[sdiy] VCS3 oscillators

[James Patchell]

Yes, they definitely are...I was just making sure the original poster was 
aware of that fact...(which he was).

I wish we could talk THAT into making a clone of the CA3280 with vertical 
PNPs....nice fantasy I guess...

At 06:53 AM 10/2/2004 -0700, Scott Bernardi wrote:
>I'm pretty sure all the OTA chips are made with lateral pnps, although 
>they may be large ones optimized for current handling capability.  I'm 
>sure the 3080 uses a standard analog (i.e. opamp) manufacturing process.
>The Beta of lateral pnp transistors drops the more current you put through 
>them. The Wilson current mirror configuration used in the OTAs has a built 
>in mismatch of approximately 1/Beta^2.  So the mismatch is going to get 
>worse when you are running higher control currents - one of the causes of 
>control voltage feedthrough. And Beta  is _very_ process dependent. And 
>that's why you would have to go through and "cherry pick" 3080's for 
>minimum control feedthrough.
>My guess is that they learned from the experience and really put some 
>effort into optimizing the matching and current handling capability of the 
>pnps in the CA3280, which is why it has superior control feedthrough specs.
>The best conceivable OTA you could make would be with dielectrically 
>isolated pnps like in the THAT transistor arrays - where the pnp 
>transistors have specs comparable to the npns.
>
>James Patchell wrote:
>
>>You will need to be careful with the CA3096.  The PNP transistors are 
>>lateral PNP's and have very poor characteristics (very low Beta, for 
>>example).  You may not find that the circuit functions the same as it 
>>would with discrete parts.  One way to solve this would be to use the 
>>THAT340 transistor array which has two PNP and two NPN transistors, and 
>>they are all vertical BJTs...very good performance.
>>
>>But, the THAT340 is pricey.  (~$4).
>>
>>At 11:25 AM 9/30/2004 +0200, Yves Usson wrote:
>>
>>>Hello (2nd post)
>>>
>>>My questions are aimed at those among you who built a VCS3 or SynthiA clone
>>>or who are lucky enough to own one of these beasts.
>>>
>>>Does the shape control of osc2 and osc3 acts on the shape of the ramp
>>>signal or does it act only on the pulse shape ?
>>>
>>>What I understood from the schematic of the oscillator 1 (that can
>>>be found on internet) and from the reading of Jurgen Haible's messages
>>>posted some years ago on this list, the shape control of osc1 only
>>>changes the shape of the sine signal and not of the ramp signal.
>>>
>>>I want to build a clone of the VCS3 and I intend to use CA3046 and CA3096
>>>transistor arrays in order to reduce the number of parts and simplify the
>>>design of the PCB.
>>>Do you consider this would be a complete heresy or an acceptable compromise
>>>that would preserve the original sound ?
>>>
>>>Thanks for your comments.
>>>
>>>Yves
>>>
>>>Yves Usson
>>>email : yves.usson at imag.fr
>>>http://www-timc.imag.fr/Yves.Usson/personnel/SDIY
>>
>>
>>         -Jim
>>***************************************************************
>>http://www.oldcrows.net/~patchell
>>
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>>
>>
>

         -Jim
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http://www.oldcrows.net/~patchell

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