DEFINITION:
Reflex Circuits: "A reflex amplifier is one which is used to amplify at two frequencies - usually intermediate and audio frequencies". - F. Langford-Smith, Radiotron Designers Handbook, Australia, 1953, P.1140
The reflex system consists of using an IF amplifier stage and after detection to return the audio portion to the same stage where it is then amplified again. Since in fig. 1. two signals of widely different frequencies are amplified, this does not constitute a "regenerative effect" - Oh gee whiz! and the input and output of these stages can have split audio/IF loads.
Figure 1 - reflex receiver block diagram
In fig. 2. the IF signal (e.g. 455 Khz) is fed through T2 to the detector circuit D1, C3 and VR1. The detected audio appears across the volume control VR1 and is returned through C4 to the cold side of the secondary of T1.
Since the secondary of transformer T1 consists of only a few turns of wire, it is essentially a short circuit at audio frequencies. C1 bypasses the IF signal otherwise appearing across the parallel combination of R1 and R2. The emitter resistor R3 is bypassed for both audio and IF by electrolytic capacitor C2.
Figure 2 - Reflex I.F. - Detector Circuit
After amplification, the audio signal appears across R4 from where it is then fed to the audio output stages. C5 bypasses R4 for IF frequencies and the primary of transformer T2 is essentially a short circuit for the audio signal.
The advantage of reflex circuits is that one stage produces gain otherwise requiring two stages with resulting savings in cost, space, and battery drain.
The disadvantages of such circuits
are that the design is considerably more difficult, although once a satisfactory
receiver has been designed, no outstanding difficulties should be encountered.
Other significant difficulties are a somewhat higher amount of "playthrough" (i.e. signal output with volume control at zero setting), and a minimum volume effect. The latter is the occurrence of minimum volume at a volume control setting slightly higher than zero. At this point, the signal is distorted due to the balancing out of the fundamentals from the normal signal and the out-of-phase playthrough component.
BIBLIOGRAPHY
1. F. Langford-Smith, Radiotron Designers Handbook, Australia, 1953
2. G.E. Transistor
Manual 1964, 1969