Two Transistors AM Radio Receiver
Using only two active components, we can build an AM radio receiver working on MW (Medium Wave) Band. The circuit uses straight receiver configuration, where an LC resonator circuit is used to filter the received RF signals and then directly rectified and further filtered to obtain the carrier’s envelope signal, which is the modulating information signal we want to hear.
The tuning coil is made from 50 turning of enameled 24 SWG wire on ferrite rod. OA91 is a high frequency germanium diode for rectifying the received RF signal (the modulated carrier). A germanium diode is used since it has low forward bias voltage, since the received RF signal should reach a level greater than this value to make the rectification works. The transistor is a high gain type, and you can replace with many other type with gain 300 or more. MW band frequency is not so high for almost any type of transistor, so don’t worry about the frequency characteristic of the transistor, just pick the highest gain one. The aerial should be 8 meters long wire at minimum. The earpiece should have high impedance to avoid using impedance matching transformer for low impedance speaker.