Author Topic: Current-feedback vs Voltage-Feedback  (Read 357 times)

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Offline LoveLaika

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Current-feedback vs Voltage-Feedback
« on: October 19, 2021, 10:15:40 pm »
In a previous design of mine, I used the TI LM7171 voltage feedback op-amp in an inverting summing circuit, taking two inputs and adding them up and inverting them. I'm revisiting the design with different requirements, so I'm looking at different topologies as a result. I was wondering, when is it recommended to use a current-feedback op-amp over a voltage-feedback op-amp for a circuit? I see that CFAs have high slew-rates and wide bandwidths, and I read about how their BW isn't dependent on gain (compared to the VFAs and the GBP), but as a result, you have more noise with CFAs. Is there a way to reduce the noise with a CFA so you get the benefit of a CFA with less noise like a VFA?
 

Offline magic

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Re: Current-feedback vs Voltage-Feedback
« Reply #1 on: October 20, 2021, 06:21:39 am »
Ahem, there is a subtle difference between voltage feedback opamps and "voltage feedback opamps". For whatever reasons, which you would have to ask marketroids about, if those words are spelled out explicitly in the datasheet it usually means a current feedback amplifier with built-in unity gain buffer at IN-.

So you have already used a CFA as far as noise is concerned, but without the GBW benefits ;)
« Last Edit: October 20, 2021, 06:25:44 am by magic »
 


Offline magic

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Re: Current-feedback vs Voltage-Feedback
« Reply #3 on: October 20, 2021, 04:47:40 pm »
I will add that exact noise specifications should of course be included in the datasheet of any given part.

Given the nature of the beast (bipolar transistors running at fairly high bias) I expect CFA voltage noise to be quite competitive, but IN+ current noise may be a few dB higher due to the combined base currents of two transistors. Off the top of my head, not sure how much current noise there is on IN-.

Many integrated instrumentation amps use current feedback in their first stage to achieve stable performance across a wide range of closed loop gains. Some of those ICs exhibit very low noise at low source impedance.
 


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