I recently purchased several OTAs (Lm13700) but have had quite a bit of trouble using them. Going through the TI datasheet and a Nuts & Volts tutorial it's occured to me that transconductance amps don't really function in the same manner as "normal" op amps. Excluding the obvious currrent output in place of a voltage output the input configurations for many of the example circuits I've seen don't seem to follow normal op amp rules. Most pointedly, while I understand that hysteresis isn't a problem thanks to controllable gain I don't understand why so many circuits feature resistors attached to both input pins. It would seem to me that to achieve differential voltage gain one simple had to apply a voltage to the non-inverting input (perhaps with a voltage divider) and tie the other to ground. However, I tried this on a breadboard without success and only one circuit I found actually suggested this. Would someone mind explaning why OTA amplifier circuits differ so radically from ordinary op amps?
Operational Transconductance Amplifier Input Configurations
amplifiercurrentdifferentialimpedanceinput
Related Topic
- Electronic – When should I be considering an operational transconductance amplifier (OTA)
- Electrical – Designing Common Source Amplifier with NMOS transistors
- Electronic – Understanding Operational Transconductance Amplifiers
- Electronic – Determining Impedances of an Op-Amp Circuit
- Electronic – Phase shift near saturation mitigation in input stage amplifier — How does this work, why does this work
Best Answer
I didn't really want to provide this as an answer, but haven't figured out on the iPad app how to add it as a comment to your question. I am learning about OTAs right now and only recently within the past few months got the hang of regular op amps. Here is a YouTube video that I found helpful regarding the OTA and specifically how it differs from op amps. I've watched this video a bunch of times now and between that, playing around with the circuits on the data sheet, and reading what I can on the chip, I am getting the hang of basic functionality as a current controlled amplifier.
https://youtu.be/I8v0VGsxElA
As I understand it, here are a couple things that make the OTA different from a traditional op amp.
Another resource I would recommend is Ray Wilson's book on DIY analog synthesis, "Make: Analog Synthesizers". There is a section dedicated to the LM13700 that is very approachable and has practical circuits for anyone interested in building their own synth. I found it a lot easier to read than the Nuts and Volts magazine article on the LM13700 - which is still not bad considering that it pretty much goes through the datasheet schematics in more detail than the data sheet does.
I like the design where you can set the gain from a pin, but I need to better understand voltage to current conversions, and current sources before really using it for analog applications.
I wish you good luck with the OTA!