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I need some help with analyzing an opamp

space

Apr 18, 2016
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So I had an experiment (few weeks ago) where I had to measure certain things of an opamp used as an active filter. One of the questions was to measure the bode plot by measuring the ratio of the AC peak to peak values and the phase shift between two signals for a certain set of frequencies (50 Hz - 10MHz). I also had to explain this measured bode plot with the one from my preparation.

There are two problems now.

1) I didn't manage to draw the bode plot for my preparation.

2) My measured bode plot is nowhere close to the one from the spec sheets or the one that I expect from my preparation. I'm afraid it isn't even close to being a bode plot of any kind. Can't be helped much I guess.


Bode plot from my preparation

So to draw the bode plot, I figured I'd have to start with the transfer function.

In the picture I included are two opamps visible. If I remember correctly, the second one could be ignored as it was only used as a buffer or something. So I'll only focus on the setup indicated by the red rectangle.

Via KCL & ideal opamp I get:
transfer.png


I made the substitution s= jw. I've got a zero for s = -R1/C1 and two poles s = 0 and s = 1/C2R2. Can anyone confirm if I'm correct here. If not, where's my mistake? Probably something stupid. When I fill in the values for my resistors (R1 = R2 = 22k) and capacitors (C1 = 220pF, C2 = 100pF), I get absurdly high values. Nothing that resembles the bode plot from the spec sheet.

I should be able to draw the bode plot following the instructions here. But the above results need to be correct first.

I'll edit my post to take a look at the (possible) problem with my measurements once the above issue is cleared. It might give me inspiration on how to solve the problem.
 

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LvW

Apr 12, 2014
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"So to draw the bode plot, I figured I'd have to start with the transfer function."

The BODE plot of a filter is a plot of the transfer functions magnitude and phase vs. frequency (log scale).
And the transfer function is H(jw)=Vout/Vin (not: Vin/Vout).
Where is your transfer function?
 

space

Apr 18, 2016
4
Joined
Apr 18, 2016
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"So to draw the bode plot, I figured I'd have to start with the transfer function."

The BODE plot of a filter is a plot of the transfer functions magnitude and phase vs. frequency (log scale).
And the transfer function is H(jw)=Vout/Vin (not: Vin/Vout).
Where is your transfer function?


Yeah my apologies, it should be Vout/Vin, the rest of the formula should be correct though.

Isn't that my transfer function in the picture?
 

space

Apr 18, 2016
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I've found an error in my transfer function. It should be:

H = - (1/R1C1 +s)/(1/R2C2 +s)
 

LvW

Apr 12, 2014
604
Joined
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I've found an error in my transfer function. It should be:
H = - (1/R1C1 +s)/(1/R2C2 +s)

Sometimes it is recommended to check if such a transfer function is in accordance with the drawing.
For example:
For s=jw=0 the capcitor values must not play any role.
However, setting s=0 in your expression for H(s) we have H(w=0)=R2C2/R1C1.
So - something must be wrong.
Can you find the error?
 

space

Apr 18, 2016
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Apr 18, 2016
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Yeah, I wrote a bit too fast and forgot to add the factor C1/C2. It should be H = -C1/C2 (1/R1C1 +s)/(1/R2C2 +s)
For s = 0 we get R2/R1, which confirms the system's independence from the capacitors.

It's a good tip though, I'll remember it. Apologies for the stupid error.
 
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