Two Transistor (Boy's Radio) Schematic and Theory of Operation

The schematic and circuit description were provided by Ronald Rissel:

It has been interesting analyzing these radios.  You have to give the Japanese credit for even making a radio that can drive a speaker with only 2 transistors!  Basically, the first transistor (Q1) performs double-duty.  It first acts as an RF amplifier, with some of the signal being fed back to the antenna coil to provide some regeneration for better selectivity and sensitivity.  (This also results in a non-linear amplification of the signal which results in noticeable distortion.)  The RF (radio frequency) signal is then rectified ("detected") by the diode, and then the resultant audio signal is fed back to the base of the first transistor where it acts as the first AF (audio frequency) amplifier. (This is called a "reflex" circuit.)  The audio signal is then fed through an interstage transformer to Q2 (the second transistor).  The transformer provides impedance matching.  The
second transistor acts as a power amplifier, with the output signal going through an audio output transformer (to provide impedance matching again), and then finally to the speaker.  Everything about this circuit is designed to provide maximimum gain; consequently there is no AGC (automatic gain control), and stronger stations come in a lot louder than weaker stations; unlike 6-transistor (or more) radio circuits which have an AGC circuit or function.

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