# 3dB frequency of inverting amplifier

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?

• ask yourself: "GPB" is an abbreviation for what? – Marcus Müller Jan 12 '20 at 23:16
• 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. – João Silva Jan 12 '20 at 23:29
• I would say your answer is correct - the 299kHz is wrong. – Kevin White Jan 12 '20 at 23:31
• Really? Isn't the "DC gain" used for anything? – João Silva Jan 12 '20 at 23:37
• 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 – Tony Stewart EE75 Jan 12 '20 at 23:56

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.

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

1. Read the frequency on the X axis.