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> The output of an opto shows a 3us slope in both edges with a 4n26 in
follow
> emitter configuration (not 4n32 as stand on the diagram). I've used this
> configuration to prevent signal inversion.
> Is it possible to reduce this slope?

FETS needs high peak gate drive currents to get good switching times. Your
circuit uses only the 1K gate pull down to turn the FET off which will lead
to very slow turn off times.

In the attached GIF replace Q6 with your opto.
Q8 is something like BC337 (NPN, 500 mA plus, beta ~= 300).
The opto now provides active turn off via D1 and Q8 provides active turn on.
R6 typically about 1K
R4 typically about 10R

This driver

- Provides active drive high and low
- Provides good FET drive (set largely by R4 and how much current the
transistors will handle.
- Is much faster than passive drive systems.

A problem MAY be how much current the opto will provide to turn off the FET.
Depending on CTR (current gain) and how hard you drive it the opto may only
supply 10 - 100 mA peak whereas the FET would like 1A or so peak gate
currents to get good switching times.

If you need more drive replace D1 with a PNP transistor (emitter to Q8
emitter, collector to ground, base to Q6 collector. You now have high
current drive in both directions.)




        Russell McMahon


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