"johna@m" wrote in message
oups.com...
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 ?

Regards,

John.


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 M
MasonDG44 at comcast dot net (Just subsitute the appropriate characters in
the address)

Never take a laxative and a sleeping pill at the same time!!