Mike Monett wrote:

Excellent description - thanks.

Only one small problem - as Win pointed out, Bob Pease feels a
diode-connected 2N3904 has lower leakage at low voltage than a 1N4148:

"What's All This Comparator Stuff, Anyhow?"

http://www.elecdesign.com/Articles/A...9517/9517.html

Does this mean a 2N3904 has a shallower slope than a 1N4148 through zero, or
perhaps one or the other has an offset, such as the Agilent Zero Bias
Schottky Detector Diodes shown in AN969?

http://www.spelektroniikka.fi/kuvat/schot8.pdf

Regards,

Mike Monett

I'm not sure what you mean by an "offset" -- all diodes cross through
the origin of the I-V curve, when excited by DC, anyway -- unless they
contain a battery. In the reverse direction, the current pretty much
levels off beyond a small reverse voltage. The current of this level
part is the saturation current.

Again, don't think that good DC characteristics make for a good RF
detector. A number of other factors, which have been discussed here, are
very important. As I recall, only transistors designed as saturated
switches (2N918 comes to mind, but it's been a long time, so don't quote
me) are gold doped. Ones which aren't, and I'm quite sure the 2N3904 is
in that category, will have long reverse recovery times so will make
poor RF rectifiers. Circuits became too fast for saturated switches
long, long ago, so I'd be surprised if gold doping is done any more
except for replacement transistors in very old equipment.

You can learn a lot with a very simple setup consisting of nothing more
than a variable amplitude signal generator, a diode, load resistor and
capacitor, and a meter or scope. SPICE should also show these effects
provided you use good models.

Roy Lewallen, W7EL