The LM13700 (co-invented by Bill Gross and Don Sauer) 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: each has 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 output 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.
Darlington pair buffers
Pins 7 and 8 are the inputs to a pair of Darlington pair buffers. These have nowhere as high an input impedance as a JFET input op-amp. When an external op-amp is used instead, the unused buffer pins can safely be left floating.
Equivalents and clones
- Make: Analog Synthesizers by Ray Wilson, Maker Media, 2013, ISBN 1449345220, Appendix B. The LM13700
- The LM13600/LM13700 Story
- A more accurate LM13700 SPICE subcircuit
- AS13704, LM13700 functional replacement (without output buffers) by Alfa Rpar AS in SSOP24 package.
- LM13700 - new discovery (at least for me), Mod Wiggler forum, Aug 2012, discusses the data sheet is easily misunderstood and why external op-amps are usually preferred over the internal buffers.