John Miles wrote...

Winfield Hill wrote...
http://www.picovolt.com/win/elec/com...de-curves.html ...
that part of my measurements cries out for further bench exploration.
It represents only one part, and is unconfirmed. Also, what happens
if the voltage is reversed? Are we to believe the diode is a 10M
resistor, shunted by a diode? I'm not comfortable with that.

I'm confused. Is there some reason to expect the semiconductor material
to be a perfect insulator with no resistivity at all? Nothing's
perfect, and those diodes probably aren't made in the most exacting
processes.

I would be blown away if you *couldn't* measure some ohmic current flow
in a diode at any particular voltage level.

Agreed. It's the rather low 10M value that raises my eyebrows.
Hence my suggestion that the measurements be revisited. Picked up
by John Jardine, who obtained similar values, copied below:

Test on a 1N4148.
ForwardV DiodeR
+50mV 8megs.
+30mV 9megs.
+20mV 10megs.
+10mv 12megs.
+5mV 21megs.
ReverseV
-5mV 21megs.
-10mV 30megs.
-30mV 270megs.

John also suggests the measurements may need further refinement.

Oops! I can think of several circuits I've designed over the years
using diodes for discharge protection that might not work exactly as
I intended, given this observation. And I recall several circuits
where I intentionally back biased the diode a few hundred millivolts
to insure an open circuit.


--
Thanks,
- Win

Winfield Hill wrote...

John Miles wrote...

Winfield Hill wrote...
http://www.picovolt.com/win/elec/com...de-curves.html ...
that part of my measurements cries out for further bench exploration.
It represents only one part, and is unconfirmed. Also, what happens
if the voltage is reversed? Are we to believe the diode is a 10M
resistor, shunted by a diode? I'm not comfortable with that.

I'm confused. Is there some reason to expect the semiconductor material
to be a perfect insulator with no resistivity at all? Nothing's
perfect, and those diodes probably aren't made in the most exacting
processes.

I would be blown away if you *couldn't* measure some ohmic current flow
in a diode at any particular voltage level.

Agreed. It's the rather low 10M value that raises my eyebrows.
Hence my suggestion that the measurements be revisited. Picked up
by John Jardine, who obtained similar values, copied below:

Test on a 1N4148.
ForwardV DiodeR
+50mV 8megs.
+30mV 9megs.
+20mV 10megs.
+10mv 12megs.
+5mV 21megs.
ReverseV
-5mV 21megs.
-10mV 30megs.
-30mV 270megs.

John also suggests the measurements may need further refinement.

Oops! I can think of several circuits I've designed over the years
using diodes for discharge protection that might not work exactly as
I intended, given this observation. And I recall several circuits
where I intentionally back biased the diode a few hundred millivolts
to insure an open circuit.

And others where I used a transistor collector or JFET gate instead.

Pease Porridge in the Feb 3rd issue of Electronic Design mentions
this problem, and Bob suggests using a transistor. "Using 2n3904s
as diodes is very important because most ordinary diodes are much
too leaky around +/-60mV to work well. Ordinary gold-doped 1n914s
and 1n4148s are quite unsuitable..."


--
Thanks,
- Win

I haven't followed this thread very thoroughly, so this might not be
directly relevant. But it should be of interest to anyone trying to
detect small signals with a diode.

There are several reasons why diodes do poorly with small AC signals.

The first is, of course, the forward drop. However, this can in theory
be reduced to an arbitrarily low value by reducing the current to a low
enough value (by, for example, making the load impedance high enough).

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.

The third reason is the diode capacitance. This shunts the diode,
effectively lowering the reverse impedance. It also lowers the forward
impedance, but when the forward Z is lower than the reverse Z, the net
effect is to further degrade the forward/reverse impedance ratio.

You can make all the DC measurements you want, but they only tell half
the story. When you apply AC, you charge the load capacitor during half
the cycle according to the diode's forward impedance, and charge is
removed from it during the other half according to the diode's reverse
impedance. As the forward/reverse impedance ratio degrades due to the
two effects mentioned above, the net charge you get in the load
capacitance decreases, hence the voltage it's charged to decreases. This
ends up looking like a larger diode forward drop.

I spent a lot of time thinking about this some years ago when designing
a QRP wattmeter, and some of the conclusions I came to appear in the
resulting article, "A Simple and Accurate QRP Directional Wattmeter",
published in QST, February 1990. See the analysis on p. 20, "Ac v Dc:
Why the Difference?"

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
I haven't followed this thread very thoroughly, so this might not be
directly relevant. But it should be of interest to anyone trying to
detect small signals with a diode.

I spent a lot of time thinking about this some years ago when designing
a QRP wattmeter, and some of the conclusions I came to appear in the
resulting article, "A Simple and Accurate QRP Directional Wattmeter",
published in QST, February 1990. See the analysis on p. 20, "Ac v Dc:
Why the Difference?"

Roy Lewallen, W7EL

Your article sounds interesting. Is there a link available to see it?.
The simplest approach I've seen, was is in the 'Levell TM6A broadband
voltmeter'(UK). Designer chopped the low level diode output at 20Hz,
allowing a 1mVac FSD.
regards
john

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.

[...]

Roy Lewallen, W7EL

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

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

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

In article ,
Roy Lewallen wrote:
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.

Don't people still use 2N2369As, or at least the plastic version?
If not, what do they use instead?

(Does gold doping work for PNP transistors? I don't see why it wouldn't,
but I've never seen a specific reference to a gold-doped PNP.)