At 09:01 PM 3/7/2010, you wrote:
> > I'd just use a VCO made with op-amps. ...
>
>That's what I had in mind when I asked about frequency etc.
>The attached circuit is copied from an LM324 data sheet.
>(Page 13 here http://www.national.com/ds/LM/LM124.pdf )
>
>LM324 is getting rather tired at 3 Volts but some other opamps would 
>do better.
>
>Parts count is far higher than using a more dedicated IC but cost
>could be good and footprint is probably OK.

Precisely-- a dual op-amp, and (maybe) replace the BJT with a MOSFET
(though Vce(sat) on a cheap BJT in this application might only be 10mV.

>Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com



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