Simple medium wave radio circuit using varactor diode and TL081 linear op amp

The figure shows the use of varactor diode VD2 as the tuning component to replace the variable capacitor. By changing the voltage applied to the varactor diode through the potentiometer RP1, its capacitance can be changed. Thus achieving the purpose of tuning. The tuning signal is detected by the diode VD1 and then applied to the operational amplifier A1 with a field effect transistor input terminal. After amplification, the output is connected to a 400-2000Ω load (headphones, small speakers or power amplifier input terminals, etc.). Adjust the potentiometer RP2 to change the amplification factor and thus the volume.

Some component specifications:

VD1: Germanium diode AA139 or similar device

RP1: 47kΩ wirewound potentiometer

VD2:Vianker diode BB312

RP2 1MΩ wirewound potentiometer

A1: Operational amplifier TL 081