Re: OTA revisited


"joe" <volt.wide.web@xxxxxxx> wrote in message
news:1138150000.899612.12850@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
> hi Danny
>
> Yes, I know about these linearization technique which was part of RCA
> CA3280 dual OTA
> which I designed into some audio gain-control-circuitry in the early
> 80s of the last century.
> At that time, there existed some fine application notes from RCA and it
> might be a good idea to search them, perhaps you are lucky and some
> people have scanned them in.
> Nowadays I use CA3080 without linearization to achieve a perfectly
> smooth
> distorting guitar PreAmp. Anyway, you might put your question and I can
> try to answer them.
>
> Joe

I'm just concerned about the input current (Is) calculation to the amplifier
input as well as the peak-to-peak voltage at the same input .
Correct me if I'm wrong:

With current (Id) injected into the diodes, data*** says the amplifier now
behaves as a current amplifier because the transfert function is:
Iout = Is * 2 * Iabc / Id where Is is the input current into OTA
input.

Data*** says Is should not exceed Id/2. I use Thevenin to find out Is and,
once simplified, the final equation I use is:
Is = Vin / Rin where Rin is the limiting resistor betwwen the OTA
input and the voltage source.

I pick Vin peak value to find Is peak value. I assume it is the peak value
that is of concern. For example, if Vin=1Vp-p and Rin=10K, then Is peak =
0.5V / 10K or 50uA.
There is also a very small value resistor called Rp shunting the input to
ground and it does not seem to do anything in that calculation. The value of
Is seems to be related to Rin only. According to Thevenin...

Even though they describe the OTA as an current amplifier when diodes are
used, it seems to me that we still have to limit the input voltage. No
(apparent) word about this in data***. I assume this because of a small
graph showing harmonic distorsion with and without diodes, related to
peak-to-peak input voltage.

My final assumption is that Rin must meet two conditions:
1) limit Is below Id/2
2) limit peak-to_peak input voltage lower than 100mVpp (with the help of Rp
to do a voltage division)

Hope my explanation is coherent and correct...! :)

Danny


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