Mike Monett wrote...

Roy Lewallen wrote:

The second is that the ratio of reverse to forward current increases as
the signal gets smaller and smaller, reaching one at the limit. This can
be observed by looking at the I-V curve of a diode. At the origin, the
curve is a straight line - the diode behaves just like a resistor. ...

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?

No, it means its a better diode at low currents. See my curves again,
http://www.picovolt.com/win/elec/com...de-curves.html Note the
1n458 and the JFET diodes, which follow the theoretical 60mV/decade rule
down to very low currents. As for Roy Lewallen's "ratio of reverse to
forward current" argument, there is no reverse current for these fine
fellows, at least for DC and reasonably low frequencies. It's the very
crummy gold-doped 1n4148 that falls over. Awwkk!


--
Thanks,
- Win

Winfield Hill wrote:

No, it means its a better diode at low currents. See my curves again,
http://www.picovolt.com/win/elec/com...de-curves.html Note the
1n458 and the JFET diodes, which follow the theoretical 60mV/decade rule
down to very low currents. As for Roy Lewallen's "ratio of reverse to
forward current" argument, there is no reverse current for these fine
fellows, at least for DC and reasonably low frequencies.

Sure there is. All diodes have reverse current.

It's the very
crummy gold-doped 1n4148 that falls over. Awwkk!

The gold doping is done to dramatically reduce charge storage time.
Without it, the voltage across a diode continues to be in the forward
direction for some time after you reverse the current through it. While
a non-gold-doped diode might look good in DC tests, it makes a lousy
rectifier of RF. In the extreme case, it acts like a PIN diode (which is
simply a diode designed intentionally to have a long charge storage, or
reverse recovery, time).

Alas, life is full of tradeoffs.

Roy Lewallen, W7EL

I need to clarify this. My comments apply only to junction diodes, which
virtually all silicon diodes are. Schottky diodes don't exhibit this
charge storage effect. That's one reason they're often used in high
frequency switching supplies. Their leakage current is, however, much
greater than silicon diodes.

Roy Lewallen, W7EL

Roy Lewallen wrote:

The gold doping is done to dramatically reduce charge storage time.
Without it, the voltage across a diode continues to be in the forward
direction for some time after you reverse the current through it. While
a non-gold-doped diode might look good in DC tests, it makes a lousy
rectifier of RF. In the extreme case, it acts like a PIN diode (which is
simply a diode designed intentionally to have a long charge storage, or
reverse recovery, time).

Alas, life is full of tradeoffs.

Roy Lewallen, W7EL