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(độ nhạy) and selectivity (độ chọn lọc)of the receiver are good.

Circuit 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 compound transistor pair (cặp transistor liên hợp )
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 (mạch theo điện áp, cực phát), Q2
common emitter, self stabilizing bias(tự phân cực) 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(radio frequency)
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(không ổn định) producing a "howling
sound"(tiếng rít ). Insufficient feedback (hồi tiếp không đủ) 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 resistor 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 (giải điều chế ) 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.

Antenna and Tuning Coil

The tuning coil, L1 can be salvaged (tận dụng) 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 0,95cm
ferrite rod about 7,62cm long. This will create a tuning inductor (cuộn cảm điện) 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 schmematic) 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.

Tuning Range
The frequency coverage or tuning range is controlled by L1 and VC1. If VC1 is fully
meshed (closed) then its capacitance will be about 500pF. The resonant frequency is
given by:

where F is frequency in hertz, C capacitance in Farads and L the inductance in Henrys.

With a meshed 500pF variable capacitor and 200uH coil the lowest frequency works out
to be:
When the vanes are open a small capacitance is still present (about 40pF). The coil
connections add a slight amount of stray capacitance which may be 7 or 8pF. With 48pF
capacitance and a 200uH coil, the highest frequency will be about 1624kHz. Some
variable capacitors, have built in trimmers to adjust the highest frequency. For any coil
and capacitor that tunes too high, a 50pF trimmer may be added in parallel with VC1 to
control the highest tuneable frquency.

Tuning Coil for 500 to 1700KHz

The coil details below were kindly submitted and tested by David from Germany and
tunes 500 - 1700kHz with a 500pF capacitor. Construction is shown below 35 turns of 32
swg enamal covered wire are wound 30mm from one end of a 10mm diater ferrite rod.
Now a papaer sleeve about 20mm wide is looped around the ferrite rod. The coil is
continued winding a further 40 turns, the start of the 36th turn being approximately 50
mm from the same end of the ferrite rod on the paper sleeve see image below.

The MW coil described above results in an inductance of approximately 200uH. If

coupled with a 500pF capacitor (full mesh) will tune to about 500kHz and open mesh
(about 43pF) tunes to 1700KHz. This covers the top part of the MW band used in North
America and some European Pirate stations. If desired a LW coil can also be made on the
same rod, this is 330 turns of 32swg wire starting 70 mm from the same end of the ferrite
rod.

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 junction 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

Finished Receiver
A finished receiver made on veroboard is shown below. This one is built by David in
Germany and has received all medium wave stations in David's locality.
More of David's radio work can be seen in my Pics section in the Practical Pages.