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I pulled some LM13700 OTAs from an old board and tested them. All voltages, currents, and resistances were measured. I had the expected (approximate) voltage gain using the equation \$ A=(R_2/(R_1+R_2 ))*g_m*R_L \$ where gm equals \$ 19.2*Iabc \$.

The problem begins when I use the linearizing diode on pins 2 or 15. The gain goes down substantially, the higher the current on these pins, the lower the output. I have used Marston’s equation from the old Nuts and Volts article, \$ I_{out}=2*I_S*(I_{bias}/I_D ) \$ but the result are way off.

Does anyone know of a better way to calculate gain in an LM13700 when the linearizing diodes are used?

UPDATE - I got a couple of new LM13700s and got the same results.

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  • \$\begingroup\$ If they are old, you may have to consider that they are damaged. \$\endgroup\$
    – a concerned citizen
    Commented May 12, 2021 at 21:26
  • \$\begingroup\$ It might pay off to conduct some tests, yourself, plot some graphs, then see if you can modify or make your own formula. Looking at its internal schematic, those diodes provide a way to over-bias the inputs. So if you increase the current, the differential input might be driven into saturation, therefore the gain decreases. \$\endgroup\$
    – a concerned citizen
    Commented May 13, 2021 at 22:22

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