[sdiy] discrete/no-OTA VCO

[DAF]

thanks, I will check those out - but wasn't there a simpler one, if I am 
remembering correctly, using a JFET as the transconductance element?  Also I 
only really need triangle output waveforms.

thanks,
Dave


----- Original Message ----- 
From: "Veronica Merryfield" <veronica.merryfield at shaw.ca>
To: "synth DIY" <synth-diy at dropmix.xs4all.nl>
Sent: Sunday, December 28, 2008 6:12 PM
Subject: Re: [sdiy] discrete/no-OTA VCO


> 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
>
>
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