[sdiy] optocouplers/Vactrols

[Scott Gravenhorst]

Paul Higgins <higg0008 at tc.umn.edu> wrote:
>You meant LED/FET opto, right?  I think that's what the H11Fx parts are.

Yes.  I'll blame the constant barrage of new acronyms, especially 3
letter acronyms.

>
>The LDR (photoresistor) optocouplers have much better linearity specs 
>than the FET ones, but are also much slower.  I know that there are 
>some techniques for using the FET optos with better linearity, usually 
>involving putting the FET at a summing node or other virtual ground 
>point.  But the LDR optos (like the Vactrols, etc.) can handle much 
>larger signal swings.  Mesa/Boogie uses them all over the place in 
>their tube guitar amps to do very high-level signal switching.  (I 
>don't think they're actually intended to do that, but they do work).

In the case of an SVF, the FET connects to an integrator summing node and
the input source.  Since the source is always at zero volts, you're stuck
with the drain being at whatever voltage the input signal is.  I think
the techinique you're referring to is:

http://www.uni-bonn.de/~uzs159/switch.html

But this won't help a SVF.

The only thing I can think of is to attenuate the signal, process it and
then amplify.


>On Tuesday, October 19, 2004, at 01:41 PM, Scott Gravenhorst wrote:
>
>> I would add:
>>
>> #4 linearity
>> At least for H11Fx parts, (this is a LDR/FET optocoupler) there is a
>> linearity issue when the input causes a larger than 100mv signal 
>> across the
>> FET terminals.  This linearity problem caused me to abandon a filter 
>> design
>> using it.  But I may revisit this with the low signal caveat, since 
>> that
>> seems to cure it in large part.  In fact, the OTA methods have a 
>> similar
>> issue, I note that an OTA SVF often uses a 100:1 (or similar) voltage 
>> divider
>> before the signal hits the OTA input terminals.
>

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