[sdiy] Re: 4007 Voltage Controlled State Variable Filter (off list)

[Scott Gravenhorst]

=?ISO-8859-1?Q?Ren=E9_Schmitz?= <uzs159 at uni-bonn.de> wrote:
>Hi Scott,
>
>> I'm looking at the schematic for this filter and I have some questions:
>> 
>> What kind of opamps are you using?
>
>TL07x.
>
>> And what power supply configuration did you use?
>
>15V bipolar supplies.
>
>> To my (somewhat clouded) understanding of these things, it would seem the
>> circuit would have problems with opamps powered by a bipolar supply because
>> the drains of the MOSFETs would go both positive and negative with respect
>> to each one's source.  However, if the opamp was an LM324 (or other single
>> supply opamp) I can see this would work just fine.  Or am I not seeing
>> something (again)?
>
>At first glance you might think so. But what really isn't visible from 
>the schematic is the signal levels. The input stage is configured so 
>that the signals inside the filter are smaller than 600mV peak. The 
>MOSFETs can handle an excursion below the negative supply as long as the 
>internal back diode (D-S) doesn't conduct. Thats the idea here.

OK, I get this, but is the gate effect any different on the D-S conductivity
(or dynamic resistance) when the D-S bias is reveresed?  My guess is no, that
the MOSFETs are somewhat ambidexterous that way.  Interesting that if not for
the static protection diodes, the filter could be operated at a much larger
signal level, which I think would improve noise characteristics.

This goes to helping me with my still to be finished Sallen-Key experiment
because the same thing will be apparent there.


>> If you think this is good for the list, feel free to forward it out there.
>
>Just done so.
>
>Cheers,
>  René
>
>-- 
>uzs159 at uni-bonn.de
>http://www.uni-bonn.de/~uzs159
>
>
>

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