Quote:
Originally Posted by Synchrodyne
The attachment shows the AM demodulator that was used in the McKay Dymek series of HF receivers of the late 1970s:
To me, it looks like the bipolar transistor version of the infinite impedance demodulator. ,
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When transistors are used in this manner all that happens is the B-E junction is used as the detector diode, but the current in that "diode" is reduced by the transistors hfe, because the bulk of the emitter current is sourced from the transistors collector and power supply, not the base. So its not really an "infinite impedance" it just behaves like a diode where the input impedance is elevated and the output impedance is lower.
In the circuit you attached they have at least biased the B-E junction of the transistor correctly. They have created a voltage reference from the B-E junction of another transistor (which has the same tempco and voltage drop as the detector transistor's B-E), to set up the initial DC conditions for the B-E junction of the detector transistor. This way the B-E junction forward voltage drop is largely negated and the detector will work down to low input signal levels with a lot less distortion.
It is a very old idea though, Sony invented this biasing scheme in 1956, I recently attached the schematic in the Homebrew section in a thread on detectors of Sony's TR-72 radio that biases the detector in this manner, but they cleverly used the B-E voltage drop from one of the IF transistors to do it !
see post #27 here:
https://www.vintage-radio.net/forum/...=132589&page=2