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Subject: Re: Improving Rise/Fall times on Opto
Date: Fri, 06 Dec 2002 22:15:25 +0000
X-Newsreader: Forte Agent 1.92/32.572
>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
>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?
If you just want it sharp use a schmidt trigger like others say.
If you want it faster use some extra components.
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Something like above. The phototransistor sees little collector to base
voltage change and the diode prevents saturation significantly improves
turn off speed.
The resistor from the phototransistor base should be high maybe 100k+ just
there to remove leakage. The low level from the output transistor might be
marginal for TTL levels.
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