Electronics Determining Op Amp Gain.

[OmniNewton]

Homework Statement

Determine the Op amp gain of the following circuit.

V1 = 1v Vo = 4 V

Homework Equations

A = Vo/ VI

The Attempt at a Solution

Using voltage division I determined the voltage for the node at the +ve terminal

VI = V1(1k/ ([1000 + 1]k)) = 1/1001 V
Vo = 4v Given

Therefore A = Vo/VI = 4/(1/1001) = 4(1001)

However the answer is A = 4/(4/1001) = 1001

So somehow I'm off by a factor of 4. I am really unsure why VI is 4/1001.

Thank you so much for your time.

 

[Charles Link]

The circuit does not make sense to me. Are you sure you don't have a feedback resistor somewhere? Also, are you sure the "-" terminal is connected to ground? Perhaps someone else can figure it out, but to me, it looks like it is missing something.

 

[OmniNewton]

The circuit does not make sense to me. Are you sure you don't have a feedback resistor somewhere? Also, are you sure the "-" terminal is connected to ground? Perhaps someone else can figure it out, but to me, it looks like it is missing something.

I strongly felt the same way as well Charles. But this is a problem directly from my textbook and it has not been modified for the last 3 editions so I'm assuming it is correct. Thank you so much for the response though.

EDIT: It is definitely very strange compared to the Operational amplifier problems I am used too. I am seeking enlightenment

 

[gneill]

The circuit is okay, it's just that the op-amp depicted is not an ideal op-amp. Perhaps it would be better to think of it in terms of some general amplification stage with a finite gain. The idea is to find that gain given the "measured" input and output voltages.

 

[Charles Link]

I strongly felt the same way as well Charles. But this is a problem directly from my textbook and it has not been modified for the last 3 editions so I'm assuming it is correct. Thank you so much for the response though.

EDIT: It is definitely very strange compared to the Operational amplifier problems I am used too. I am seeking enlightenment

Taking the circuit as is, for a small positive input voltage, the output voltage would be pinned at +12 Volts or whatever bias you use for it. For a small negaive input voltage, the output would pin itself at -12 volts.

 

[OmniNewton]

Charles thanks for taking to the time to help out as well. I think gneill has the right idea of how to approach this problem so if we are to approach the problem assuming it is not an ideal OP amp what would I have to change in my solution?

 

[gneill]

From what is given it appears to me that your result of A = 4004 is correct. Is the question complete as shown or were there other "characteristics" of this amplifier, such as input impedance?

 

[OmniNewton]

From what is given it appears to me that your result of A = 4004 is correct. Is the question complete as shown or were there other "characteristics" of this amplifier, such as input impedance?

Yes you are absolutely right my A will be 4004 but the correct answer is indicated to be 1001 in all previous editions. the circuit provided is all information. The only additional information says Vo = 4 V and v1 = 1.0 V

 

[gneill]

There is nothing provided in the given circuit to account for a factor of 4 other than the gain itself, so I'm thinking that there's an error in the book.

 

[Charles Link]

Charles thanks for taking to the time to help out as well. I think gneill has the right idea of how to approach this problem so if we are to approach the problem assuming it is not an ideal OP amp what would I have to change in my solution?

I had difficulty reading the line in your OP where you have $V1=+1.0$ Volts and $V_{out}=+4.0$ Volts. I am glad that @gneill was able to help get the correct solution.