[sdiy] Waveform/source mixers in classic polysynths

[Neil Johnson]

Hi

On 6 October 2017 at 13:32,  <mskala at ansuz.sooke.bc.ca> wrote:
> On Fri, 6 Oct 2017, mark verbos wrote:
>> > conversion is done by the common base transistor feeding the iABC pin
>> > of the OTA.  The common base configuration also happens to have a high
>> > output impedance which is precisely what a current source is.
>>
>>
>> Isn’t that a linear current source?
>
> Linear in applied current if you feed it with a current source, but
> exponential (inverse log) in applied voltage if you feed it with a
> voltage.

Having just run up the CA3080 driven by common base transistor circuit
used in the Oberheim SEM
page 7 here:
http://www.synthfool.com/docs/Oberheim/Oberheim_SEM1A/Oberheim_SEM_1A_Schematics.pdf

in LTSpice I can see it is pretty linear once you get over the Vbe forward drop.

Neil
-- 
http://www.njohnson.co.uk