[sdiy] expo converter

[Scott Bernardi]

Doh! You're right - especially when you get down to trying to measure
30nA or so. That 470k resistor seems rather large, and limits the upper
range of the VCO to about 10kHz, if I'm figuring correctly. (~33uA/330pF
= 100,000v/sec, 10v/100kv/s = .0001sec or 10kHz). At 30nA of charging
current, the freq will be 10Hz.
I think the VCO could have a wider range (ie, from subaudio thru audio)
by increasing the charging cap to 3300pF, and changing the 470K to 22K.
A max charging current of 500-600uA for the top freq range is OK, and
you can get much lower freq response (down to 0.1Hz at ~3nA charging
current).
Any comments, TomG? This is your circuit.



harry wrote:
> 
> Hi Scott...
> 
> The max current is about 30uA... he better have a damn good ampmeter !!!  ;^)
> 
> H^) harry
> 
> Scott Bernardi wrote:
> 
> > Your attached schematic has the expo converter driving an OTA through a
> > 470K resistor. The current input of an OTA just looks like a couple of
> > diode drops above the V- supply, so just run your 2N3906 collector to
> > 470k into two diodes to V-. You can put an ammeter in series, or measure
> > the voltage across the 470k and use ohms law to get the current.
> >
> > Benjamin Smith wrote:
> > >
> > > Whilst constructing VCO5 B from Tom G's cookbook, encountered several
> > > problems, hence I have isolated the exponential converter and constructed it
> > > seperatley.
> > >
> > > How would I go about loading the output to test if it works or not?
> > >
> > > Thank you,
> > >
> > > Ben
> > > _________________________________________________________________________
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> > >
> > >   ------------------------------------------------------------------------
> > >                    Name: vco5b_sd.gif
> > >    vco5b_sd.gif    Type: GIF Image (image/gif)
> > >                Encoding: base64
> >
> > --
> > Scott Bernardi
> > sbernardi at home.net

-- 
Scott Bernardi
sbernardi at home.net