[sdiy] Filter Structure Question

[Scott Gravenhorst]

Magnus Danielson <cfmd at bredband.net> wrote:
>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:
[snip]
>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.

Ok, I think I see this.  We use a single supply, say 5 to 8 volts (I want to be
able to overdrive the CMOS amps easily).  Then one inverter is used the way you
described to create a new "ground" line that will be about 2.5 volts away from
what will then be the rails (+/- 2.5Volts).  ??  The CV is applied across the
MOSFET gates and this new created ground ??

So this way, I can use standard 4069UB integrators (not the hipass thing I
proposed) in a state variable configuration.  

I think I understand the resistors in supplying power to the 4069UB, but I'm not
sure why they're needed.  The ground creating inverter will seek a point about
1/2 way between the rails, but that inverter will draw only 11 ma without the
resistors.  Maybe that's too much?  With the resistors, will there be power
supply coupling between the amplifiers?  How big would these resistors have to be
(I don't see a total power dissipation spec on the datasheet)?  Guess: 10K ?

>> 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)

I suppose so, but I have this love affair with the 4007UB as well, it's just a
MOSFET array to me and the accessibility of extra electrodes is attractive.

>> 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.

Cheapertronics, MML, etc., is all fine, I _want_ this thing to distort, but I
want the CV to be reasonably useful.  Doesn't have to be exactly linear, just not
wildly nonlinear.  If the 2 resistor gate-drain thing will work, then fine.

>
>> 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.

That's what I'm after.  I like the idea of being able to overdrive the 4069UB
inverters by using a large enough input.  This is said to be one of the charming
things about the WASP (I know this only from reading, not experience).  I just
don't want to put bipolar OTAs in this.

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

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