Page 139 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

), the input resistance of Q4 is about 108 k

. But when VRl is set to 100 k

, the gain of amplifier is lowered, but the input resistance is raised from 108 k

to 108 k

1 (50 1 1) x 10.7 k

= 654 k

. The 10.7-k

number is the parallel resistance of 100 k

and 12 k

The output of Q4 via its collector is connected to the low-side tap of the primary
winding of T3 and coupled to the second IF amplifier Q5 via C17. The second IF
amplifier's input resistance is similar to that of the Q4 amplifier and equa,ls about
100 kV or more.
A germanium diode (D3) demodulates the 455-kHz AM signal at the collector of Q5 ..
R8 and R9 provide about 100 mV of forward voltage bias to the germanium diode

detector. Audio signals from the cathode of D3 are amplified further by Q6, an
inverting audio amplifier, to drive the crystal earphone via e15.
Alternative Low-Power Superheterodyne Radio Design

Figure 9-3 presents a slight modification to the preceding design. By using a twin
variable capacitor, the two-transistor oscillator circuit can use an IF transformer
with its internal capaCitor removed or, better yet, a standard variable inductor.