Op Amps: The Operational Transconductance Amplifier (OTA)

In this video we examine the operational transconductance amplifier, or OTA. One example is the LM13700, a dual OTA in a single package. OTAs are useful in the design of voltage controlled amplifiers and filters, among other applications.
References: Operational Amplifiers: Theory and Application, Chapter 6 section 7.
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[@oldblokeh]

Thanks for this. I never really understood OTAs before.

[@andymouse]

Fascinating device and a great tutorial....cheers.

[@danieleden1856]

Great video

[@JurekPrzezdziecki]

Great video and another one interesting to watch. Please analyse a LM3900 Norton amplifier which is quite uncommon these days.

[@ElectronicswithProfessorFiore]

Ah, the old 3900... quite a unique device that was very popular among hobbyists back in the 70s and 80s. A weird beast. I must say that I never liked them for a variety of reasons. Might be an useful video for someone repairing an old design, though.

[@JurekPrzezdziecki]

I would love to know those reasons 😉

[@ElectronicswithProfessorFiore]

@@JurekPrzezdzieckiReasons why I didn't like them? First, the slew rate wasn't particularly good. Second, the slew rate was asymmetrical (different for positive slope vs. negative slope, and by an order of magnitude). Third, it did not have a high Zin (actually, it was asymmetrical as the circuit used a current mirror instead of a diff amp for the first stage). Fourth, biasing was non-standard. Fifth, the Zout was rather high. There are other issues, but as an audio amplifier or precision amplifier, it had serious issues. I think the popularity came from the fact that they were very inexpensive and could run on a single polarity power supply. If you just needed some manner of amplifier to trigger a circuit or some such, I guess they were fine. Not what I was interested in, though.

[@mahmouda1920]

Thank you

[@burakg7246]

What is the purpose of the potentiometer at the minus input? And how does this result in a differential input when one end of the potentiometer is connected to the AC input signal?

[@ElectronicswithProfessorFiore]

That's there for DC balance. It is adjusted so that the DC output is zero. One issue with a circuit like this is that, like the desired input signal, DC offset will also be controlled by the control voltage. The end result is that the control voltage appears to "leak" into the output. This is minimized by nulling out the DC offset. You can see the effect of this by building the circuit in the simulator and adjusting the pot to a value that will make the balance worse (like, say 70% instead of 50%). Examination of the load voltage will show the expected output now riding on what appears to be the control (or modulating) signal. Normally, not a situation you want.

[@Hellcommander245]

Is it practical to build one of these with discrete devices?

[@ElectronicswithProfessorFiore]

Many modern analog circuits rely on things like current mirrors, which in turn, rely on close part matching for highest performance. That's relatively easy to do on an IC but a royal pain for discrete.