UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Interesting Armstrong AM detector circuit.

Argus25 · 28th Oct 2017, 10:28 pm

Yes I agree it's just a matter of proportions. But I would guess even for small signals out of the IF transformer the voltage swing at the plate would be much higher (well over a few hundred millivolts) than the value required to operate the detector diode in its linear region and the AC component of the plate current drives the detector diode via the coupling capacitor.

It might have been even better with an RFC as the plate load rather than the 10k resistor, then it would have much better approximated a current source drive as the inductance as a load tends to resemble a current source as a load.

In this circuit it's very important that the coupling capacitor has no leakage.

28th Oct 2017, 10:28 pm	#15
Argus25 No Longer a Member Join Date: Oct 2016 Location: Maroochydore, Queensland, Australia. Posts: 2,679	Re: Interesting Armstrong AM detector circuit. Yes I agree it's just a matter of proportions. But I would guess even for small signals out of the IF transformer the voltage swing at the plate would be much higher (well over a few hundred millivolts) than the value required to operate the detector diode in its linear region and the AC component of the plate current drives the detector diode via the coupling capacitor. It might have been even better with an RFC as the plate load rather than the 10k resistor, then it would have much better approximated a current source drive as the inductance as a load tends to resemble a current source as a load. In this circuit it's very important that the coupling capacitor has no leakage.