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I have an amplifier with the following frequency response:

enter image description here

It is an inverting amplifier with a gain of about -12. I would like to know what would be the correct approach for identifying its bandwidth. I know it should be measured at the -3dB point, but is that relative to the passband gain (M2 on image) or relative to the reference 1V level (M3 on image)? What is the bandwidth of this amplifier?

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3 Answers 3

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Bandwidth is measured relative to the passband gain. While the 3dB bandwidth is most common, other points are also used. 1dB, 6dB for example. The 3dB bandwidth on you plot is at M2, ~2.3MHz

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The graph you produced is a full definition for the bandwidth and is probably the most useful information for any engineer looking at your design.

But, if you wanted to state the 3 dB bandwidth (M2) or the unity gain bandwidth (just above M3) or, some other bandwidth, just make it clear (using words such as \$BW_{-3dB}\$ for example) what it is you are defining.

Most folk assume you mean \$BW_{-3dB}\$ when BW is stated on its own.

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  • \$\begingroup\$ Wouldn't the unity gain bandwidth be at the 0dB point instead of at M3? \$\endgroup\$
    – Ching Chung
    Commented Aug 18, 2022 at 18:19
  • \$\begingroup\$ @ChingChung oops yes. I meant just above M3 \$\endgroup\$
    – Andy aka
    Commented Aug 18, 2022 at 19:09
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Bandwidth is related to the "loss" of 3 dB
-> M2 -> thus 2.27 MHz if I read well because in this case, it is a low-pass filter.
So only one point is needed.

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