op-amp

Discussion in 'Homework Help' started by uofmx12, Mar 8, 2011.

Mar 8, 2011
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if I have a ideal op-amp of an instrumentation amplifier with 2R1, R2, R3, and R4. with v1=Vcm-Vd/2 and v2=Vcm+Vd/2.

Ad=.5[(R4/(R2+R3) ) (R4/R1 + 1) + (R4 / R1) ]
Acm=(R4/ (R2+R3) ) (R4/R1 + 1) ( R4/R1)

Would those equations be correct?

or Acm=0?

2. hgmjr Retired Moderator

Jan 28, 2005
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It would help us better understand your equations if you would post a diagram of your circuit.

hgmjr

3. Ron H AAC Fanatic!

Apr 14, 2005
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Agreed. I am baffled by posts like these. What are they thinking?

Mar 8, 2011
55
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I figured everyone knew what an ideal op-amp instrumentation amplifier looked like. Let me google it for you:

http://www.electronics-tutorials.ws/opamp/opamp21.gif

With R1=2R1.

Last edited: Mar 8, 2011
5. beenthere Retired Moderator

Apr 20, 2004
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Link does not work. How about a schematic with all those resistors placed and labeled?

Mar 8, 2011
55
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Apr 14, 2005
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674

8. Ron H AAC Fanatic!

Apr 14, 2005
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I know what the circuit looks like, but I haven't memorized the resistor reference designators. In fact, I doubt that they are standard.

Mar 8, 2011
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my link works when I click on it in my post. So not sure what the problem is.

10. Ron H AAC Fanatic!

Apr 14, 2005
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When I click on your link, I get a blank page with this text in the upper left corner:

11. Ron H AAC Fanatic!

Apr 14, 2005
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OK, I got the schematic.
In your original post, what does 2R1 mean?
You can find the gain equation on the Internet.

Mar 8, 2011
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Ok, so if 2R1=4k, R2=100k, R3=10k, and R4=10k, Find differential gain and common mode gain, would I use those equations in first post? Assumming V1 and V2 what I put.

13. Ron H AAC Fanatic!

Apr 14, 2005
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I still don't get the 2R1 thing. Why didn't you just say R1=2k?

Mar 8, 2011
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that is the case, by my circuit has 2R1 instead of R1. SO R1=2k

15. Ron H AAC Fanatic!

Apr 14, 2005
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Have you searched for the gain equation?

Mar 8, 2011
55
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No, Acutally I believe Ad=R4/R3(1+R3/R2). I have a problem in my book that states.

For instrumentation amplifier a designer proposes R2=R3=R4=100k, 2R1=10k. For ideal components, what is difference mode gain, common mode gain.

The solution uses Ad above to get 21V/V and Acm=0. So, not sure what Acm equation is but doesn't look like my equations on original post are correct.

17. Ron H AAC Fanatic!

Apr 14, 2005
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That equation doesn't even contain R1, so it can't be correct.
The correct equation is all over the internet.

Mar 8, 2011
55
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Apr 14, 2005
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674