Many detectors using diodes have an active bias voltage/current applied.
An Autek WM-1 peak reading watt meter is a great circuit of said.

cheers
skipp
http://sonic.ucdavis.edu

: Clifton T. Sharp Jr. wrote:
: I was going through my bookmarks when I came upon one about diodes
: (http://uweb.superlink.net/bhtongue/7diodeCv/7diodeCv.html). In his
: third paragraph, the author asserts:

: For RF diode detectors to work, one needs a device that has a
: non-linear V/I curve. In other words, the slope of the V/I curve
: must change as a function of applied Voltage. The slope must be
: steeper (or shallower) at higher voltages and shallower (or steeper)
: at lower voltages than at the quiescent operating point.

: If there's any truth to this, someone please enlighten me. I've always
: been of the belief that an envelope detector diode would be most perfect
: if the diode was a perfect switch, i.e. zero attoamps reverse current
: and perfectly linear forward current (as though the diode was a wire
: during the forward conduction period). I don't see how a changing slope
: during forward conduction could do anything other than distort the
: demodulated waveform, especially on tiny signals.

: --
: The function of an asshole is to emit quantities of crap. Spammers do
: a very good job of that. However, I do object to my inbox being a
: spammer's toilet bowl. -- Walter Dnes