AM Receiver

Posted on Feb 5, 2014

This is a compact three transistor, regenerative receiver with fixed feedback. It is similar in principle to the ZN414 radio IC which is now replaced by the MK484. The design is simple and sensitivity and selectivity of the receiver are good. All general purpose transistors should work in this circuit, I used three BC549 transistors in my prototyp

AM Receiver
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e. The tuned circuit is designed for medium wave, but the circuit will work up to much higher frequencies if a different tuning coil and capacitor are used. I used a ferrite rod and tuning capacitor from an old radio which tuned from approximately 550 - 1600kHz. Q1 and Q2 form a compound transistor pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. Q1 operates in emitter follower, Q2 common emitter, self stabilizing bias is via the 120k resistor and the tuning coil. As Q2 operates in common emitter its base volatge will be a Vbe drop higher than ground or about 0. 71V in my test sample. The volatge at Q1 base will be this Vbe drop plus the voltage drop across the 1k resistor and Q1`s own Vbe drop, this amounted to 1. 34V from base to ground in my test circuit. For audio amplifiers, Q2 collector would be biased near half supply voltage, however the input signal levels at RF are tiny, typically 50uV appearing across the coil being amplified by Q2 and being about 5mV RF across the 2k2 load resistor. The 120k resistor provides regenerative feedback, between Q2 output and the tank circuit input and its value affects the overall performance of the whole circuit. Too much feedback and the circuit will become unstable producing a "howling sound". Insufficient feedback and the receiver becomes "deaf". If the circuit oscillates, then R1`s value may be decreased; try...

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