[sdiy] Re: Leapfrog

[Magnus Danielson]

From: "jhaible" <jhaible at debitel.net>
Subject: Re: Leapfrog
Date: Wed, 7 Jan 2004 02:31:07 +0100
Message-ID: <000401c3d4bd$ef4e4f20$8976b9d9 at debitel.net>

> > So, I am confident that the ortonormal ladder filters is a pretty sane
> design
> > method, given that you can take the expense of 2 op-amps (one summer and
> one
> > integrator, sadly enought you need both) per state. Hmm... or maybe one
> can do
> > with one less... Hmm! (Evil thoughs going through my brain!).
> 
> My evil thoughts lead to a one opamp/state variable design.
> No idea if this will be good in practice - simulations look promising,
> though.
> 
> First step is that you need a signal inversion for each feedback loop. Now
> you could build the whole chain with 2 inverting integrators and one
> inverting
> amp (3 opamps total) per two state variables, and then each feedback loop
> will comprise one signal inversion, as intended. (because the feedback loops
> always go over two state variables - always speaking of a LC LPF)
> 
> The really evil step is to get rid of the extra opamp by building a
> non-inverting
> integrator. So the whole chain will be
> inverting integrator - noninverting integrator - inverting integrator -
> noninverting ...
> and so on.
> 
> The noninverting integrator is built by a RC lag circuit (passive) plus a
> negative
> impedance converter that un-dampens the RC lag circuit by loading it
> with -R. This need a single opamp, which also buffers the passive circuit.
> Then split the R into two R's for getting a summing input. Both R's will
> load each other, so the signal is divided by 2. Set the negative impedance
> converter such that you have a gain of 2 at the opamp output (where you
> take the buffered signal anyway.)

Hmm... I see what you are saying, but I am not sure that your solution is the
best way to acheive it. The unstability issue remains as a little worry, but
I agree the feedback loops in existence might be an aid here.

Wouldn't it be simpler to use an active LR there? I.e. use an inductor
instead of an capacitor. The thing is, you can let the inductor be the negative
sign in relation to the capacitor, but then you use the resistors to set the
critical parameters. So what's the gain from using a whole bunch of inductors
to begin with? You only need one value since everything else is scaled
correctly! However, there are certain things you don't want to do with
inductors, you don't want to put them in the feedback path of that op-amp,
since that will certainly put you at risc wakening the dominant pole of the
op-amp and wheee what a selfoscillation we got ourselfs...

But again, sometimes the use of inductors isn't necessarilly an evil. You can
get small and neat surface-mount inductors these days. Through scaling we can
make sure we only use up a single value.

I'm toying with a few schetches to see if we can come up with a single op-amp
non-inverting integrator.

Cheers,
Magnus