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Suppose I have a carrier signal with low-ish RF frequency (a few MHz) that I can reasonably amplify with an op-amp. How can I estimate the added phase noise due to the amplifier?

For microwave frequencies, gain blocks often have specifications of their phase noise (examples on p. 75). Op-amps do not, presumably because phase noise rarely is a concern. Can I derive the phase noise from the opamp's parameters that are specified?

For the wideband (white) noise, I think I know how I might proceed. The amplifier's noise (op-amp + feedback network) is equally distributed across both of the carrier's quadratures (corresponding to amplitude and phase). Given the carrier's input amplitude I can compute the phase noise density.

I am, however, unsure how I might estimate the close-in phase noise. By my understanding, the 1/f phase noise that an amplifier contributes is effectively its own 1/f noise, upconverted to the carrier frequency. I read that some ways by which this upconversion can happen are:

  1. non-linear gain of the amplifier;
  2. modulation of the gain with temperature, supply voltage or other fluctuations (AM -> PM);
  3. modulation of the delay (phase) of the amplifier.

In essence, all are due to a closed-loop gain that fluctuates. By design, an amplifier constructed around an op-amp is incredibly linear. With enough loop gain, the closed-loop gain only depends on the feedback network. Resistors are really quite linear (~ ppm?).

Is a feedback amplifier at these frequencies essentially that good in terms of phase noise that no one bothers?

My question is for educational purposes, I am not facing an immediate design challenge.

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  • \$\begingroup\$ There's not many op-amps that are suitable for amplification above 1MHz. Are you really using op-amps with GBW products above 50MHz with all of the layout problems that brings? \$\endgroup\$
    – TimWescott
    Aug 21 at 21:45
  • \$\begingroup\$ And can you please edit your question with some part numbers for microwave amplifiers that have phase noise specifications? I'm suspecting that they're class C or A-B or other classes where significant nonlinearities occur. \$\endgroup\$
    – TimWescott
    Aug 21 at 21:47
  • \$\begingroup\$ Added an example. While most opamps have GBW of < 10 MHz, there are plenty that are faster. I have seen opamps (oftent the current feedback type) used at > 100 MHz. \$\endgroup\$
    – polwel
    Aug 22 at 6:16
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    \$\begingroup\$ Yes the 1/f noise becomes the close-in noise. If the closed-loop gain is low, then this is a rather simple exercise, because the fluctuation of the op-amps gain does not matter. You will have to consider the 1/f noise of the gain setting resistors instead, however. If you run the opamp at close to its open loop gain, though, I guess matters become much more complicated. \$\endgroup\$
    – tobalt
    Aug 22 at 7:34
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    \$\begingroup\$ @polwel I'm afraid I can't. The details of how to get from the noise density to phase noise are still an "exercise", i.e. I don't know them currently. I just wanted to comment that it is indeed the low frequency noise that is upconverted as you supposed. \$\endgroup\$
    – tobalt
    Aug 22 at 18:15

1 Answer 1

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Op-amps don't really work well in frequencies above 1 MHz with some exceptions of high precision Op-amps. Are you sure your design is correct?

If you do a Bode plot of the gain of the op-amp with the frequency of the signal, a frequency above 1 MHz is usually above the frequency range for full power bandwidth. You get full amplification only in some frequency range and that is most likely not above 1 MHz. Your op-amp becomes a low pass filter beyond that. You will have signal attenuation outside the full power bandwidth frequency. Look at terms like full power bandwidth, gain bandwidth product of the op-amp you are using in its datasheet.

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  • \$\begingroup\$ I don't think the slew rate limits the phase noise. Certainly not when the signal amplitude is small enough. \$\endgroup\$
    – polwel
    Sep 22 at 5:11
  • \$\begingroup\$ Can you show the diagram of your circuit? Which op amp are you using? \$\endgroup\$
    – Amit M
    Sep 22 at 5:23
  • \$\begingroup\$ Per my question: "My question is for educational purposes, I am not facing an immediate design challenge." \$\endgroup\$
    – polwel
    Sep 22 at 5:25
  • \$\begingroup\$ But look here, page 208, for an example of low-phase-noise amplification of a 10 MHz signal with an ADA4895-1. Or AD8037 a page later. \$\endgroup\$
    – polwel
    Sep 22 at 5:32
  • \$\begingroup\$ The noise you talked about seems to increase with frequency in non low-noise op amps and total noise may be reasonably high at frequencies over 3 MHz.. You have to choose a op-amp with total noise which is under some range. Look at a op amp as the one below. Look at terms like total noise, phase noise, noise figure. Your op amp selection and design may be based on the above criteria. . analog.com/media/en/technical-documentation/data-sheets/… . \$\endgroup\$
    – Amit M
    Sep 22 at 5:38

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