Abstract: A YIG oscillator circuit including a closed loop control system which maximizes loop gain during start-up and minimizes loop gain after start-up to minimize gain compression. In one embodiment the closed loop control circuit includes a PIN diode coupled between the collector and base of a BJT, and comparison circuitry for comparing the YIG oscillator output voltage level with a reference voltage and providing a resulting feedback current to the PIN diode at the collector of the BJT. In operation, during start-up when the output voltage of the YIG oscillator is less than the reference voltage, the feedback current will be minimal and the PIN diode will reverse bias to maximize loop gain. When the output voltage approaches the reference voltage level, the feedback current will increase to forward bias the PIN diode to reduce loop gain and minimize gain compression. In a second embodiment, the PIN diode is not included, and the closed loop control circuit output signal is coupled to the emitter of the BJT.