[sdiy] Filter Structure Question

[Magnus Danielson]

From: Scott Gravenhorst <music.maker at gte.net>
Subject: Re: [sdiy] Filter Structure Question
Date: Fri, 16 Jul 2004 10:02:22 -0700
Message-ID: <200407161702.i6GH2MZ06562 at linux6.lan>

> Magnus Danielson <cfmd at bredband.net> wrote:
> >From: Scott Gravenhorst <music.maker at gte.net>
> >Subject: [sdiy] Filter Structure Question
> >Date: Fri, 16 Jul 2004 08:01:34 -0700
> >Message-ID: <200407161501.i6GF1YZ32465 at linux6.lan>
> >
> >> This may seem daft to many, but I have evil, subversive and nefarious
> >> reasons for asking:
> >> 
> >> Are there lowpass voltage controlled filters that instead of using the
> >> standard integrator, use a high pass filter structure in the feedback loop
> >> of an OpAmp?  If so, are there viewable examples?
> >
> >Try something like this:
> >
> >buffer the highpass filter with an op-amp and let the buffered version drive
> >the negative input of the opamp. Replace the resistor with an OTA which has 
> >its neg input grounded and its pos input properly divided from the buffered
> >signal. This should make the OTA operate as a resistor to ground with CC
> >(Control Current) which needs the classical drive.
> >
> >Why you ask for it is beyond me... but I am curious to find out.
> >
> 
> Well, because I'm evil, subversive, nefarious and twisted.  

Yes, yes, yes and yes... but what else? ;O)

> It's more CMOS stuff.  I looked at the WASP filter and I didn't like the idea
> of using OTAs to control the integrators (just because they're not CMOS).  I
> though about using 4007 transistors, but this method has problems because the
> virtual ground created by the 4069UB inverters isn't ground, even if a dual
> supply is used.  I.e., one can't guarantee the summing node will be at ground
> potential and stay there as a reference for the MOSFET's  source electrode.

What you could do is to use one of the 4069UB inverters as voltage-reference 
and regulate the powersupply so that it will sitt at ground. The evil way of
doing this (if I may be even more evil and subversive and teach you some really
evil tricks) is to let one inverter of a 4069UB have both its input and output
tied to ground and then have both supply feeds through resistors (which is good
for current limiting/over-current protection) and with a cap over the local
voltage. This will DRIVE the supply (with feedback) so that the virtual grounds
of the other inverters is at close proximity to ground. This is both evil and
minimalistic.

> So I've mentally played with the idea of using H11F1s for the voltage
> controlled resistors.  This solves the virtual ground problem, but I really
> wanted this to be a CMOS filter.  The H11F1 method is close and I may try it
> anyway.

You could be using another 4069UB, but you know that, right? ;O)

> What I had in mind was to create inverting lowpass "blocks" by using a 4069UB
> inverter something like this (pardon the ASCIImatic):
> 
> 
>                  |\                                                           
>         R        | \                                                          
>                  |  \                                                         
> -----\/\/\/\--+--|   >--+----                                                 
>               |  |  /   |                                                     
>               |  | /    |                                                     
>               |  |/     |                                                     
>               \         |                                                     
>               /         |                                                     
>             R \         |                                                     
>               /         |                                                     
>               \         |                                                     
>               |    C    |                                                     
>     4007      |   | |   |                                                     
>     MOSFET    +---| |---+                                                     
>           |   |   | |                                                         
>         | |---+                                                               
>         | |                                                                 
>                          
>  CV ----+ |---+                                                               
>           |   |                                                               
>               |                                                               
>              GND                                                              
>                                                                               

You need to linearize the MOSFET. A pair of resistors will do the trick.
That is, if you don't want to be cheapertronics-evil, in which case you just go
forward blindfolded and it will be OK.

> And then arrange them in a state variable configuration.  This takes care of
> the "source needs to be connected to ground" issue.  I don't have a clue at
> this point if 1) it will work and 2) what R should be.  High, I think, like 1
> meg or more.  Linearity fixing resistors should be applied to the MOSFET
> perhaps or an opamp with a MOSFET in the feedback loop to fix gross CV
> nonlinearity.

OK. Well, I will think some more about it. We certainly would like a 4069UB
clean device and not the WASP hybrid.

In a 4069UB clean design all active processing (OTAs included) is in 4069UBs.

Cheers,
Magnus