Audio Oscillators

So far, we have considered the oscillators which use L-C tuned circuit that causes a phase shift of 180° due to inductive or capacitive coupling in addition to a 180° phase shift produced by the transistor itself. The oscillators employing L-C elements, called the L-C oscillators, are very popular for generating high frequency oscillations but they cannot be employed for generation of low frequency oscillations as they become too bulky and expensive. R- C oscil­lators are commonly used for generating audio-frequencies as they provide good frequency stability and waveform. Also, with the advent of IC technology, R-C network is the only feasible solution, as it is very difficult to make a too high value inductance in an integrated circuit. Two com­monly used R-C oscillators are (i) R-C phase shift oscillator and (ii) Wien bridge oscillator. Another commonly used audio-oscillator is the beat frequency oscillator (BFO). So these three audio-oscillators will be discussed here.

For producing oscillations in an oscillator circuit we need positive feedback which means that the voltage signal feedback should be in phase with the input signal. For providing a positive feedback at one particular frequency, an inverting amplifier may be used with a feedback network that causes a phase shift of 180° at the desired frequency of oscillation, as shown in fig. (a). The 180° phase shift in the feedback signal can be obtained by a suitable network consisting of three R-C sections, as shown in fig. (b).

When a phase-shift network such as that indicated in fig. is used in a phase shift oscillator, the R’s and C’s must be selected so as to produce a phase shift of 180° at the desired frequency of oscillation. The output of the voltage amplifier is fed to the input to the phase-shift network. Thus V₁ = V_out. The output resistance of the amplifier is designed to be very small in comparison to the input impedance of the phase-shift network. The output voltage of the phase-shift network, V₂ is fed into the input of the amplifier i.e., V₂ = V_in. The amplifier’s input impedance must be much larger than the output impedance of the phase-shift network.

Alternatively, a positive feedback can be obtained by using two stages of amplifiers each giving a phase shift of 180°. A part of this output is fedback to the input through a feedback network without causing any further phase shift. Wien bridge oscillator operates on this principle.