104. Operational Transconductance Amplifiers

This time we cover the unheralded brother of op amps, the Operational Transconductance Amplifier, or OTA. Using the jellybean of OTA's, the LM13700, we cover the characteristics of an OTA and how to set one up.

[@mortenrobinson5421]

I am so much in awe at the wealth of applications for transconductance amplifiers. I'm very into synthesizers and electronic music equipment. I think it's possible to design an analog synth almost entirely using nothing but lm13700s, which would make for a very interesting and probably a crazy and great sounding DIY project. These chips are absolutely incredible for anything voltage or current controlled. You can use them for designing Voltage Controlled Amplifiers, Voltage Controlled Oscillators, Voltage Controlled Filters. You can even turn them into voltage controlled resistors, so you can basically take any circuit with pot controls and swap the pots with these beasts to make it true voltage or current controlled.

[@meiserhans]

Just came across your channel, great content and awesome way to explain the topics and pointing out the challenges in real life 👍

[@MasterMindmars]

Very good information. I am thinking about to use the Butterworth biquad showed in that datasheet for achieve an 10th order filter, by cascading 5 of that circuit.

[@RvnKnight]

I miss having you as an instructor. Your class was honestly my favorite at CTC.

[@TheOffsetVolt]

Thank you very much. Sadly, it's hard to put a name with the face, especially when it's an enigmatic channel name. Hope things are going well for.

[@RvnKnight]

@@TheOffsetVolt It was years ago, so you probably wouldn't even remember the name. I hope everything has been well for you and yours too.

[@TheOffsetVolt]

@@RvnKnight Email me at theoffsetvolt.gmail.com if you get a chance.

[@RvnKnight]

@@TheOffsetVolt Email sent

[@andymouse]

Wonderful tutorial !...cheers.

[@TheOffsetVolt]

Thank you! Cheers!

[@peterschmidt9497]

Great tutorial, will start breadboarding a State Variable Filter based on your input. Isn't the voltage shift on the output to be expected as the Darlingtons of the buffer introduce two diode drops?

[@HazeAnderson]

Just in time for my brain! :) I recently assembled a compressor guitar pedal that uses a CA3080 OTA. I have one more CA3080, one AS3080 "clone" and a handful of the easier (and cheaper) to obtain LM13700 ICs.

[@TheOffsetVolt]

It's good to feed the brain. Let me know how the pedal turns out!

[@HazeAnderson]

Thanks -- it was a build success! The most recent video on my KZbin channel shows me putting it together in the final stages and performing a little music with it, should you feel so inclined to seek it out. ;)

[@robertogabriel6430]

Hello. How do you selected the R out resistor? (47 K)? You do not told us... Thank You very much for this class!!!

[@alexandrostranoudakis4601]

Hello! Whats the Iout without using the diode? You dont mention that. Thanks!

[@jonathanwogerbauer2703]

I wonder how you get good resulsts when using the diodes. You are calculating I_S by (V_in/R_1), to be precise acutally it would be (V_in/ (R_1 + R_2)) but this is neglegible. However this current is not the current going into the the OTA. Most of it will go straight into ground and only a small percentage will be the real I_S. In your case this would be calculated by a current divider of R_2 and the impedance of pin 14. In the datasheet at the figure "linearizing diodes" you can clearly see that I_S is the current running into the circuit. The thing that is confusing me , is that in the datasheet there is no input impedance or what so ever given. I am not sure how one should calculate I_S. I would be glad if you had any idea.... By the way: you could measure the current running into the OTA. As i understand you should measure something around 11 micro Amps peak to peak! Looking forward to your reply ;)