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From: email@example.com (Dale Chisholm)
Subject: Re: Improving Rise/Fall times on Opto
Date: 6 Dec 2002 10:35:50 -0800
NNTP-Posting-Date: 6 Dec 2002 18:35:50 GMT
"thorin" wrote in message news:...
> 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?
> thanks in advance
I smell this same problem coming across my desk in the next month or
two, so I did a little pre-emptive surfing & library work. That is -
I haven't actually tried this, but there are published claims for
Put the phototransistor in a cascode configuration with your favorite
gen'l purpose BJT (2N2222, MPSA56, 2N3906, etc). The polarities &
connection details will depend on what's available in your
application, but in my case it looks like:
- Phototransistor (NPN) collector to +12V
- Phototransistor emitter to emitter of MPSA56 (PNP)
- Base of MPSA56 to +5V (maybe through few kilohms of isolation
resistance; bypassed with 0.1uF)
- Collector of MPSA56 thru load resistor to common; select a value
to get the sensitivity you need.
Signal output comes off the MPSA56 collector. Swing is from a few mV
above ground with the phototransistor dark (leakage currents times
load resistor) to a few hundred mV above the +5V rail with the
Phototransistor base is left floating, or lightly loaded (100's of
kilohms to phototransistor emitter) to fine-tune speed & sensitivity.
In this configuration the phototransistor operates almost as a pure
current amplifier: its collector and base voltages don't change much,
so the Miller & junction capacitances can't slow you down.
If you can breadboard something like this I'd sure like to know how
well it works . . .
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