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The so-called “infinite impedance detector” is a circuit that was commonly used in the old days of vacuum tubes. Since vacuum tubes can be somewhat considered to be heated JFETs, it is evident that such a circuit can also be built using a more modern, silicon-based approach. This article covers my first experiments with a BF256B based infinite impedance detector.

General Overview

Infinite impedance detectors got their name from their extremely high input impedance. Of course, the real input impedance is anything but infinite. But it is relatively large. Vaccum tube based infinite impedance detectors have been around for a long time. They promise high-fidelity and low harmonic distortion compared to simple diode detectors when used for AM-demodulation

The working principle is rather simple: The active device, in this case a BF256B JFET, is self-biased by R103 in a way, that the gate to source voltage is essentially equal to the pinch-off voltage of the JFET. Therefore, the negative half-wave of the incoming AM-signal will have little to no effect on the voltage seen over R103. The positive half-wave, however, allows the JFET to conduct more. Thus the source voltage will follow the positive half-wave of the AM-signal applied to the gate of the JFET. The impedance “seen” by the AM-source is solely the (typically very high) gate impedance of the JFET in parallel with R102 (100 K). The previous stage is, therefore, not loaded all too much by the infinite impedance detector.”

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