[sdiy] LM13700 as VC resistor

[Didier Leplae]

BTW, the SNICSter looks super interesting!

> On Feb 8, 2020, at 9:19 PM, Ian Fritz <ijfritz at comcast.net> wrote:
> 
> 
> It looks like the reply I sent earlier didn’t make it through. I used an OTA as a VC resistor a while back. Didn’t have any trouble with it, although IIRC the range was a bit limited. Certainly did not need AC coupling. Does anyone else have actual experience in incorporating one in a design?
> http://ijfritz.byethost4.com/sy_cir10.htm
> 
> Ian
> 
> 
>> On Feb 8, 2020, at 6:25 PM, Didier Leplae via Synth-diy <synth-diy at synth-diy.org> wrote:
>> 
>> René,
>> Sorry for my ignorance, but when you say “ Use Iabc to get gm (19.2*Iabc = gm), and the rest should be easy.”
>> 
>> What is “labc”?
>> 
>>>> On Feb 8, 2020, at 11:51 AM, René Schmitz <synth at schmitzbits.de> wrote:
>>> 
>>> Hi Didier and all,
>>> 
>>>> On 08.02.2020 10:58, Didier Leplae via Synth-diy wrote:
>>>> I am curious to know anyone’s opinions on using the LM13700 as a voltage controlled resistor.
>>>> The LM13700 datasheet has a couple schematics which I have tried building, with mixed results. I’m having a hard time knowing what sort of resistance I am getting as the multimeter gives strange and inconsistent readings.
>>>> The information on the datasheet is somewhat incomplete in my opinion and it is not easy to find much info online either.
>>> 
>>> Yes, it doesn't tell you the fact that the resistor is AC coupled (single ended circuit). See the Cap at the output...
>>> So poking around with a multimeter is not going to give meaningful results.
>>> But the formula for Rx is given. Use Iabc to get gm (19.2*Iabc = gm), and the rest should be easy.
>>> 
>>> 
>>>> It doesn’t seem like it is common practice to use these as VC resistors. Is that because it doesn’t work very well? Or use too many parts?
>>> 
>>> Wouldn't be my first choice due to complexity and parts count.
>>> 
>>> Best,
>>> René
>>> 
>>> --
>>> synth at schmitzbits.de
>>> http://schmitzbits.de
>> 
>> 
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