillators produce non-sinusoidal waveforms such as square, triangular or sawtooth shapes by repeatedly charging and discharging a capacitor between threshold levels. The choice between them depends on whether a clean tone or a switching, timing waveform is needed.

The harmonic group includes the well-known LC designs. The Colpitts oscillator takes its feedback from a capacitive voltage divider, the Hartley from a tapped inductor, and both are common at radio frequencies. The Wien-bridge oscillator uses a resistor-capacitor network to produce a low-distortion sine wave at audio frequencies. A related category, the negative-resistance oscillator, sustains oscillation using a device whose current falls as its voltage rises, cancelling the losses in a tuned circuit.

Relaxation oscillators work by switching rather than resonance. An RC relaxation oscillator charges a capacitor through a resistor until a threshold is reached, then discharges it, repeating to make a square or sawtooth wave; the multivibrator is the classic transistor example. The blocking oscillator uses a transformer to provide strong positive feedback that drives the active device hard into conduction and then abruptly cuts it off, producing short, sharp pulses with a long off-time between them. Such pulse and ramp generators are widely used in timing, sweep and triggering circuits.

Frequently asked questions

What is the difference between a harmonic and a relaxation oscillator?

A harmonic oscillator produces a sine wave from a resonant feedback circuit, while a relaxation oscillator produces square, triangular or sawtooth waves by repeatedly charging and discharging a capacitor.

How does a blocking oscillator generate pulses?

It uses transformer feedback to switch the active device fully on, then quickly off, producing brief pulses separated by a relatively long interval.

Which oscillators are used at radio frequencies?

LC-based harmonic oscillators such as the Colpitts and Hartley are common at radio frequencies because their tuned inductor-capacitor circuits work well there.