This is a compact three transistor, regenerative receiver with fixed feedback. It is similar in principle to the ZN414 radio IC which is now no longer available. The design is simple and sensitivity and selectivity of the receiver are good.
Notes:
All general purpose transistors should work in
this circuit, I used three BC109C transistors in my prototype.The tuned
circuit is designed for medium wave. I used a ferrite rod and tuning
capacitor from an old radio which tuned from approximately 550 -
1600kHz. Q1 and Q2 form a compund transistor pair featuring high gain
and very high input impedance. This is necessary so as not to unduly
load the tank circuit.
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 68k. If there
is a lack of sensitivity, then try increasing R1 to around 150k. R1
could also be replaced by a fixed resisor say 33k and a preset resistor
of 100k. This will give adjustment of sensitivity and selectivity of
the receiver.
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 will directly drive
high impedance headphones or can be fed into a suitable amplifier.
Construction:
All
connections should be short, a veroboard or tagstrip layout are
suitable. The tuning capacitor has fixed and moving plates. The moving
plates should be connected to the "cold" end of the tank circuit, this
is the base of Q1, and the fixed plates to the "hot end" of the coil,
the juction of R1 and C1. If connections on the capacitor are reversed,
then moving your hand near the capacitor will cause unwanted stability
and oscillation.
Finally here are some voltagee checks from my breadboard prototype.This should help in determining a working circuit:-
All measurements made with a fresh 9volt battery and three BC109C transistors with respect to the battery negative terminal.
Circuit diagram
Parts
Q1 (b) 1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V
Q3 (c) 3.87V
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