[sdiy] Filter Structure Question

[Scott Gravenhorst]

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.  

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.

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.

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                                                              
                                                                              


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.







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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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