The figure shows a schematic for an oscillator amplitude-control servo system. The circuit creates a closed-loop system that supplies a fixed and adjustable peak-to-peak amplitude ac signal centered around 0 V.
A 1-kHz sine wave, labeled AC_INPUT, is ac-coupled to an AD633 multiplier chip. Then the AC_INPUT is multiplied by the SERVO signal. The multiplier's output is a scaled version of the AC_INPUT. This 1-V sine output signal is fed into a peak detector, and the peak of the signal is compared to a user-supplied reference voltage.
The error integrator integrates the difference between the peak and the reference. This generates the servo signal that feeds the multiplier. The servo signal will force the peak amplitude of the AD633 output to match that of the user-supplied dc reference at the error integrator.
The ac-coupling of the input signal and the frequency of the input signal are not fixed requirements. The circuit was successfully tested using a 1-kHz sine wave for the input with an amplitude of 4.5 V, and it produced a 1-V amplitude sine-wave output using a 1-V reference for the set point.
See the figure