Electronically tuned high-frequency signal amplification circuit

Shown is an electronically tuned high frequency signal amplification circuit . This circuit is an electronically tuned AM radio circuit. Varactor diodes VD1, VD2, and VD3 are provided in the high-frequency
input resonant circuit, local oscillator resonant circuit, and high-frequency output resonant circuit . When selecting a station, the 1.5~8.5 V tuning voltage sent from the tuning circuit is filtered (Olqin capacitor) and current limited (150 kn resistor). Provide reverse bias voltage to the varactor diodes VD1, VD2, and VD3. When the junction capacitance of the varactor diode changes, the resonant frequency in the loop changes, thereby achieving tuning (channel selection) and tracking.