Infinite Impedance detectors.
The thread about the Armstrong circuit using a non-AGC-controlled triode stage feeding the diode detector has reminded me of one underlying question I've had about the design of AM detectors in 'upmarket' receivers: why so few of them use the 'infinite impedance' detector?
The II detector looks at first like a cathode-follower but it's not Class-A biased and so it detects. The two big features are (1) it provides an essentially infinite impedance to the preceding stage (hence the name) and (2) it also has what amounts to 100% audio negative-feedback.
(1) means that the secondary-winding of the IFT driving it can be designed for higher loaded Q and so deliver a higher IF voltage to the grid of the triode, meaning higher potential audio output.
(2) means low distortion.
The only downside of it that I can see is that it doesn't provide AGC voltage - but I *have* seen a published design using a pentode in which the cathode/control-grid/screen-grid form the triode for the II detector, and the amplified IF signal appearing at the anode is then rectified using a diode to provide AGC: I guess this is modelled after the pentode "electron-coupled oscillator" where the screen-grid serves as the oscialltor anode?
So - why wasn't the II detector more popular?
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