AM radio high frequency amplifier circuit

Shown is the high-frequency amplifier circuit of an AM radio . Its function is to amplify the weak signal received by the antenna. In addition, it also has a frequency selection function.     As can be seen from the figure, the core device of the amplifier is the transistor VT1. The signal is input from the base, and the amplified signal is output from the collector and coupled to the mixing circuit through the resonant transformer.     The signal induced by the antenna is added to L. The resonant circuit composed of , cl and VD1 can change the resonant frequency by changing the parallel capacitor cl of the coil Ll. This circuit uses the electrical tuning method of a varactor diode. The varactor diode VD1 is equivalent to a capacitor in the circuit, and the value of the capacitor changes with the reverse voltage applied to it. Changing the voltage changes the resonant frequency. In addition, a varactor diode VD3 is also used in the parallel capacitor of the primary coil of the output transformer of the high-frequency amplifier , which changes synchronously with VD1. Cl and C2 can be fine-tuned to fine-tune the resonance point.