AM Receiver
Description:
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.
Notes:
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 compund 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.
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 voltage 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.
Q1 (b)
1.31V
Q2 (b) 0.71V
Q2 (c) 1.34V
Q3 (b) 0.62V
Q3 (c) 3.87V
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