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Subject: Re: Improving Rise/Fall times on Opto
Date: Fri, 6 Dec 2002 21:17:14 +1100
X-Newsreader: Microsoft Outlook Express 6.00.2600.0000
"thorin" wrote in message
> 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.
> 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
The secret is to use a circuit configuration the minimises the amount of
voltage swing C-E. That translates into loading the opto with a very low
If you have a -ve rail, connect the base of an NPN to GND, the emitter to
the collector of the opto and the emitter of the opto to -ve supply
(say -5V). The collector of the NPN goes to +ve supply via a load resistor.
This is called a cascode connection. All the photo current of the opto wil
lappear across the load on the NPN transistor, but Vce of the opto will
remain virtually constant. A large value R (1M or so) across Vbe of the opto
will also help speed it up. Make sure the transistor doesn't saturate. This
scheme can be adapted to single rail by lifting the NPN base off GND with a
resistive divider, at the expense of output swing.
You can also use a grounded emitter NPN, with a turn on resistor base to +ve
supply, and simply have the opto "short" the base to ground.
Another arrangement is to use fast-ish bifet op-amp, like an LF356. With a
single supply bias the + input to midrail with equal R's. Connect the opto
between - input and gnd, and use a 100K pullup to +ve supply. Connect back
to back 1N4148s from opamp o/p to - i/p. The diodes prevent the opamp from
ever hitting the rails and saturating. You'll only get about 1v pp output
swing, but a comparator stage after that will get you back to logic levels.
BTW, a 4-pin opto such as PC811 is preferrable, because the base pin on a
4N35 is very prone to noise pickup.
Alternatively, buy a fast opto :-)
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