[sdiy] exponential vs linear vco cores

[Scott Nordlund]

http://www.art-sci.udel.edu/ghw/phys245/05S/classes/images/opamp-integrator.gif



This is the integrator circuit that makes up the "heart" of every VCO
that I know of.  Of course this can also be realized with discrete
transistors.



The op amp's inverting input acts as a "virtual ground", so the circuit
can be fed either a voltage (with the resistor in the diagram) or a
current (omitting the resistor).  Since the exponential part 
outputs a current (you're basically getting I = exp(V) from a PN junction), an exponential VCO will usually put a current straight into the op amp, while a linear VCO will use a voltage from the mixed CV inputs.  It's really still a current going in, in any case.

The negative feedback of the op amp forces the input current through the capacitor, and from the relation I = C*dv/dt, the output voltage is the (inverted) integral of the input current.

I guess some variant would be possible using inductor's V = L*di/dt relation, but this doesn't strike me as a fantastic idea for a number of reasons... it might need a spark gap or something.


----------------------------------------
> Date: Fri, 26 Feb 2010 21:24:04 +0100
> Subject: Re: [sdiy] exponential vs linear vco cores
> From: cheater00 at gmail.com
> To: jays at aracnet.com
> CC: synth-diy at dropmix.xs4all.nl
>
> Jays and David,
> sorry to hijack your SSM2164 thread :P but I have a follow up question
> to the topic of I/V stage designs.
>
> Is the exponentialization always integrated with I/V or does it
> sometimes happen purely in the voltage domain or alternatively purely
> in the current domain?
>
> Aaron,
> thanks a lot for the link. I can't have a look right now being on
> GPRS, but as soon as I have some sort of useful internet connection
> I'll make sure to dig up the link.
>
> Cheers
> D.
>
> On Fri, Feb 26, 2010 at 21:06,  wrote:
>> David,
>>
>> Several quesions if you don't mind. Thanks!
>>
>> If I remember right you need to get a magic voltage good to 3 digits (down to mV) for your expo to work right.
>>
>> Is this voltage used for the expo converter or the temp comp for the expo converter?
>>
>> How hard is it to get that tweaked in for proper operation with cheap test equipment (in this case for me cheap means < $50 DVM)?
>>
>> Thanks
>> Jay S.
>>
>> David G. Dixon wrote:
>>
>>>
>>>> Tempco class discussion based on Rene's website, with a side note that if
>>>> you really want to go down the rabbit hole, you should check out Ian
>>>> Fritz's website.
>>>
>>> Didn't want to go down the hole, or even near the hole.  That's why the
>>> 2164-based approach was so appealing to me.  No gluing of expensive and hard
>>> to find parts (tempcos) to other expensive and hard to find parts
>>> (super-matched pairs)!
>>>
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