Amplitude stability and distortion in thermistor-controlled oscillators

The linearised theory of the control of the amplitude of oscillators is put into a form which makes it possible to draw on the results of the general theory of closed-loop control systems. The transfer function of a thermistor as an amplitude-controlling element is derived according to a simple uniform-temperature model. An expression for the 3rd-harmonic distortion introduced by the thermistor is also derived. The theory is illustrated by reference to a simple Wien-bridge oscillator. Measurements on the oscillator, of envelope damping and the 3rd-harmonic distortion, show that the simple theory of thermistors is inadequate for this application. An explanation of the observations is suggested.