"Roy J. Tellason" wrote to "MIKE ROSS" (01 Jan 03  07:07:10)
 --- on the topic of "DIGITAL CLOCKS"

 RJT> MIKE ROSS wrote in a message to ROBERT SAYRE:

 MR> The problem is that the capacitance of a diode increases as its
 MR> reverse bias voltage becomes smaller.

 RJT> Why is diode capacitance a problem?

Because of the Miller multiplication effect in for example the basic
common emitter transistor configuration. Where a portion of the output
is fed back through the collector-base capacitor to the input. That
current is a direct function of stage voltage gain. Thus an inverting
stage with a capacitance of 5pF in the feedback path and a gain of 100
(re: Av+1) will in effect see a virtual capacitance of 500pF at the
input due to the Miller multiplication effect. This virtual capacitor in
parallel with the input will slow down a pulse edge as it charges
through a non-zero source resistance. (The Miller effect isn't exclusive
to transistors, any inverting voltage amplifier will have it.)


 MR> The diode capacitance actually becomes a maximum just as it starts
 MR> to conduct.

 RJT> Just before,  actually.  The junction still needs to be slightly
 RJT> reverse-biased.  And the minimum reverse-bias gives the same effect as
 RJT> having those capacitor plates *real* close together.

So how do you suppose the capacitance varies between zero and the
threshold of conduction at around 0.5 volts for a Si diode? (which is
fully conducting at say 0.7 volts, ie what happens between 0 and 0.5v?)


 MR> The choice of Schottky diodes is because they have a low forward
 MR> voltage and can be made very tiny with very little capacitance.

 RJT> Tiny in terms of the area of the junction?  Or something else?

Yes, because the metal to oxide junction can be made as a parasitic
diode of the point-contact type. This type of metal-semiconductor point
contact diode has very little capacitance because it is basically a
single capacitor plate with a needle tip real close and the area under
the needle tip can be very tiny. OTOH as you probably know a diffused
junction has a much larger area by comparision.

 Mike
 ****

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