DaveM wrote...

"johna@m" wrote ...
Should not we expect that the current, even at very small level, to
be half rectified by a diode, since the reverse resistance of the
diode is supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

There is nothing wrong with the simulator... the problem is with your
idea of a diode. The general definition of a diode is a component that
conducts normally in one direction, but does not conduct in the other.
That definition only applies to a "perfect" diode. The reality of
semiconductor diodes is that a 'barrier potential" exists across the
junction. In germanium diodes, this is around 0.3 volts; in silicon
diodes, it's around 0.6 volts. In order for the diode to conduct, this
barrier potential must be exceeded by an externally applied voltage.
Until that potential is reached, the diode is said to be reverse biased,
and only a very small leakage current flows. When the barrier potential
is reached, the junction becomes forward biased and conducts heavily.

The small signal voltage that you are trying to simulate may not be enough
to reach the barrier potential of the diode junction, thus, no conduction
(rectification) in either direction. The simulator is aware of the barrier
potential of the diode. If the peak value of your signal voltage is less
than the barrier potential, no rectification occurs. If you increase the
amplitude of the signal applied to the defined barrier potential of the
particular diode in your model, you will see rectification begin. The
higher the signal amplitude, the more rectified signal appears on the
output.

You can make a diode rectify a signal amplitude lower than the barrier
potential by applying a forward voltage that is just under the barrier
potential, so that the signal doesn't have to overcome the full barrier
potential. Fer instance, if you apply a 0.5 volt DC voltage to a silicon
diode, it will start to rectify signal levels as low as 0.1 volts.

Dave, your considerable effort to explain the nuances of diodes to John
is commendable, but your explanation is rather misleading. It's not true
that for a diode to conduct, the "barrier potential must be exceeded,"
and "the junction becomes forward biased and conducts heavily." Instead
the diode current has an exponential relationship to the voltage across
it, and gradually turns on over many hundreds of millivolts, not abruptly
at say 600mV. Here, examine some diode measurements I made a long time
ago, http://www.picovolt.com/win/elec/com...de-curves.html

For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win

The difficulty is "what to do with that 1uA current". To put it to practical
use, a signal processor is needed that has a useful output. For example, a
MOSFET amplifier with a 10 megohm input resistance and negligible input
capacitance (for low frequency sigs) could be used.

In this case it would be better to rethink the project.
In addition to the diode, some system design is indicated.

Bill W0IYH

"Winfield Hill" -edu wrote in
message ...
DaveM wrote...

"johna@m" wrote ...
Should not we expect that the current, even at very small level, to
be half rectified by a diode, since the reverse resistance of the
diode is supposed te be far greater than the forward resistance?

Why can't we found this result in smulation. Is it a flaw in the
simulator (Simplorer) or is the theoric behavior of a diode that
changes in case of very small input ?

There is nothing wrong with the simulator... the problem is with your
idea of a diode. The general definition of a diode is a component that
conducts normally in one direction, but does not conduct in the other.
That definition only applies to a "perfect" diode. The reality of
semiconductor diodes is that a 'barrier potential" exists across the
junction. In germanium diodes, this is around 0.3 volts; in silicon
diodes, it's around 0.6 volts. In order for the diode to conduct, this
barrier potential must be exceeded by an externally applied voltage.
Until that potential is reached, the diode is said to be reverse biased,
and only a very small leakage current flows. When the barrier potential
is reached, the junction becomes forward biased and conducts heavily.

The small signal voltage that you are trying to simulate may not be
enough
to reach the barrier potential of the diode junction, thus, no conduction
(rectification) in either direction. The simulator is aware of the
barrier
potential of the diode. If the peak value of your signal voltage is less
than the barrier potential, no rectification occurs. If you increase the
amplitude of the signal applied to the defined barrier potential of the
particular diode in your model, you will see rectification begin. The
higher the signal amplitude, the more rectified signal appears on the
output.

You can make a diode rectify a signal amplitude lower than the barrier
potential by applying a forward voltage that is just under the barrier
potential, so that the signal doesn't have to overcome the full barrier
potential. Fer instance, if you apply a 0.5 volt DC voltage to a silicon
diode, it will start to rectify signal levels as low as 0.1 volts.

Dave, your considerable effort to explain the nuances of diodes to John
is commendable, but your explanation is rather misleading. It's not true
that for a diode to conduct, the "barrier potential must be exceeded,"
and "the junction becomes forward biased and conducts heavily." Instead
the diode current has an exponential relationship to the voltage across
it, and gradually turns on over many hundreds of millivolts, not abruptly
at say 600mV. Here, examine some diode measurements I made a long time
ago, http://www.picovolt.com/win/elec/com...de-curves.html

For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win

Hi,

I have re-done the simulation with the diode 1N41481, and it rectified.
Thanks.

For the 600mv, it did give indeed around 0.5mA. I even tried at 100mv
and at 100uv, and it still conducts (respectively 60uA and 25nA).

Regards,

John.

"Winfield Hill" -edu wrote in
message ...
For example, these plots show that an ordinary 1n4148 class of silicon
signal diode, which conducts about 0.5mA at 600mV, is still working at
250mV, conducting 1uA in my measurements. In fact, this diode was still
conducted at 100mV. See http://www.fairchildsemi.com/ds/1N/1N4148.pdf
where Fairchild's datasheet also shows this exponential relationship,
albeit drawn with a draftsman's straight line.

So, as others have pointed out, diodes can rectify very small signals.
They may not be very efficient, but they will work. These plots also
show how Schottky diodes (e.g., 1n6263 and 1n5819) are better than
ordinary silicon diodes at low voltages, even below 100mV. The 1n6263
may be hard to get, but other parts, like the sd101 or bat17 may not.
http://www.vishay.com/docs/85629/85629.pdf

There are other diodes that work well at very low voltages, notably
some made by Agilent (see an1090), but we won't go into them here.


--
Thanks,
- Win


Must be my lucky week!. I needed to make some similar readings. Thanks!.
I still puzzle over the oft quoted "up to about 30mVrms the output from the
diode offers a square law response and will approximate a true RMS
measurement.
As the OP found, there's very little happening down there. Who makes these
sweeping statements?.
regards
john

Must be my lucky week!. I needed to make some similar readings.
Thanks!.
I still puzzle over the oft quoted "up to about 30mVrms the output
from the
diode offers a square law response and will approximate a true RMS
measurement.
As the OP found, there's very little happening down there. Who makes
these
sweeping statements?.
regards
john

No it's true. Look at the curves and notice that the current scale on
the x axis is a log scale. When the RF input is very small, the DC out
is proportional to the log of the RF level i.e. the RF in dB. This is
how the normal power meter works. It also provides a true RMS value
for modulated RF signal. Once the signal gets too big and the diode
begins to work as a converntoin rectifier, this relationship no longer
holds true. Notice the curves break upeards. When the RF volatge to
log I curves are straight line, this is rhe square law region where the
diode current gives you true RMS readings of the RF voltage.
Think of it as a voltage in dB to current converter.

Mark

"Mark" wrote in message
oups.com...



Must be my lucky week!. I needed to make some similar readings.
Thanks!.
I still puzzle over the oft quoted "up to about 30mVrms the output
from the
diode offers a square law response and will approximate a true RMS
measurement.
As the OP found, there's very little happening down there. Who makes
these
sweeping statements?.
regards
john

No it's true. Look at the curves and notice that the current scale on
the x axis is a log scale. When the RF input is very small, the DC out
is proportional to the log of the RF level i.e. the RF in dB. This is
how the normal power meter works. It also provides a true RMS value
for modulated RF signal. Once the signal gets too big and the diode
begins to work as a converntoin rectifier, this relationship no longer
holds true. Notice the curves break upeards. When the RF volatge to
log I curves are straight line, this is rhe square law region where the
diode current gives you true RMS readings of the RF voltage.
Think of it as a voltage in dB to current converter.

Mark

I know where you're coming in from, but whichever way I look at Win's graphs
I'm seeing a straight line for that 1N4148 at 0-30mv levels. Yes, the scales
are log-log but the constant of proportionality is dead straight linear. I.e
1mv RF in gives 1e-10 amps and 10mV in gives 1e-9 amps and pro-rata for all
points in between (you did see the double decade increments?). This agrees
with the 10Mohm value that's marked on the graph.
It shouldn't matter if the RF signal is swept over a 1 to 30mV range or just
a 5 to 5.01mV range, the DC out will be directly proportional to the 'DC' in
and no distortion of the waveform can occur, hence no dc offsets or
harmonics.
The graph next door though , the 1n5819, looks like it could offer up a tad
of rf dB-I rectification. Though to my eyes it still looks way more like a
resistor than anything with a square or log law response.
There'll be better devices out there that offer (say) quadratic like
classical responses at these low levels but it's moving out of the 'common
or garden' playground.
regards
john

john jardine wrote...

I know where you're coming in from, but whichever way I look at Win's graphs
I'm seeing a straight line for that 1N4148 at 0-30mv levels. Yes, the scales
are log-log but the constant of proportionality is dead straight linear. I.e
1mv RF in gives 1e-10 amps and 10mV in gives 1e-9 amps and pro-rata for all
points in between (you did see the double decade increments?). This agrees
with the 10Mohm value that's marked on the graph.

Right, but 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.


--
Thanks,
- Win

"Winfield Hill" -edu wrote in
message ...
john jardine wrote...

I know where you're coming in from, but whichever way I look at Win's
graphs
I'm seeing a straight line for that 1N4148 at 0-30mv levels. Yes, the
scales
are log-log but the constant of proportionality is dead straight linear.
I.e
1mv RF in gives 1e-10 amps and 10mV in gives 1e-9 amps and pro-rata for
all
points in between (you did see the double decade increments?). This
agrees
with the 10Mohm value that's marked on the graph.

Right, but 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.


--
Thanks,
- Win

Yes. Ive been mulling that over. If there is no static bias current flowing
through the diode and it magically switches to 'open circuit' when the
incoming voltage reverses sign, then it would still make a perfect 'average
respsonding' rectifier even at these mV or even uV levels. Just how does the
reverse current start to act in the reverse direction?. On the face of it, a
IN4148 seems easy enough to check out.
regards
john

In article , hill_a@t_rowland-dotties-
harvard-dot.s-edu says...
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.

-- jm

------------------------------------------------------
http://www.qsl.net/ke5fx
Note: My E-mail address has been altered to avoid spam
------------------------------------------------------

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