Gain Calculation for LM13700 When Using Linearizing Diodes

[Scr1bbl3]

Hello everyone,

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=(R2/(R1+R2))∗gm∗RL 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 (lower resistor values) on these pins, the lower the output. I have used Marston’s equation from the old Nuts and Volts article, Iout=2∗IS∗(Ibias/ID) 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 just received some new LM13700's and got the same results.

[Terry Bites]

see page 5-6
https://www.google.co.uk/url?sa=t&rct=j&q=&esrc=s&source=web&cd=&ved=2ahUKEwiBmOe8gc7wAhVk8-AKHRGTAFsQFjAPegQIERAD&url=https%3A%2F%2Fwww.experimentalistsanonymous.com%2Fdiy%2FDatasheets%2FLM13700.pdf&usg=AOvVaw3UAxJCx4wz4VvVYgwSiDlS