Using Op amps to reduce signal

Discussion in 'Homework Help' started by RyanKim, Mar 24, 2013.

1. RyanKim Thread Starter Member

Sep 18, 2011
37
1
Hey guys. I had a question regarding using Op-amps in a signal conditional amp circuit. Assuming I have some input to the cct, and I need to reduce that signal by a gain factor of 0.555 or so, is this doable without any reprocussions?

I ask this because the input span of SCA is around 143mV and the output of my circuit needs to be around 80mV. I've really only dealth with "amplifying" using op amps and SCA's. By amplifying I mean making a signal larger.

Thanks guys. Let me know if theres more info needed to answer this question.

*Edit*

Also these are DC signals only. No AC involved

Ive also attached the circuit i will be using. The operation equation for my circuit is Vout = 0.55613 (V(+)-V2(-)) + Bias

Im just wondering if its okay to have a gain of less than 1 in this case.

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Last edited: Mar 24, 2013
2. mlog Member

Feb 11, 2012
276
36
What's an SCA?

Apr 5, 2008
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4. RyanKim Thread Starter Member

Sep 18, 2011
37
1
Thanks. An SCA is a signal condition amplifier circuit. Using opamps to "amplify" or condition a signal. For instance I am using a RTD (Resistive Temperature Device) that will measure temperature. At 0 Degrees it will give me some voltage, and at 80 degrees C it will give me a different voltage (e.g 400mV to 543mV) in my case. I need to take this signal and "condition it" so that I can input a smaller signal into a digital display. The display requires the signal range from 0 to 80 mV.

5. WBahn Moderator

Mar 31, 2012
23,201
6,993
Yes, you can use an opamp to attentuate a signal -- although the stability of an opamp goes down as the gain goes down. Since you are only talking about ~0.5, you should be okay as long as your opamp is "unity gain stable".

6. Ron H AAC Fanatic!

Apr 14, 2005
7,049
674
I don't get it.
Your first amplifier has a gain of 50. The second amplifier has a gain range of -2.45 to -3.6. Why do you say your gain is 0.55613?

7. Ron H AAC Fanatic!

Apr 14, 2005
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674
Just as an aside, an inverting amp can have a gain of zero, and it will still be stable, as long as the op amp is unity gain stable.

8. WBahn Moderator

Mar 31, 2012
23,201
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Ah. I wasn't sure if that was the case or not. Good to know.

9. Ron H AAC Fanatic!

Apr 14, 2005
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674
A zero gain inverting amp has Rf=0 when Rs is finite. Rf/Rs=0. This has the same loop gain as a unity gain noninverting amplifier.

10. WBahn Moderator

Mar 31, 2012
23,201
6,993
Makes sense.

All of the opamp designs I've done haven't required me to get into looking at loop gain and so I just don't think in those terms off the bat.