# Frequency Response Project

Discussion in 'Homework Help' started by Heavydoody, Apr 3, 2011.

1. ### Heavydoody Thread Starter Active Member

Jul 31, 2009
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I have this project due tomorrow (attached pdf) and am pretty satisfied with what I have come up with. However, there is a slight discrepancy in the upper cutoff frequency and I was wondering if I had made a math error somewhere, or if I am simply missing some characteristic of the opamp. With some creative rounding the discrepancy disappeared, but I don't know why. Or maybe I am being too picky...

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2. ### Heavydoody Thread Starter Active Member

Jul 31, 2009
140
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Bumpity bump?

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
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Looking at your schematic for the simulation and the overall model in the question you seem to have CA & C1 in the wrong positions.

CA should be in parallel with Ri and C1 in series with Rs. They appear to be "swapped".

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4. ### t_n_k AAC Fanatic!

Mar 6, 2009
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This was a design I came up with - values seemed OK. This was however with an ideal op-amp.

My values agreed reasonably well with yours. My method has a frequency 'fudge factor' of 1/√(√2-1)=1.5538 .... compared with your 1.55

• ###### Frequency Response Project.png
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5. ### t_n_k AAC Fanatic!

Mar 6, 2009
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It's also possible that your LT1226 is marginally rolling off in gain at the 5MHz mark. I haven't checked the gain-bandwidth product for this device.

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6. ### Heavydoody Thread Starter Active Member

Jul 31, 2009
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Thanks for the responses t_n_k. I swapped CA and C1 (can't believe I did that). It had no noticeable effect on the response, I guess because when either one effects the response the other one is "invisible," but I am not sure.

As to the GBP, we are just now learning this in class (along with slew rate and some other really fiddly stuff), so I did not take it into consideration for this project. To be honest, I am rather behind in that regard, because I was sick and missed a couple days of class. Our professor tends to assign projects a bit beyond where we are at in class, which is a nice challenge. The datasheet lists a gain bandwidth of 1 GHz at 1 MHz for this application. We have another test coming up in a week and a half on GBP, so I will investigate this further as I get it all figured out. The next project is an audio amplifier which takes all these new paramters into account...

Its encouraging to see my math was correct though .

Thanks Again

7. ### Heavydoody Thread Starter Active Member

Jul 31, 2009
140
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So, from what I can tell:
$BW_{CL}=\beta \times GBP=\frac {R_1}{R_1+R_f} \times GBP=\frac{1,000}{1,000+120,000} \times 1 GHz=8.26 MHz$
Thus, I do not think this is the problem. I did notice in the data sheet that feedback resistors over 5kΩ are not recommended unless a cap is placed in parallel. I will play with that when I get more time tomorrow...

8. ### Heavydoody Thread Starter Active Member

Jul 31, 2009
140
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Well, I gave up fooling around with that last project. This is the audio amp project. If anyone has time to check my work, I would display my appreciation with a click of the thank you button

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Jul 31, 2009
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