LM13700
The LM13700 in an operational transconductance amplifier, or OTA for short, that is almost identical to the LM13600 (the darlington output buffer is the only difference). The LM13700 is like a standard op-amp, but it is voltage in and current out and is programmable via the IABC pin. 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 OTA chip utilizes a differential input stage and current mirrors to to achieve its unique circuit characteristics.