[sdiy] discrete/no-OTA VCO

[Veronica Merryfield]

On 28-Dec-08, at 2:08 PM, Magnus Danielson wrote:

> 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.

The circuit at http://www.mikrocontroller.net/attachment/13360/ca3080.pdf 
  does not show additional current mirrors but does show an  
interesting output drive. The long tail pair and bias could be used as  
a starting point for a 3 transistor OTA. With resistors in the  
collector lines, this could be followed with an op-amp.

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

Veronica