# Op amp and 3-db frequency problem

## Homework Statement

An inverting amplifier with the DC gain of -9 (V/V) is built with an op amp having its
DC gain of 105
and corner frequency of 30 rad/s.
▪ Determine the 3-dB frequency of the inverting amplifier.
▪ Sketch the magnitude Bode plot of the voltage transfer function of the inverting amplifier.

X

## The Attempt at a Solution

X

Hello.
So I am dealing with this homework and I have absolutely no idea how to approach this problem.
Just to clear things out I do not want someone to do this for me as you'll all guess. But I am kinda asking for help on how to approach and start this problem. Also I am not sure what the question is. I know that that 3-db frequency is the half power point but because this is a DC gain I would think that I am dealing with a DC circuit and therefore I would not expect any frequency in this problem (but I am probably wrong). I am also having a hard time understanding why there are 2 given gains, the -9 V/V and the other 105
What are the difference in those gains. So far we've learned that an op amp can be represented with a simple circuit consisting of 2 resistors and Voltage-Controlled-Voltage source (VCVS).

So basically my question is how do I approach this and am I suppose to simplify the op amp with a simple amplifier circuit with 2 resistors and VCVS?

berkeman
Mentor

## Homework Statement

An inverting amplifier with the DC gain of -9 (V/V) is built with an op amp having its
DC gain of 105
and corner frequency of 30 rad/s.
▪ Determine the 3-dB frequency of the inverting amplifier.
▪ Sketch the magnitude Bode plot of the voltage transfer function of the inverting amplifier.

X

## The Attempt at a Solution

X

Hello.
So I am dealing with this homework and I have absolutely no idea how to approach this problem.
Just to clear things out I do not want someone to do this for me as you'll all guess. But I am kinda asking for help on how to approach and start this problem. Also I am not sure what the question is. I know that that 3-db frequency is the half power point but because this is a DC gain I would think that I am dealing with a DC circuit and therefore I would not expect any frequency in this problem (but I am probably wrong). I am also having a hard time understanding why there are 2 given gains, the -9 V/V and the other 105
What are the difference in those gains. So far we've learned that an op amp can be represented with a simple circuit consisting of 2 resistors and Voltage-Controlled-Voltage source (VCVS).

So basically my question is how do I approach this and am I suppose to simplify the op amp with a simple amplifier circuit with 2 resistors and VCVS?

The high gain is the open-loop low-frequency gain. The -9V/V is the closed loop low-frequency gain with the negative feedback. Show us the Bode plot for the open loop gain of an opamp (find it via Google Images or scan your textbook). Then show us how that open-loop Bode plot of gain is altered when you close the loop with negative feedback...

This is the only thing that was given in the problem. There is no image or anything else. But shouldn't I first determine the 3 db frequency before sketching up the Bode plot?

Also I am not quite sure how to draw the circuit. I've completed one sketch but I am not sure if it is the right thing to do.
I will link the sketch that I've drawn.

#### Attachments

• Homework 1.pdf
265.4 KB · Views: 149
Last edited:
berkeman
Mentor
This is the only thing that was given in the problem. There is no image or anything else. But shouldn't I first determine the 3 db frequency before sketching up the Bode plot ?

I was just asking for some visual support for this problem. It helps a lot to see the Bode plots of opamps with and without feedback when working this type of problem. As I said, use Google Images or some other way to find some Bode plots that you can use to help you illustrate and work this problem.... :-)