[sdiy] discrete/no-OTA VCO

[Magnus Danielson]

DAF skrev:
> Hello - can someone point me to a circuit for a  voltage-controlled lfo 
> that uses discrete or other easy to source parts.  I have used an 
> OTA-based design in the past, but I want to design around more generic 
> parts.  I seem to remember a vco that used a discrete JFET?  I don't 
> neet expo response or temp compensation.  I am looking for about a 2 to 
> 10 Hz response range.  I don't want to use any specialty chips or OTA's, 
> just discrete transistors and op-amps.   Any help greatly appreciated.

You can build a simple OTA with 4 transistors, two as a input 
differential pair and one as a current-mirror on top of the collectors 
of the differential pair. You find this in the Buchla 258 oscillator for 
instance:
http://rubidium.dyndns.org/~magnus/synths/companies/buchla/Buchla_2580_1_200.jpg

You can use that transistor setup in place of traditional OTAs. However, 
for a propper dynamic range you would need two additional current 
mirrors to re-create the internal layout of the CA3080 or similar.

But then again, LM13700 and NE5517 is around, but I think just that 
chunk of 4 transistors might be what you need.

Cheers,
Magnus