I'm using the LM13700 in an automatic gain control configuration to adjust the volume of a speaker based on the audio level perceived by the microphone. Both amplifiers on the chip are used: one to handle the incoming microphone signal, and the other to adjust the gain of the speaker accordingly. Most of the examples I've seen (datasheet, etc) show this configuration with only one OTA, but I'm using both since I thought it was a convenient way to perform AGC on the mic, while using that output current to modify the gain of the speaker (Notice Idiode is controlling both diode pins). The effect is such that the first output voltage will increase at a large signal compression (~20:1) with an increase in the input voltage (ie mic volume), while the speaker voltage will decrease by a proportionate amount. I'm using a Teensy 3.2's DAC to generate the input to the second OTA, however this leads to pulsing and instability in the output waveform, even with a unity gain buffer between the Teensy and the remaining circuit. I've verified that replacing the Teensy with a function generator leads to a very clean signal, so it seems like the issue lies with the microcontroller: my question is why? I plan to replace the Teensy's DAC with an external one with the hope that the external chip will be more isolated, but I'd still like to know what's happening here, and if there's a fairly simple solution.
Please let me know if I've left out any important information, I've been staring at this screen for much too long. If you're confused why I've taken such a convoluted approach to automatic gain control, it's mainly because I don't know a better way, but also because I'm trying to reverse engineer a patent from the 70s.
