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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