TIM distortion and slew rate in IV opamp

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#22
I think the rationale went something like, "It's beyond linear, well compensated, multi-tanh is highly effective before adding global feedback, and really really fast".

Might have been primarily the advantage of the multi-tanh compensation of the input pair.
 
#23
Multitanh would certainly help, but how do you know it has a multitanh input stage? I can't find anything about it in the datasheet and the ratio of slew rate to gain bandwidth product is close to the 4 pi kT/q expected with a plain old bipolar differential pair.
 
#24
MarcelvdG said:
Any op-amp of which the input stage is a bipolar differential pair without emitter degeneration can only handle a couple of millivolts more or less linearly, say 10 mV or so.
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How can this value be visible in datasheet? How much can handle AD828 (which has open loop output impedance of 8 ohm and a degenerated NPN differential pair at its input)?
 
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#25
MarcelvdG said:
Multitanh would certainly help, but how do you know it has a multitanh input stage? I can't find anything about it in the datasheet and the ratio of slew rate to gain bandwidth product is close to the 4 pi kT/q expected with a plain old bipolar differential pair.
Click to expand...

It was a tip from Scott, I think. The ADA4899 might use it also, I forget, or at least something similar.

I'd be interested if someone could measure an impact in the audio band. There are plenty of DACs out there with NE5532s in this position.
 
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#27
MarcelvdG said:
Multitanh would certainly help, but how do you know it has a multitanh input stage? I can't find anything about it in the datasheet and the ratio of slew rate to gain bandwidth product is close to the 4 pi kT/q expected with a plain old bipolar differential pair.
Click to expand...

https://www.diyaudio.com/forums/sol...p-amps-linear-barrie-gilbert.html#post2622071

I apologize as I both haven't *done* the critical analysis JCX has, nor am I sure I would be the best (or even a decent) party *to* do such an analysis. I may have an EE background by degree, but I'm more of a confused materials scientist. 🙂

Edit: I went through to JCX's link, which was partially broken by the forum's new way of referencing posts, here it is: John Curl's Blowtorch preamplifier
 
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#28
MarcelvdG said:
Assuming a traditional op-amp with high open-loop input impedance on both inputs (that is, not a so-called CFB op-amp):

Very often the feedback network around the op-amp consists of a resistor and a capacitor in parallel, so the transimpedance amplifier (or current to voltage converter, whatever you like to call it) makes the first pole of the analogue reconstruction filter.

With such a circuit, after each step of the DAC output current, the error voltage across the op-amp's input jumps from a near-zero value to the magnitude of the current step multiplied by the open-loop output impedance of the op-amp. Then the feedback kicks in and reduces the error voltage to a near-zero value again.

In this case, you have to ensure that the maximum DAC current step multiplied by the op-amp's open-loop output impedance is smaller than the maximum voltage the op-amp's input stage can handle without significant distortion. Any op-amp of which the input stage is a bipolar differential pair without emitter degeneration can only handle a couple of millivolts more or less linearly, say 10 mV or so. It can be much larger for FET op-amps, especially those that have a high ratio of slew rate to gain-bandwidth product.
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I received some private questions related to this post that I could not answer in plain text, so I wrote a little document about it. It is attached just in case anyone else finds it useful. It's just two pages with basic op-amp theory.
 

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