# operational amplifier dc input ?

#### Xufyan

Joined Aug 3, 2010
114
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 #### Jony130

Joined Feb 17, 2009
5,243
But if circuit don't have any capacitor in signal path.
The DC gain is equal to AC gain
Av = 1+ Rf/Ri

#### Xufyan

Joined Aug 3, 2010
114
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'

#### praondevou

Joined Jul 9, 2011
2,942
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 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? #### Jony130

Joined Feb 17, 2009
5,243
Maybe it is time for you to show us the diagram.
Are we talking about the same circuit ?? • Xufyan

#### Xufyan

Joined Aug 3, 2010
114
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? 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:

#### Xufyan

Joined Aug 3, 2010
114
Maybe it is time for you to show us the diagram.
Are we talking about the same circuit ?? yeah, the same circuit

#### Jony130

Joined Feb 17, 2009
5,243
yeah, the same circuit
If so the DC gain = AC gain = 1+ R2/R1
So for Vin =1.7V
Vout = -1.7V * (1 + 120K/39K) = -6.93V

• Xufyan

#### Xufyan

Joined Aug 3, 2010
114
If so the DC gain = AC gain = 1+ R2/R1
So for Vin =1.7V
Vout = -1.7V * (1 + 120K/39K) = -6.93V
how DC gain could be equal to AC gain :S

#### Jony130

Joined Feb 17, 2009
5,243
• Xufyan

#### Xufyan

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

Joined Jul 9, 2011
2,942
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
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