Opinions on OTA's?

I've seen some audio circuits using OTA's (Operational Transconductance Amplifier), and I've seen a lot of interesting applications such as VCO's, VCA's, voltage controlled filters and so on. It's all interesting, but there seems to be a downside from what I can see and that is THD, most of the circuits I've seen are essentially open loop and use input linearizing diodes which work as a local feedback to linearize the amplifier, input signals are restricted to a couple tens of milivolts if distortion is to be kept low.

Am I missing something? what are your thoughts on OTA's?

user 37518 said:

I've seen some audio circuits using OTA's (Operational Transconductance Amplifier), and I've seen a lot of interesting applications such as VCO's, VCA's, voltage controlled filters and so on. It's all interesting, but there seems to be a downside from what I can see and that is THD, most of the circuits I've seen are essentially open loop and use input linearizing diodes which work as a local feedback to linearize the amplifier, signals are restricted to a couple tens of milivolts if distortion is to be kept low.

Am I missing something? what are your thoughts on OTA's?

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I, and many designers came to the same conclusion. They end up being too noisy for pro audio usage. They are OK for MI, though. There are palliatives, such as running several in parallels or pre distortion, but the meagre gain in performance does not justify the cost and complexity of such arrangemenst.

abbey road d enfer said:

I, and many designers came to the same conclusion. They end up being too noisy for pro audio usage. They are OK for MI, though. There are palliatives, such as running several in parallels or pre distortion, but the meagre gain in performance does not justify the cost and complexity of such arrangemenst.

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I understand, yes I was thinking more on the lines of using it as a VCO or VCF for synth work. For pro audio work, quality aside, it does seem as an easy way to achieve total recall on parameters such as EQ's and Gain stages.

user 37518 said:

I've seen some audio circuits using OTA's (Operational Transconductance Amplifier), and I've seen a lot of interesting applications such as VCO's, VCA's, voltage controlled filters and so on. It's all interesting, but there seems to be a downside from what I can see and that is THD, most of the circuits I've seen are essentially open loop and use input linearizing diodes which work as a local feedback to linearize the amplifier, input signals are restricted to a couple tens of milivolts if distortion is to be kept low.

Am I missing something? what are your thoughts on OTA's?

Click to expand...

yup.. the distortion is a trade off between linearity and noise floor...

OTAs were good technology for several decades ago. I first saw them used in the 70s making companding NR around noisy BBD delay lines, but BBDs are even noisier than OTAs.

There are ways to use the OTA for gain reduction where the OTA is not in the direct path when not limiting.  I made a quad noise gate limiter (LOFT) back in the early 80s with great path performance when not limiting, and while limiting distortion/noise floor is not a paramount concern.

At Peavey the iconic DDT amplifier clip limiting is based on an OTA (again not in the direct path while not limiting). The part we used at Peavey was similar to the old 3080 OTA but a semi-custom house number selected for low control feed through (another problem with OTAs).

Also in the early 80s I made a voltage (actually current) controlled sine wave generator for my LOFTECH TS-1 test set based on a dual transconductance amp configured in a SVF configuration. 

So long answer shorter, modern VCAs (like THAT corp) are so much better that OTAs are not considered for premium audio paths.

JR

PS: If you can live with the poor fidelity go for it... for onesy twosey, designs with trims they should be usable.  I am probably irritating some people but DPOTs and digital control seems far more predictable for recalling parameters.

Thanks for that John, looks like these suckers are good if THD and noise are not paramount. Maybe cheap VCF's or VCO's  and so on. For VCA's, like you mention, THAT corp should be the way to go.