[sdiy] Bending a Sum

[Phil Macphail]

While it's true that the linearising circuit is acting as a log-amp, the
temperature compensation is due to the two amplifiers being on the same
piece of silicon (and therefore nominally isothermal). If the two circuits
were in different devices the temperature stability would be at best
'variable'.
Unfortunately the same goes for any other attempt to linearise the control
signal with external transistors. The simulation will look good as the
temperature used in the simulation is common to all devices, but that
isn't realistic in practice. The original circuit is pretty hard to beat,
unfortunately, so you are probably better served finding space for another
2164, which isn't much larger than a transistor and a couple of resistors
anyway.


Phil.


On 18/11/2013 20:13, "David G Dixon" <dixon at mail.ubc.ca> wrote:

>Tom, the linearizing part of the Irwin circuit is just a logarithmic
>amplifier, so....  Of course, it also perfectly compensates the TC of the
>2164 cell doing the business.
>
>> -----Original Message-----
>> From: synth-diy-bounces at dropmix.xs4all.nl
>> [mailto:synth-diy-bounces at dropmix.xs4all.nl] On Behalf Of Tom Bugs
>> Sent: Monday, November 18, 2013 5:30 AM
>> To: Synth-DIY DIY
>> Subject: [sdiy] Bending a Sum
>> 
>> Just wondered if anyone may have a brainwave on this little problem...
>> 
>> It is around the area of linearizing the response of an
>> SSM2164 - but, at the moment (blah blah) I wanted to see if I
>> can achieve a rough linearization WITHOUT using the well
>> documented Irwin EDM technique.
>> 
>> http://bugbrand.co.uk/images/SumBend.jpg
>> 
>> I just have a simple opamp sum stage to generate 5V to 0V
>> from a 0V to 10V control - that's the green line.
>> The standard linearization technique achieves the blue line.
>> How could I bend green to blue without too much extra
>> complexity and, crucially, without using another 2164 cell?
>> 
>> Thanks!Tom
>> 
>> --
>> BugBrand LTD
>> UK company No. 07199808
>> VAT No. GB 988 2629 57
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>> 
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