Amplitude modulation circuit

Claim 1

An amplitude modulation circuit having a D.C. voltage source and comprising: a modulated stage adapted to receive a carrier wave; a resonant circuit connected at one end thereof to said modulated stage to provide a modulated carrier wave; a modulator connected to the other end of said resonant circuit to apply thereto a modulating signal; a voltage regulator connected between said D.C. voltage source and the other end of said resonant circuit to regulate a D.C. voltage applied to said resonant circuit at a predetermined constant level; and a coupling capacitor connected between said modulator and the otheer end of said resonant circuit; said voltage regulator including control means having its input coupled to said D.C. voltage source and its output coupled to the other end of said resonant circuit, an error amplifier to generate a control signal when a potential difference exists between the output of said control means and said predetermined constant level, and a low pass filter connected between the output of said voltage regulator and an input of said error amplifier and having a cut-off frequency selected below a lower limit of a frequency band of the modulating signal, said control means being operative in response to said control signal to zero the potential difference for thereby regulating the D.C. voltage applied to the other end of said resonant circuit to said predetermined constant level when said frequency band is below said cut-off frequency and operative to provide high impedance for the modulating signal when said frequency band exceeds beyond said cut-off frequency of said low pass filter.