[sdiy] NICs, GICs, NLOs and all that

[Magnus Danielson]

From: Ian Fritz <ijfritz at earthlink.net>
Subject: [sdiy] NICs, GICs, NLOs and all that
Date: Fri, 03 Jan 2003 22:04:39 -0700

> Hi listers --
> 
> It seems that there has not been much use of NICs and GICs (negative and 
> generalized impedance converters) and related circuits in the synth-diy 
> arena.  Recently JH reminded us that synthetic inductors constructed from 
> classical gyrator circuitry may have important advantages in constructing 
> fixed-frequency filter banks.

I have allways wondered why they are not used more. Much of the theory is
however on fixed curcuits and not on variable curcuits, something which much
(but not all) of the Synth-DIY needs.

>  I've collected some interesting general 
> information and circuit ideas in this area from the web and thought I would 
> pass them along.  Of course, much of the basic information is also 
> available in textbooks on network theory.

Thanks for all the links, will look them through...

> None of these references incorporate voltage control of parameters, but 
> using OTAs as variable resistors to tune synthetic inductors and capacitors 
> looks feasible.

I where just about to say that I've designed both single and double ended
gyrators with OTAs for this purpose! It took a little thinking, but I was able
to derive the full expressions for dimensioning it. If you wish I could dig it
up and present it here...

By a little head-scratching and a little scribbeling on a piece of paper you
should be able to have CV controlable resistors, capacitors and inductors and
both linear and inverse responces should be possible. Having those available in
both double and singleended variants should provide a nice little design-base
for many different designs.

One issue with OTAs though... the offset of the OTA output may be a real issue.
I would not expect even near rail-to-rail responce without serious distorsion.
I don't know if it has been discussed. In many cases OTAs is being used to
drive virtual ground setups, and then you have a fixed voltage offset for the
OTA output to operate in. Then, in other setups (especially in phasers) OTAs
really operate on a varying output voltage offset. Most cases where this
happends you can modify the surrounding curcuits, but this would be against the
general idea about "floating" double-ended gyrators.

Cheers,
Magnus - floating around in ideas and knows how to walk the water!