Sinusoidal Oscillators

It was shown in section 7.5 that positive feedback applied to an amplifier augments any spurious input signal and if the magnitude of the loop gain is greater than or equal to unity, that is |β∗A∗| ⩾1, oscillation ensues. This means that an oscillator can be made by applying positive feedback round an amplifier provided its gain is sufficient to overcome any losses arising from the feedback network. The amplifier should be adequately decoupled from the supply and ideally its output impedance should be zero and its input impedance infinite. The last two requirements ensure that the amplifier action is not affected by loading and that the feedback network itself is not loaded by the amplifier input. For many applications an operational amplifier is particularly suitable. The frequency of oscillation may be controlled by choosing a suitable frequency-selective feedback network.