Breadboard Transistor Radio Circuit
Description
All the voltages assume a battery voltage of 9.0V.
RF Amplifier Stages
The ferrite rod antenna has a single high Q tuned circuit which
picks up the magnetic component of radio signals in the
conventional way. The output from the rod is coupled into
the radio input from a smaller, lower impedance coupling coil via
the d.c. blocking capacitor C1. Q1 and Q2 are identical
cascaded RF amplification stages. At low signal levels the
d.c. bias point is initially set by R5 and R6 giving 1.1V at the
base of PNP emitter follower Q3. This results in Q3 giving
1.7V at the emitter. The base bias currents through R14 and
R15 drop about 0.5V each to give about 1.2V on each base of Q1 and
Q2. This directly results in 0.6V at Q1 and Q2
emitters. The operational collector currents of Q1 and Q2
are then set by the values of R1 and R3, that being approximately
0.6V / 1K = 0.6mA. With that collector current, the values
of R2 and R4 are set to put the collectors of R2 and R4 somewhere
close to or a little above the mid-rail voltage. This
typically works out at about 4.7V. C2 and C4 bypass the
emitter resistors to maximise the gain of the two stages.
Demodulator and AGC
As well as setting the initial d.c. operating point of the RF
amplifier stages, Q3 acts as both demodulator and buffer for the
amplified radio signal. As the RF voltage hits minimum peaks
at the base of Q3, the emitter tracks this voltage down strongly
with a low impedance drive. As the RF voltage at the base
increases above its previous minimum, capacitor C6 holds the
previous negative peak and the voltage only rises at a rate
determined mainly by R7. This produces the required
demodulated audio signal on C6, and the loading on the previous RF
stage is minimised by the current gain of Q3. The audio
appears across VR1 and the wiper picks off the desired level.
The longer term average level of the audio signal plus the 1.7V Q3
emitter voltage appears on C7, low pass filtered through the 10K
track of VR1 and C7. As it is the negative RF envelope peaks
that are detected, the average d.c. value on C7 drops with large
signals. As this is the voltage used to bias the previous
stages, the collector currents of Q1 and Q2 will drop and the RF
gain of the circuit will drop. This provides automatic gain
reduction on strong signals and contributes to the radio behaving
more like a traditional superhet with AGC.
Audio Amplifier
C8 provides d.c. blocking and couples the audio signal into the
audio amplifier stage made up of Q4, Q5 and Q6. C8 must must
be a non-polar type as the bias voltages on either side of it are
about the same. The audio stage is a very conventional class
B output made up of Q5 and Q6 with Q4 being a driver which also
exhibits significant audio frequency voltage gain. Q4 is set
up for a collector current of about 2.1mA, an operating point at
which it shows high gain and will have sufficient drive for the
output pair without using too much current. R9 and R10 set
up the d.c. voltage at Q4 base for 1.8V and the resultant emitter
voltage is 1.1V across R11 470R. That sets the Q4 collector
current. So, inline with the 0.7V diode drops of D1 and D2,
a 1.8K resistor is required for R12. This conspires to
result in 4.5V at Q5 base which sets the output at the emitters of
Q5 and Q6 to about 3.9V. That is almost mid-rail and allows
the best output swing into the loudspeaker. The top of the
output swing is limited by the current drive for Q5 from R12 and
by the Vcesat of Q4 at the bottom end.
D1 and D2 might ideally be mounted in physical contact with the
transistors so that base bias voltage, and so the quiescent
battery current tracks with the device temperature. R13 is
present to reduce the quiescent current to a minimum, saving
battery power. In a more complex design the quiescent bias
current would typically be adjustable. This has been avoided
to reduce the tweaking required.
This output stage configuration with these devices can have a full
power bandwidth of up to 400kHz into an 8 Ohm resistor if no steps
are taken to limit this. Such a high bandwidth is
undesirable, as small amounts of RF signal will get through the
demodulator stage, be amplified by the audio output stage and feed
back into the antenna causing high frequency oscillation.
The bandwidth of the driver transistor Q4 is reduced by adding
collector-base feedback capacitor C9 and by driving the stage
through resistor R8. Both components are necessary. C9
provides a path for high frequency negative feedback from Q4
collector to the base, but if this was driven from the low
impedance of Q3's emitter directly when at maximum volume, it
would be ineffective because the output of Q3 would
dominate. The result would be a system where the bandwidth
was reduced when the volume control was in the middle range, but
which increased again with the wiper at either end of the
track. R8 ensures that there is always some resistance in
line driving the audio amp from the volume control, so the
variation is reduced. The resulting -3dB audio bandwidth of
the amplifier is about 6kHz. This is about right for AM
radio, and higher bandwidth would just reproduce more annoying
noise and make most stations sound excessively bright. (See
note 1)
100uF capacitor C11 couples the audio signal from Q5 and Q6
emitters into the loudspeaker while blocking the d.c.
voltage. 100uF is quite a low value to use here and this
avoids driving the small speaker with lots of bass signal which
requires a lot of power. The -3dB roll-off will start at
about 200Hz with 100uF. This saves battery current in a
small radio, but you might want to increase this value for more
bass output especially if you are using a bigger speaker which can
reproduce the lower frequency sounds.
R16 and C12 form a low pass filter for the power supply to the
sensitive RF and demodulator stages. Without this, signals
induced on the battery voltage by the high current taken by the
output stage will feed back along the power rail into the earlier
stages. The result is a low frequency oscillation at high
volume settings independent of the layout of the ferrite rod or
loudspeaker.
C13 is a general supply decoupling and reservoir capacitor for the
main power rail. It can be removed with apparent impunity
when using an alkaline battery, but is absolutely necessary when
using a weaker zinc-carbon battery with a higher internal
resistance.
Possible Problems
There is a lot of overall gain in the design and only one tuned
circuit. It is possible to get feedback and oscillations if
you put the ferrite rod near the loudspeaker and its
windings. Feedback is also possible into the wiring of the
tuning coil and the variable capacitor. It is best to keep
the tuning capacitor and coil at one end of the radio, and to have
the loudspeaker at the other. Feedback might sound like a
howling noise or it could be that the radio sounds very hissy or
quiet, and moving the tuning control just producing whistling
noises. Q2 collector and Q3 base are the parts of the
circuit which have the most radio frequency energy in them, so the
component leads in that area need to be short and you need to keep
the ferrite rod and coils away from there. The layout shown
in the pictures works well.
Limitations
A TRF design is never going to be quite as good as a
superheterodyne design for station selectivity and rejection of
strong off-station signals. This radio is designed to work
just off the ferrite rod and batteries and it will suffer from
picking up strong short-wave and long-wave signals if you connect
an earth, an untuned aerial wire or a mains power supply. If
you live in a particularly mountainous valley where a normal radio
struggles, you will have problems here too.
Note
1: AM
band modulation bandwidth in Europe is specified to have -3dB at
6.3kHz. (and -40dB at 9kHz). Some years ago the -3dB point
allowed was substantially narrower, more like 4.5kHz. This
fitted well with the IF bandwidths of radios made at the
time. IF bandwidths of most AM radios haven't changed a
great deal though, as many are still made in the same way.
The approximate 4.5kHz bandwidth that you get out of them is
inherent in the simple design. Consequently many AM broadcasters
reduce the level of their low and mid-range audio transmission
somewhat and gradually increase the treble above 3kHz. This
compensates for the limitations of many receivers. This is
called pre-emphasis. Our radio does not have a traditional
IF strip where this bandwidth limitation occurs. The mildly
reduced bandwidth of the audio amplifier is deliberate to fit in
with most broadcasts. There is another reason for
this. The channel spacing on the AM bands is specified as
9kHz in Europe by international agreement. The result of
imperfections in any radio system mean that all the signals
interact a little in each of the stages. One consequence of
this is that a tone at the channel separation frequency, and at
harmonics of that frequency tends to be created. This is
especially prevalent at night when many strong stations appear on
channels adjacent to each other. Adult males above the age
of 40 tend to ignore this tone and hear it as the "traditional AM
sound," but in a more sophisticated radio you might choose to put
in a 9kHz notch filter specifically to remove it. Children
and dogs may appreciate this especially, as they have better
hearing at high frequencies. In our radio, the gentle
roll-off of the amplifier bandwidth above 4.5kHz provides some
relief.
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All content © Henry J.
Walmsley 2014