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From: email@example.com (Tim Shoppa)
Subject: Re: Improving Rise/Fall times on Opto
Date: 6 Dec 2002 11:07:36 -0800
NNTP-Posting-Date: 6 Dec 2002 19:07:36 GMT
Robert Baer wrote in message news:<3DF05E03.8438230E@earthlink.net>...
> thorin wrote:
> > Dear Group, could anyone suggest an effective ciruit that will vastly
> > imrpove the rise and fall times on the output of a 4N35 or equv
> > opto-isolator.
> > curently I am using a 68K from base to emitter in the output transistor. The
> > output transistor is connected as emiitter-follower to prevent the circuit
> > from inverting the input.
> > Is there a way of improving the sharpness of the pulse?
> > thanks in advance
> > lh
> Well, i do not know the schematic of that unit, but if E, B and C are
> all available, i fail to see whyyou are purposely "killing" the
> sensitivity with that E-B resistor.
Without an E-B resistor the "fall time" can be very very long. Think of it as
a pulldown resistor.
Properly choosing the pulldown is an important matter, and 68K could be the
wrong value. Or it might be right. It depends on the rest of the circuit.
The classic answer to "how do I improve the rise and fall times" is a
Schmitt trigger, of course. But maybe he also wants to increase the bandwidth
too (he didn't say).
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