Non-linear techniques with filters

[Grant Richter]

Thank you Juergen, that makes a lot of sense. I was not thinking about the
frequency dependence of the feedback path.
>> 
>> One example immediately springs to mind. A CA3080 has a very limited
> linear
>> range. To extend the range, diodes are used as compensation on the input
>> side. But what if the compensation were done on the OUTPUT side?

A little more "puddle of consciousness" design ;^)

The easiest way to generate an inverse non-linear function would be to put
an identical section in the feedback loop of an op-amp. 3080s don't match
well, but then I remembered that 13700s are monolithic duals that should
inherently match at all Iabc.

According to the datasheet, using a 15K diode bias and keeping the same THD
you can only push the input from 20 mv p-p to 100 mv p-p. Not that great of
a S/N improvement compared to a CA3280. But 3280s are kind of expensive, and
unless JRC picks them up, they'll probably end of life before the 13700.

If you could use the second section of the 13700 to do inverse compensation,
you might be able to drive the input very hard, but keep a low THD. This
would allow a VCA design with improved S/N ratio over the diode compensation
for only the cost of the second section (you probably have an op-amp for a
buffer already).

Another option is to use this as a control voltage "twister" using different
Iabc currents for the two sections. With pots for the diode currents you
should be able to generate all kinds of wacky transfer functions with a
fairly low parts count.