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

[René Schmitz]

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.

> 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