Page 21 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

In a parallel-capacitor-coil resonant circuit (aka parallel-capacitance-inductance
circuit) for a tunable RF filter, the quality factor, Q indicates that the higher the
selectivity, the better is the separation of radio stations. A low Q in an antenna coil
and variable-capacitor resonant circuit will allow unwanted adjacent stations to
bleed into the tuned station. But a higher Q allows the RF signal of the desired
tuned radio station to pass while attenuating RF signals from other stations. The Q
in a parallel tank circuit is affected by the input resistance of the amplifier to which

it is connected. The higher the input resistance, the higher the Q is maintained. So
an amplifier with an input resistance on the order of at least 100 k

(e.g., typically 500 k

or more) allows for a high Q to be maintained. If an amplifier has a moderate input
resistance (e.g., in the few thousands of ohms), tapping the coil with a

stepped-down turns ratio allows the Q to be maintained, but at a tradeoff of lower
signal output. For example, if an antenna coil has a 12: 1 step-down ratio or 12: 1
tap, the signal output will be 1/12 in strength, but when connected to an amplifier of
3 k

of input resistance, the effective resistance across the whole coH and variable

capacitor is 12 x 12 x 3 k

, or 432 k

, which maintains a high Q.
Transistor Q1 serves a dual purpose as the RF amplifier and detector. The

(coUector) output signal of Q1 is connected to an audio transformer T1 that
extracts audio signals from detector Ql, but Ql's collector also has amplified RF
signa:ls, which are fed back to coil Li via the extra winding. By varying resistor Ri,
the gain of the Q1 amplifier is varied, and thus the amount of positive feedback is
varied. The user tunes to a station and adjusts R1 to just below the verge of
oscillation. Too much positive feedback causes the squealing effect. But when
adjusted properly, the circuit provides very high gain and increased selectivity,

Reflex Radio

In a reflex radio, which also uses a recirculation technique, an amplifying circuit is
used for purposes: (1) to amplify detected or demodulated RF signals and (2) to
amplify RF signals as well. The demodulated RF signal, which is now an audio
signal, is sent back to the amplifier to amplify audio signals along with the RF