Page 134 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

The turns ratio of the 42IF100, 42IF300, or 42IF110 oscillator coil is at least 20: 1,

which is required for the one-transistor oscillator/converter circuit of Chapter 8.
However, if an extra transistor is added to form a two-transistor oscillator circuit,
then the turns ratio can be lower, something like 4:1,3:1,2:1, or even 1:1. The
extra transistor "buffers" the oscillator signal to allow less loading on the oscillator's
tank circuit such that the oscillator signal is not stepped down or attentuated as
much. By avoiding stepping down or attenuating the oscillator signal too much, the
two transistors with lower transconductance still will provide sufficient gain to

sustai n reliable oscillation.
Another way to analyze the oscillator/converter circuits of Chapter 8 is that the load
resistance for the collector of the transistor at resonance is low because of the high
step-down ratio. This low-valued load resistance (in the few kiloohms) requires

higher transconductance.
In contrast, if the load resistance at resonance is m uch higher, a lower
transconductance is required because gain is related to the load resistance
multiplied by the transconductance. Thus, using a differential-pair oscillator circuit
allows for a higher load resistance at resonance to "make up" for the lower
transconductance of the transistors.

Turning to the mixer, the same type of mixer will be used as in Chapter 8, but at a
much lower current. And the IF amplifier circuits will be similar to the ones used in
Chapter 8 but again at a lower current. Because the IF amplifier circuits will be
running at much lower transconductance or gain, coupling from one stage of the IF
signal to another will not use the secondary winding of the IF transformer. The

signall voltage from the secondary is stepped down from the primary winding so as
to allow loading into a lower input resistance of the IF amplifier. However, when
the IF amplifier is run at a lower operating current such as 20 IJA, the input
impedance is sufficiently high (e.g., approximately 50 kV to 100 kV) to allow
coupling from the transistor's collector output terminal of the previous stage to the
input of the next amplifying stage. By skipping the secondary winding and using the
signal voltage at the primary of the IF transformer, more IF signal' voltage is

provided.
lLow-Power Detector' and Audio Circuits

A germanium diode will be used for demodulation or detection of the
amplitude-modulated (AM) signal from the last IF stage. However, since the
secondary winding of the IF transformer provides a lower signal voltage, the diode
will rectify the IF signal at the primary winding instead.

To drive the crystal earphone, a low-power audio amplifier will be used. This audio
amplifier will have an input resistance of at least 100 k

to maintain the Q or selectivity characteristic of the last IF transformer.