[sdiy] Filter Structure Question

[Scott Gravenhorst]

harrybissell <harrybissell at prodigy.net> wrote:
>Hmmm I'm not sure I followed that.  My guess would be you don't
>WANT to use more than one 4069 in the same dc coupled circuit.
>You could use two chips (self biased) that are ac coupled.

Yes, I'm sure that's true, but not a problem since this is not meant as a high
performance filter.  This is a goofy idea for hopefully generating some interesting
harmonics or effects with a minimalist CMOS approach.  It could wind up on the ash
heap faster than it took to think of.  Also, my first attempt at this will be a
single 4069UB, and will be powered without the power supply resistors required only
for the multi IC application of this.

>
>The technique of using a spare inverter to estimate the bias point
>is an old (and useful one).  If I followed correctly, you want to tie the
>output of that inverter to "real" ground ?  (to the center of a pair
>of resistors connected to the positive and negative rails).  I don't
>know about that part. The output of the inverter would have only a
>very limited ability to move that point to the correct one.

Yes, and it could actually be used with a dual supply with the psuedo-ground
connected to the real ground, but that would require using power supply resistors
which I want to avoid, at least initially.

>You might buffer the inverter with an opamp, but that would spoil
>the philosophy :^P

Trooly.

>The magic (as Magnus pointed out) is that the on-chip matching is excellent,
>and the variation from part to part, lot to lot, and mfgr to mfgr becomes
>increasingly dismal !!!

Trooly too.  If this works for a simple one-chip SVF, and sounds good, it will make
a good box (self powered) that I can shift from one FatMan to another and also use
with an electric guitar.  The operative phrase being "sounds good".  If it ain't,
it goes back where it came from...

>(been away for a week so I have not caught up on all 280 back e-mail, maybe this
>was said :^)
>
>H^) harry
>
>Scott Gravenhorst wrote:
>
>> 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 20:59:54 -0700
>> >Message-ID: <200407170359.i6H3xsZ22023 at linux6.lan>
>> >
>> >> Yeah, I know it looks strange, but there's no connection between that ground
>> >> and any power supply common.  That "floating" inverter isn't really unused.
>> >
>> >It IS used, alot! It's part of the powersupply setup! ;O)
>> >
>> >>  It creates a signal ground approximately halfway between the 4069UB Vss and
>> >> Vdd pins.  If a symmetrical CMOS inverter has it's input connected to it's
>> >> output, the output seeks a voltage approximately halfway between the Vss and
>> >> Vdd pins.  It creates a center reference between the IC power rails which is
>> >> why it is used as a signal ground.  In this case, the ground point sits
>> >> about 2.5 volts away from either rail (which are at 0volts and 5volts).
>> >
>> >Right, the input is driven from the output in order to selfbias on the
>> >reference level whatever it is in that particular device. This feedback also
>> >uses the gain of the inverter to stabilize the output in relation to the load.
>> >This is equalent to an op-amp with the reference voltage at it's pos input and
>> >the output hooked to it's neg input, i.e. a buffer configuration. The only
>> >thing is that the gain of the inverter is much less than the open loop gain of
>> >an op-amp. It's a crude approximation, but hey, that's half the fun! ;O)
>> >
>> >> This was a suggestion from Magnus to avoid problems with the virtual ground
>> >> of the summing nodes created by the integrators.  Doing this allows me to
>> >> reference my CV signal to that ground so that it properly influences the
>> >> gate of the MOSFET feeding current to the integrator.
>> >
>> >Exactly.
>> >
>> >> Magnus's last comment was that I was "bloody close", so there still may be
>> >> something I don't yet understand.  I didn't really understand his comment
>> >> about using a dual supply, I don't know what that would be necessary.
>> >
>> >If you have a floating single-ended supply you are fine as long as you only
>> >drive THAT 4069UB with it. Hook up another one and it has a different view on
>> >where the bias-point shall be. What I wanted to propose was a way to handle the
>> >situation where you have potentially many of these in a design. Doing a bread-
>> >board test of a single one won't give away the difference. What you could do is
>> >to self-bias the inverters of a number of 4069UBs and measure their bias-level.
>> >I expect them to vary between chips, especially from different batches or even
>> >different fabs. When you compare within a chip you would probably not see very
>> >large differences, since they have virtually the same doping characteristics,
>> >but differences are there from geometry of individual design instances.
>>
>> Ok, I see what you're saying about different chips having different gains and
>> other characteristics that would change where the virtual ground point would sit
>> from one chip to another.
>>
>> I think I now understand what to do to fix it for more than one chip in a
>> system, please bear with me as I try to articulate it.  It took sleeping on this
>> to see what you were giving me, and it's pretty cool.
>>
>> We would use a dual supply as you suggest, which has it's own natural ground
>> point.  This ground point is then connected to the pseudoground that the
>> "special" inverters want to make and each chip in the system has one inverter
>> that is used this way.  This is the real reason for those resistors that feed
>> power to the chips, each chip has it's own set of power feed resistors.  Now
>> what happens is that instead of the signal ground floating around as in my
>> single supply, single chip method (which I believe doesn't really need the
>> resistors) the IC's internal power rails (Vss and Vdd pins) float around
>> instead.  So ground then will be ground, because all the grounds, created and
>> real, are all connected but Vss and Vdd at each chip will differ somewhat.
>>
>> Whew.  Thanks, I think I get it now.
>>
>> This is very good information, and I shall keep it tucked away.  I believe my
>> first experiments will use the single supply method, because at present, it
>> looks as if I can implement the whole filter with one 4069UB.  For my studio,
>> this filter will be a self contained, self powered box that I can use with
>> either of my FatMan synths, or use with a guitar.  One of the reasons I wanted
>> to use a single supply was to keep the total supply voltage rather low making
>> overdrive easier.
>>
>> >> I'm going to throw this at a solderless bread board tomorrow and see what
>> >> happens.  Maybe I'll find out...
>> >
>> >Keep us posted, I am eager to learn how it goes.
>>
>> You bet.
>>
>> >And yes, I had my HP and LP outputs swapped. Since the two stages are
>> >integrating, the lowpassiest output should be the last in the chain, and the
>> >feedback will with the summation of the input create the highpass function
>> >naturally. Sorry for the confusion, I was typing away from memory, which was
>> >obviously flawed.
>>
>> Another Whew!
>>
>> ---------------------------------------------------------
>> - Where merit is not rewarded, excellence fades.
>> - Hydrogen is pointless without solar.
>> - What good are laws that only lawyers understand?
>> - The media's credibility should always be questioned.
>> - The only good terrorist is a dead terrorist.
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>>
>> -- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
>> -- Linux Rex         | RedWebMail by RedStarWare
>> -- FatMan: home1.gte.net/res0658s/FatMan/
>> -- NonFatMan: home1.gte.net/res0658s/electronics/
>> -- Autodidactic Master of Arcane and Hidden Knowledge.
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.