# OTA with linearization diodes

Discussion in 'Electronic Design' started by R. G., Sep 21, 2003.

1. ### R. G.Guest

A small mystery (for me...)
from the following datasheet of the LM13700 at page 7, figure 3:

http://www.national.com/ds.cgi/LM/LM13700.pdf

Using the linearization diodes, input-output equation is:

Io = 2 * Is * Iabc / Id

and I assume:

Is = Vth / Rth

And if Ra << Rin then Rth = Ra
(Rb = Ra in all conditions for the sake of simplicity..)

Let's say Rin = 10K and Ra = 500 ohms as in figure 4.
Now if I reduce the value of Ra, Vth will also be reduced in the same
proportion thus Is remains *unchanged* if I read this correctly.

If so, no matter how low Ra is reduced, I should still get the same output
from the same Vth input signal.
But Vth at the OTA input will be much lower, is it what sets a lower limit
to Ra?

Ron

2. ### Tony WilliamsGuest

I think the thing that limits how low Ra can go is S/N ratio.
At the end of the day, Rin/Ra is a pot-down and is it desirable
to have at least 50 or 100mV pk across Ra.

3. ### R. G.Guest

So I was not so far from the truth after all!

Thanks!
Ron