BC560C PNP Transistor



This is the BC560C PNP transistor used in the circuit as a kind of active diode.  It operates as an emitter follower.  The negative swings of the radio frequency signal are followed strongly by the current amplifying operation of the transistor.  This drops the voltage on capacitor C6, which holds at that voltage, and is only slowly recharged by R7.  The result is a voltage which follows the envelope of the AM radio signal, and the result is amplitude demodulation.  As a secondary operation, the negative audio peaks are filtered out by VR1 and C7.  The voltage on C7 drops when the radio and audio signal is loudest.  That voltage is used to set the bias current of the radio stages.  When that reduces, the gain reduces as the stages have a harder time driving each subsequent stage.  Automatic gain control (AGC) is the result.

The need for a negative going AGC voltage is the reason for using a PNP transistor.  It works "upside down."

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