# operational amplifier dc input ?

Discussion in 'Homework Help' started by Xufyan, Aug 21, 2011.

1. ### Xufyan Thread Starter Member

Aug 3, 2010
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I have a question regarding opamp,

i have a non-inverting amlifier problem with Rf and Ri is given but the input voltage is not given however input DC voltage is -1.7 volt.

according to the formula,

Vo = (1+ Rf/Ri)x Vi(ac)

how can i solve if the DC input voltage is given but not AC

2. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
1,115
But if circuit don't have any capacitor in signal path.
The DC gain is equal to AC gain
Av = 1+ Rf/Ri

3. ### Xufyan Thread Starter Member

Aug 3, 2010
114
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why would there be a capacitor in a non-inverting amplifier :S

will it be Av = (1+ Rf/R)x Dc input ???

i have checked a non-inverting amp in multisim with a DC input voltage , the output is 'ZERO'

4. ### praondevou AAC Fanatic!

Jul 9, 2011
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489
It's not clear what you want to solve. Do you want to know the gain? Well you have the formula... The AC-gain will also depend on the frequency of the AC-signal.
Or do you want to know the AC output voltage? That would be impossible, since you don't have the input voltage.

And what's the DC- voltage doing there?

5. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
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Maybe it is time for you to show us the diagram.
Are we talking about the same circuit ??

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6. ### Xufyan Thread Starter Member

Aug 3, 2010
114
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this is what i am thinking :S why there is no AC i/p voltage mentioned ?? :S

and what does DC voltage is doing there :S

this is what question says,

Determine the output voltage of Non-inverting amplifier if Rf=120k and Ri =39k and the DC input voltage is -1.7 volts ?

Book: Fundamentals of Operational Amplifiers and Linear Integrated Circuits by Howard M.Berlin and Frank C Getz, Jr.

Problem 10 , Chapter 2

Last edited: Aug 21, 2011
7. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
yeah, the same circuit

8. ### Jony130 AAC Fanatic!

Feb 17, 2009
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If so the DC gain = AC gain = 1+ R2/R1
So for Vin =1.7V
Vout = -1.7V * (1 + 120K/39K) = -6.93V

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9. ### Xufyan Thread Starter Member

Aug 3, 2010
114
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how DC gain could be equal to AC gain :S

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,990
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11. ### Xufyan Thread Starter Member

Aug 3, 2010
114
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i didn't know that , thank you

12. ### praondevou AAC Fanatic!

Jul 9, 2011
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It doesn't say anything about AC. <they want to know the DC-output voltage.

edit: oops I guess I'm several posts too late