Audio oscillators are used in myriad electronic devices including doorbells, ambulance sirens, electronic games, telephone sets, and toys that play musical tunes. All AF oscillators are, in effect, AF amplifiers with positive feedback.
At AF, oscillators can use RC or LC combinations to determine frequency. If LC circuits are used, the inductances must be large, and ferromagnetic cores are necessary.
At RF, oscillators are usually designed to produce a sine wave output. A pure sine wave represents energy at one and only one frequency. Audio oscillators, by contrast, don’t always concentrate all their energy at a single frequency. (A pure AF sine wave, especially if it is continuous and frequencyconstant, can be annoying.) The various musical instruments in a band or orchestra all sound different from each other, even when they play the same note (such as middle C). The reason for this is that each instrument has its own unique waveform. A clarinet sounds different than a trumpet, which in turn sounds different than a cello or piano.
Suppose you were to use an oscilloscope to look at the waveforms of musical instruments. This can be done using a high-fidelity microphone, a sensitive, low distortion audio amplifier, and an oscilloscope. You’d see that each instrument has its own signature. Thus, each instrument’s unique sound qualities can be reproduced using AF oscillators whose waveform outputs match those of the instrument. Electronic music synthesizers use audio oscillators to generate the tones you hear.
The Twin T Oscillator
A twin T audio oscillator using two PNP bipolar transistors. The frequency is determined by the values of the resistors R and the capacitors C.
An audio oscillator circuit that is popular for general-purpose use is the twin T oscillator (above figure). The frequency is determined by the values of the resistors R and capacitors C. The output is a nearperfect sine wave. The small amount of distortion helps to alleviate the irritation produced by an absolutely pure sinusoid. The circuit shown in this example uses two PNP bipolar transistors. They are biased for class A amplification.
A multivibrator audio oscillator using two N-channel JFETs. The frequency is determined by the value of the inductor L and the capacitor C.
Another popular AF oscillator circuit makes use of two identical common emitter or common source amplifier circuits, hooked up so that the signal goes around and around between them. This is sometimes called a multivibrator circuit, although that is technically a misnomer, the term being more appropriate to various digital signal-generating circuits.
In the example of above figure, two N-channel JFETs are connected to form a multivibrator for use at AF. Each stage amplifies the signal in class A, and reverses the phase by 180°. Therefore, the signal goes through a 360° phase shift each time it gets back to any particular point. A 360° phase shift is equivalent to no phase shift at all, so it results in positive feedback.