AM Receiver

This is a compact three transistor, TRF receiver with fixed feedback. It is similar in principle to the ZN414 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 prototype. 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 darlington pair featuring high gain and very high input impedance. This is necessary so as not to unduly load the tank circuit. There is only one tuned circuit in this design so sensitivity and selectivity will not be as good as a regenerative or superhet design but good results can still be obtained nonetheless. The 120k feedback resistor impacts on both the gain and input impedance of the circuit so varying this value is a crude way of altering sensitivity and selectivity. In my test circuit Q2 had an emitter voltage of about 0.71V and collector voltage of 1.34V.

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 feedback resistor, between Q2 output and the tank circuit L1 affects overall performance of the receiver. The value of 120k was found to just enough gain for the receiver to work well. In strong signal areas its value may be too high and a lower value of 100k or 82k may weork better; in weak signal areas increasing the value may work better. Too much feedback and the circuit will become unstable producing a “howling sound”. Insufficient feedback and the receiver becomes “deaf”. R1 could also be replaced by a fixed resistor say 33k and a preset resistor of 100k.

Transistor Q3 has a dual purpose; it performs demodulation of the RF carrier whilst at the same time, amplifying the audio signal. Audio level varies on the strength of the received station but I had typically 10-40 mV, this is audio voltage, not RF signal level. This will directly drive high impedance headphones or can be fed into a suitable amplifier.

The tuning coil, L1 can be salvaged from an old AM receiver, or to make your own wind about 50 to 60 turns of 26 SWG enamel coated copper wire over a 3/8 inch ferrite rod about 3 inches long. This will create a tuning inductor of about 200uH. AM stations are directional so rotating the rod (or whole receiver) should allow nulling of some signals whilst boosting others.
If you are in an area of weak reception then an external antenna may be required. Wind about 4 or 5 turns (indicated as 4 or 5 T on the schematic) of 26 SWG wire onto the ferrite rod, close to the main winding and connect one end to a cold water tap or ground connection. Use several feet of flexible wire as an antenna.

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