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There is already a similar question in this website, but I still can't solve this. I have the following information:

An inverting opamp with nominal gain -20V/V, DC gain 10^4 and unit gain 10^6Hz. I need to find the 3dB frequency. I tried the following:

\$GBP = f_U = 10^6Hz\$;

\$ f_{3db} = \frac{GBP}{nominal_.gain} = 50000Hz\$.

However, the correct answer should be \$299kHz\$. What did I do wrong?

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  • \$\begingroup\$ ask yourself: "GPB" is an abbreviation for what? \$\endgroup\$ – Marcus Müller Jan 12 '20 at 23:16
  • \$\begingroup\$ Well, it's the Gain–bandwidth product. I saw people using it this way, so I tried to use it to. I don't fully understand its purpose. I'm not and electrical engeneering student, but I was put in a electrical engeneering course - I have trouble understanding many of the concepts. \$\endgroup\$ – João Silva Jan 12 '20 at 23:29
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    \$\begingroup\$ I would say your answer is correct - the 299kHz is wrong. \$\endgroup\$ – Kevin White Jan 12 '20 at 23:31
  • \$\begingroup\$ Really? Isn't the "DC gain" used for anything? \$\endgroup\$ – João Silva Jan 12 '20 at 23:37
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    \$\begingroup\$ somebody (who "shall go nameless") forgot to square 20 then add 1 to get the correct result of 20.02 so ignore the error correction and use 20 as in your answer is correct GBP/G=BW . Dont be sorry. Are you Canadian? haha \$\endgroup\$ – Tony Stewart EE75 Jan 12 '20 at 23:56
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What you have done is basically correct; the " answer " of 299 Khz is wrong. Your error, and Andy's, is with the division by 20. An inverting amplifier configuration makes the amp work at a gain of 1+20, so you should divide 10*6 by 21 for a result of 47.6 Khz. The reference to the DC gain is just to throw you off.

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  1. Get some log-log graph paper and draw an X axis up to 1 MHz
  2. Draw a Y axis that covers the region from unity gain to 100 or 1000
  3. Draw a line from (1 MHz, 1) with negative slope as per purple below
  4. draw a line a horizontal line at gain 20 (blue) until it intersects the purple line
  5. Project that intersect down to the X axis

enter image description here

  1. Read the frequency on the X axis.
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