# Op Amp and capacitors

Discussion in 'Homework Help' started by pianolife, May 20, 2013.

1. ### pianolife Thread Starter New Member

Nov 15, 2012
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Dear members of All About Circuits,

I am back to disturb you for a question I am stuck with.

I am asked to sketch a Bode Plot (magnitude only), for the circuit in the picture, showing the frequencies where the midband gain has fallen by 3dB.

I see that there are both a LPF and an HPF, so the first formula that comes into my mind is omega=1/RC. The problem is how to "relate" it to the op amp, can anyone help me?

All the best

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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You will firstly have to derive the transfer function relating the output voltage to the input voltage. If you can do this in the Laplace Transform domain it will make your job that much easier to then create the Bode plot.

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3. ### pianolife Thread Starter New Member

Nov 15, 2012
28
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Dear t_n_k,

One thing I don't get, though, is that in all the examples I have seen so far, the negative input of the Op Amp had the voltage source, whereas the positive input was grounded.

How does this affect the transfer function?

My idea was to work it out with something similar to the picture I am attaching, would that be ok?

Thank you so much!

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4. ### Jony130 AAC Fanatic!

Feb 17, 2009
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The voltage gain for non inverter amplifier is equal to
Av = 1 + Z2/Z1
So the transfer function will look different.

5. ### pianolife Thread Starter New Member

Nov 15, 2012
28
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Thanks Jony.

z2 is the feedback (1.5n || 9k) and z1 the "ground" (1k + 1u), isn't it?

Thank you!

6. ### Jony130 AAC Fanatic!

Feb 17, 2009
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Yes, you got that part right.

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