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

[rsdio at audiobanshee.com]

Thanks, Neil.

Members of the AES can access the JAES Volume 39 Issue 10 that you referenced.

Good reading.

Brian


On May 11, 2018, at 2:57 PM, Neil Johnson <neil.johnson71 at gmail.com> wrote:
> David,
> 
>> 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.
> 
> There are many ways of compensating op-amps, not just strapping a
> capacitor across the feedback resistor ("feedback lead compensation").
> 
> In the context of the 2164, the front end _is_ very much like an
> op-amp.  Or, as Doug Frey (inventor of the OVCE, of which the 2164
> contains four of them) wrote:
> 
> "The OVCE structure may be described conveniently
> with the help of Fig. 3. This structure consists of a
> high-gain differential amplifier whose front end re-
> sembles that of a standard operational amplifier, but
> whose output is a current compatible with the gain
> cell."
> The Operational Voltage-Controlled Element: Generalizing the VCA
> JAES Volume 39 Issue 10 pp. 775-784; October 1991
> 
> In the case of the 2164, the frequency of interest that we're
> concerned about is around 5.8MHz.  To give enough room for at least 45
> degrees of phase margin we drop down by a decade, to around 580kHz,
> for applying compensation.  That explains the ratio of the
> compensation components.  The resistor reduces the gain and thus
> shifts the Bode plot down to give us phase margin and hence stability,
> and because the gain also depends on the input resistor, as it is part
> of the feedback network, then the compensation resistor must be
> adjusted as the input resistor is changed.  The capacitor is there to
> only bring in the compensation when it is needed - around 580kHz.
> 
> (Note: I'm hoping for some feedback on my analysis from the original
> designer of the SSM2164).
> 
>> 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.
> 
> Gain alone is not a problem.  Gain and phase combined is the problem.
> 
> Chapter 6, in Design with Operational Amplifiers and Analog Integrated
> Circuits, Franco, McGraw-Hill.
> Or if you don't have that, here's a TI paper on the subject:
> http://www.ti.com/lit/an/snoa486b/snoa486b.pdf
> 
>> Concerning the textbooks: I don't believe that I have ever seen the 2164
>> mentioned in any textbook.
> 
> It is unlikely that you will find the 2164 in a text book.  There are
> some references on earlier versions of the OVCE (SSM2014, SSM2018),
> for example:
> http://www.gyraf.dk/schematics/VCAs_Ben_Duncan.pdf
> 
> There is also some discussion on VCAs in Handbook for Sound Engineers
> (Glen Ballou, Focal Press) although the section on VCAs is about as
> comprehensive as reading the datasheets and app.notes for the various
> devices discussed.
> 
> If you really want to understand how the 2164 works you need to read
> up about the OVCE - although it is mostly described in Frey's papers
> published by the AES.  Your university library might be able to help
> you there.
> 
>> The only source of information I have ever seen
>> on the performance of the 2164 is the datasheet, which is woefully
>> inadequate.
> 
> The SSM2164 datasheet is not that bad.  I've seen a lot worse.
> 
>> 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.
> 
> Then I suggest you read a book on how op-amps really work (down to the
> transistor level), then correlate that with the schematic of the 2164
> and then start drawing Bode plots to work out gain and phase margins.
> 
>> 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.
> 
> *rolleyes*
> 
> Cheers
> Neil
> -- 
> http://www.njohnson.co.uk