Page 87 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

total gain of about 100 is needed to bring the level up sufficiently for envelope

detection.
Transistors Q1 and Q2 form a double emitter follower circuit that has a gain of
about 1 (unity) with a very high input resistance at Q1's base so as not to load tank
circuit T1 and VC1 while providing a sufficiently low output resistance drive to
voltage amplifier Q3. The output signal of Q3 via its collector is fed to another
emitter follower Q4, which, in turn, drives a second voltage-gain amplifier Q5. And

Q5's collector output signal is connected to gain-of-1 amplifier emitter follower Q6.
The output of emitter follower Q6 then is connected to peak envelope detector Q7,
which also looks like an emitter follower but with a peak hold capaCitor CS at its
emitter.
In the first design, it was found that emitter follower stages Q1 and Q2 can oscillate

in an undesirable manner when the variable capaCitor is tuned to the top of the AM
band (e.g., around 1,400 kHz or higher). The high impedance characteristic of the
parallel tank circuit actually sets up a condition for oscillation as the variable
capaCitor is adjusted for minimum capacitance. Therefore, an alternative design
was tried out in the next design, which uses a tapped-down coil that is fed to Q1.
A second design uses the more common 140-pF variable capaCitor (Figure 5-4).

An inductance of about 680 J,JH is needed, though, to resonate with the 140-pF
variable capacitor at the low end of the AM band (e.g., 535 kHz), and the oscillator
coils mentioned earlier max out at about 500 IJH. Thus a 455-kHz IF transformer
(42IF104) is used instead because its inductance at the secondary windinQl easily
can be adjusted to 680 J,JH. Here the primary has a suitable turns ratio of about

1: 13 for a primary-to-secondary-winding ratio. Thus the external antenna is
connected to the primary, and the tap of the secondary is connected to emitter
follower Q1. By using the tap at the secondary winding, the driving impedance is
dropped by fourfold or more compared with the impedance at the full winding
(e.g., at VC2). This lowered tank impedance reduces the undesirable oscillation
from Q1. But the signal is also reduced. Therefore, to increase the gain of the Q3

amplifier, Q3's e,mitter is bypassed to ground via C3. In Figure 5-3 C3 is in series
with a gain-reducing resistor (3,300 V), R6.
It should be noted that T1, the 42IF104 transformer, may be replaced with the
capacitor taken out from a 42IF101 or 42IF102 IF transformer. See Figure 3-28 on
capaCitor removal.

At 3 volts, the current drain is about 200 IJA for the radio designs in Figures 5-3
and 5-4. The designs in Figures 5-3 and 5-4 had some oscillation problems, with
the circuit in Figure 5-4 improved over the circuit in Figure 5-3. So, other types of
circuits were tried.
The "famous" ZN414 or MK484 integrated circuit was tried as well, but osciUations

also occurred similarly when a 680-IJH antenna coil and a 140-pF variable capacitor
were used as the tank circuit.