Audio oscillatorsAudio oscillators find uses in doorbells, ambulance sirens, electronic games, and thoselittle toys that play simple musical tunes. All AF oscillators work in the same way, con-sisting of amplifiers with positive feedback.A simple audio oscillatorOne form of AF oscillator that is popular for general-purpose use is the twin-T oscillator(Fig. 25-9). The frequency is determined by the values of the resistors R and capacitors C.The output is a near-perfect sine wave. The small amount of distortion helps to alleviate theirritation produced by an absolutely pure sinusoid. This circuit uses two NPN bipolar tran-sistors. Two JFETs could also be used, biased for class-A amplifier operation.The multivibratorAnother audio-oscillator circuit uses two identical common-emitter or common-sourceamplifier circuits, hooked up so that the signal goes around and around between them.468 Oscillators25-9A twin-T audio oscillator.