Guidelines for simple ‘FET infinite impedance’ AM detectors.

[catman]

G’day all, many will know of my interest/passion for high fidelity AM broadcast band reception and particularly for the FET based infinite impedance detector. In just about every way I regard this detector as quite superior to the simple diode based detector, which can perform well but has serious limitations particularly under weak signal conditions and the necessary requirement for optimum output ‘buffering’ for minimal audio distortion.

The FET based infinite impedance detector has no such limitations and is possibly one of the most ‘uncritical’ circuits I’ve ever seen. The FET circuit is based on the old electrically equivalent triode valve circuit. However there are some tricks to watch with the FET version, particularly the actual FET used. Initially I had used the MPF102, and it is a FET that can give good results but mainly because of the very wide spreads in individual MPF102 device parameters, some work well and some don’t.

There are better FET’s to use like the 2N5457, 2N5485 and especially the J201 device which appears to be almost made for ‘infinite impedance detector' applications. The very high impedance of the gate input ‘preserves’ the Q of the preceding tuned circuit giving excellent tuning selectivity, but with much wider audio modulation bandwidth recovery than your ‘typical’ narrow banded superheterodyne AM tuner approach along with low audio distortion. In short the audio quality is a delight to listen to.

The value of the source resistor more or less defines the audio output impedance for tuner applications and values in the 27 k range or thereabouts will ‘drive’ most line level inputs quite well, however a simple additional FET common source voltage amplifier stage can be added immediately following the detector stage for a moderate voltage gain boost and a lower output impedance for improved output ‘drive’ if deemed necessary.

Other variations to the ‘basic’ generic circuit are possible, however the basic circuit works fine and is what I am presently using with excellent results. This page has lots of good information: http://sound.westhost.com/articles/am-radio.htm Regards, Felix (vk4fuq) aka catman.

[catman]

G'day all, just a little update that I thought I'd document. Regarding 'recalcitrant' MPF102's that produce distorted audio output, a technique that I've discovered produces very good audio output, is simply to parallel any two together.

I've tried this approach with otherwise poor performing MPF102's and my knowledge of FET's is not sufficiently good to say why, but it appears that combining in parallel any two MPF102 FET's, produces a vastly improved 'single' one for this application. It works very well! Regards, Felix (vk4fuq) aka catman.