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[Moved]: Dynamic range of amplifiers

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mordak

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Hi all,

I am a bit confused about dynamic range of amplifiers. Based on my understanding, dynamic range is defined as the ratio of the maximum signal the amplifier can deliver over its total noise. Now here is the problem, if you have an amplifier that has a low bandwidth, say 1 kHz, it may have less integrated noise than an amplifier with 1 GHz bandwidth.
Dynamic range of the low bandwidth amp may be better than the one with wide bandwidth (just because one has wider bandwidth). If you use the same bandwidth for both of these opamps to calculate the integrated noise, the wide bandwidth opamp may give better dynamic range.
I think in that sense, it is not fair to look at the dynamic range to compare two opamps, if they have different bandwidth. Am I missing something here?

Any help is appreciated!
 

Re: Dynamic range of amplifiers

Hi,

I agree. An amplifier with wider bandwidth (but the same noise distribution) will have less dynamic.

But is it unfair?
How fair is a comparison of a 1kHz with an 1GHz amplifier anyway?
They are made for different applications.

In doubt I recommend to compare the noise distribution charts.
Especially for lower bandwidth amplifiers (up to audio amplifiers) the noise distribution with it's 1/f corner plays a relatively large role.

In datasheets you find a noise density value, this usually is given above the 1/f corner.
For high bandwidth amplifiers maybe this value is a more "fair"way to compare amplifiers.

Klaus
 
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    mordak

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Re: Dynamic range of amplifiers

Hi,

I agree. An amplifier with wider bandwidth (but the same noise distribution) will have less dynamic.

But is it unfair?
How fair is a comparison of a 1kHz with an 1GHz amplifier anyway?
They are made for different applications.

In doubt I recommend to compare the noise distribution charts.
Especially for lower bandwidth amplifiers (up to audio amplifiers) the noise distribution with it's 1/f corner plays a relatively large role.

In datasheets you find a noise density value, this usually is given above the 1/f corner.
For high bandwidth amplifiers maybe this value is a more "fair"way to compare amplifiers.

Klaus
Hi Klaus

Thanks for your comment. To answer to your question " How fair is a comparison of a 1kHz with an 1GHz amplifier anyway?" I should say I made an exaggeration to make a point. Say you and I are in charge of designing an opamp, you design one with 2 kHz bandwidth, I do one with 1.5 kHz, and in the title of my paper I put the dynamic range as an outstanding feature. Of course (assuming same noise distribution) my number would be better than yours, but it's misleading. These papers are already out there in prestigious conferences making such comparisons between different amplifiers, and I don't know how they get accepted. But I agree, using noise density is more fair.
 

Hi,

you and I are in charge of designing an opamp, you design one with 2 kHz bandwidth, I do one with 1.5 kHz
according DN015f of linear technology.
Input voltage noise of LT1037 (10Hz...1500Hz) = 96nV
Input voltage noise of LT1037 (10Hz...2000Hz) = 112nV
they are just 1.2dB apart. This may be within the production tolerance.

But don´t forget: What you say is the max bandwidth of the amplifier. A good audio OPAMP may have a bandwidth of several MHz. But for noise calculations you take only the useable (audible) bandwidth into account (20Hz ... 20,000 Hz for example).

Klaus
 

Klaus' extract of LT's noise values correspond closely to the square root law of bandwidth vs noise in an evenly distributed noise spectrum.

Regarding the matter of presenting papers on amplifiers with unequal bandwidths, any competent engineer will - or should - be able to spot the difference in bandwidths immediately and make their assessments with that in mind. If one or both designs are presented without stating the bandwidth, the attending engineers should ask about them.
 

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