[sdiy] Query about best cap type for '2164 stability network

[David G Dixon]

Neil,

We're not talking about opamp compensation (where one typically installs a
small capacitor between the output and the virtual ground), but the 2164,
where one installs a resistor and capacitor from the virtual ground to
ground.

My understanding of the stability issues of opamps is that they stem from
the very high loop gains and the use of feedback networks.  These things
don't seem to apply to the 2164.  Are you saying that the issues are the
same?  If so, then please elaborate.

Concerning the textbooks: I don't believe that I have ever seen the 2164
mentioned in any textbook.  The only source of information I have ever seen
on the performance of the 2164 is the datasheet, which is woefully
inadequate.  I'd be the first one to admit that, when I look at the internal
structure of the 2164, I'm not enough of an electrical engineer to
understand how it might suffer from the same stability issues as an opamp.
If you really wish to be helpful, please suggest a reference (title, author,
page numbers) that will help me understand precisely what the stability
issues are in the 2164 VCA.



> -----Original Message-----
> From: Neil Johnson [mailto:neil.johnson71 at gmail.com] 
> Sent: Sunday, May 06, 2018 3:48 PM
> To: David G Dixon
> Cc: Guy McCusker; *SYNTH DIY
> Subject: Re: [sdiy] Query about best cap type for '2164 
> stability network
> 
> Hi,
> 
> David G Dixon wrote:
> > What I'd like to know is what this capacitor is actually doing, or 
> > under exactly what circumstances it does anything at all.
> 
> To understand what the RC circuit is doing you first need to 
> understand:
> 1/ what the difference is between ground and virtual ground 
> (they're not the same) 2/ what an op-amp's dominant pole is 
> (that's the problem we're trying to fix) 3/ datasheets are 
> marketing documents, so whatever is in them is there to show 
> off the product in the best way possible, not to trick you
> 
> If you could list some of the electronics text books you have 
> then we might be able to point you in the right direction.
> I would recommend Franco's book on opamps:
> https://www.amazon.com/Design-Operational-Amplifiers-Integrate
d-Circuits/dp/0072320842
> 
> I think for values of source resistance greater than 30k you 
> can stick with the values per the datasheet.  It is only when 
> the input resistance goes below 30k that you need to rejig 
> the compensator as you start pushing the input stage harder 
> with more current and so compensation becomes more critical.
> 
> Cheers,
> Neil
> --
> http://www.njohnson.co.uk