[AMRadio] infinite impedance detector vs diode

Larry Szendrei ne1s at neandertech.com
Thu Sep 4 10:33:59 EDT 2008

From: jc at pctechref.com

> The way I understand it,
> The infinite impedance detector is a device such as a tube or FET that
> normally draws output current unless reversed biased.  The device is
> reversed biased to cutoff and then input signal turns it on like class B
> service but the input does not draw current.  The output is taken from the
> cathode, or source in the case of a FET, in follower fashion.
> I am not sure how this is going to correct the slew distortion that is
> caused by the RF filtering and discharge rate of the output.  Perhaps it is
> because of the follower effect of having a low Z output with no diode at
> output.  This may mean that the discharge is at the same rate as the

I've recently been working on a quasi-restoration of a National NC-156
(like the NC100XA with a 1500Kc IF and a few other differences). This
particular one uses one half of a 6C8 dual triode as an infinite impedance
AM detector. I don't know why they did this, as it is immediately followed
with a series-diode noise limiter which I would think would introduce more
distortion than a simple diode envelope detector, even when "disabled"
(diode biased fully "on"). The cathode-grid terminals of the other 1/2 of
the 6C8 is used for this diode. This is the 1st BA receiver crossing my
bench employing an infinite impedance detector, and only some of the
NC-100 variants used this circuit. Other versions used the common "plate
detector" (triode or sharp-cutoff pentode cathode-biased to near cutoff,
with the IF ripple filtered in the plate circuit and audio taken from the
"far-side" of the filter).


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