[sdiy] OTA Iabc feeding

[Michael E Caloroso]

The common base configuration has a high output impedance, which is
exactly what a current source is.

MC

On 9/2/17, sleepy_dog at gmx.de <sleepy_dog at gmx.de> wrote:
>
>  >  Thomas Henry's book "Making Music with the 3080 OTA"
>
> Argh! I bought that book years ago, then had an electronics fiddling
> hiatus, and completely forgot about it.
> I better have a look at that then, thanks for reminding me!
>
> Guy McCusker wrote:
>> There's a decent discussion of various control circuits for the Iabc
>> in Thomas Henry's book "Making Music with the 3080 OTA". At least, it
>> seemed decent to my beginner's mind.
>>
>> On Sat, Sep 2, 2017 at 9:56 AM, Olivier Gillet <ol.gillet at gmail.com>
>> wrote:
>>> The voltage at the Iabc pin is V- + 2 diode drops.
>>>
>>> So if you want to only use a resistor between your CV source and the
>>> Iabc pin, your CV source must be offset so that its minimum level (the
>>> value that will, for example, completely shut off your VCA) is exactly
>>> V- + 2 diode drops. That's not a very convenient voltage to deal with!
>>>
>>> It's more convenient to provide a current to the Iabc pin with a
>>> current source circuit - a circuit that is able to source a current
>>> proportional to the voltage at its input, irrespectively of the
>>> precise value of the voltage at the Iabc pin.
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