LM13700

Schematic symbol for the OTA — Like the standard operational amplifier, it has both inverting (−) and noninverting (+) inputs; power supply lines (V+ and V−); and a single output. Unlike the traditional op-amp, it has two additional biasing inputs, I_abc and I_bias

The LM13700 series consists of two current controlled operational transconductance amplifiers (OTA) each having differential inputs and a push-pull output. The LM13700 is like a standard op-amp: both have a pair of differential inputs and a single output, but an OTA is voltage in and current out rather than voltage in and voltage out; and OTAs are programmable via the IABC pin. Linearizing diodes at the input reduce distortion and allow increased input levels. The darlington outout buffers provided are specifically designed to complement the wide dynamic range of the OTA. This chip is very useful in audio electronics especially in analog synthesizer circuits like voltage controlled oscillators, voltage controlled filters, and voltage controlled amplifiers. The darlington output buffers on the LM13700 are different from those on the LM13600 in that their bias currents (and hence their output DC levels) are independent of IABC pin. This usually results in performance superior to that of the LM13600 in audio applications.

The LM13600/LM13700 actually started out as a practice analog integrated circuit layout for a novice mask designer. During the late 70s, electronic organ companies were all analog. The CA3080 had just come out and was being designed into controlling the attack and decay of audio signals. Customer response was " This is great, gives us more at a lower price." National Semiconductor had just developed the 16 pin DIP plastic package at this time, and so the 5 minute circuit design revolved around laying out a dual 3080 and including output darlingtons buffers to save the customer some external transistors.This left two pins free.