Hi-fi AM detector

Posted on Feb 7, 2014

A modification to David Knight`s linear detector circuit. I wanted to build a `hi-fidelity` AM receiver and needed an ultra-linear AM detector for it. Here is the circuit I came up with: Note that I eliminated the meter circuit, and removed a lot of the bypass caps. I now have a circuit that has a finite quiescent current at the output te

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rminal but is extremely linear as you can see. The input waveform is the lower trace, upper trace being the output. My strategy is to adjust R2 so that the input signal gives the proper signal swing. In the application I will replace C4 with a better filtering system and 10 kHz. notch filter. This should do nicely and is far superior to a germanium diode detector. The simulation shown is for 455 kHz, but (subject to practical confirmation) it appears that the circuit will work to well over 5 MHz. Below is the schematic of the complete detector with 10 kHz notch filter (for the American band plan). The filter gets rid of the adjacent-channel carrier heterodyne whistle, which can be bad at night when the sky-wave comes in. It can be re-tuned for the European 9 kHz channel spacing as well. One reason for using Dave Knight`s method of linearizing the detector is that it fits well with a low voltage single supply. As you can see I powered my 455 kHz. detector from 6 V and it has a buffered output. I discovered "Hi-Fi AM" quite by accident too. It can rival FM to some extent and some of the venues on AM here in Idaho are not available on FM. The input Z is high enough so that the detector circuit can drive directly into the notch filter with no additional buffering. Supply voltage for this circuit can vary between 5 V and 12 V with no ill effects. Voltage divider R5/R6 controls the circuit Q, R3 controls the center frequency but...

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