In the modern day, few would have heard of the reflex receiver, but for a time it was a very popular circuit. The concept was that one transistor would provide RF amplification of the incoming signal, it would be detected, and then the audio fed back through the same transistor for further gain. At this point, the signal is strong enough to drive an earphone, but with a second transistor, speaker operation becomes possible.
The reflex circuit has been around since the 1920's for valve use, and was used for economy reasons. Making one valve do the job of two could make for a less expensive receiver. One must remember that electronic components were considerably more expensive than now, and this applied particularly to the valves themselves. Valve reflex circuits have been described here and here.
When transistors first appeared commercially in the mid 1950's, their cost was also very high, and thus the reflex circuit was revived once again. Because of the low component count, it suited small earphone-only personal portables, and pocket size loudspeaking radios. Throughout the 1960's, the reflex circuit was common in electronics magazine projects, particularly in the UK and Europe, as it was a simple way to make a receiver with good performance, but without the complexity and expense of a superhet. It was also a popular circuit in Japanese electronics kits - particularly the Denshi Block series.
The link to a big collection, including schematics and photos is here:
https://www.cool386.com/reflex/reflex.html