electronics is driving me nuts....

[Gibson486]

I hate asking for homework help on these forums, but this is my only hope now...


On this problem, I have an ideal Op Amp, whose input voltage is measured to be 4V while it's outpout voltage is measured to be 4V also. the question asks, what is the Op Amp Gain A? Well....ideally, gain A is Vo/Vi, so that is easily 1. That answer however, is wrong. It is supposed to be 1001 v/v. Well...the only way 2 ways i can get this is if I divide Resistor 2 by Resistor 1 then add 1. That doesn't work because you cannot just do that with no explanation. The other way is taking the sum of the two resistors and dividing it by resistor 2, but why would you do that. I tried manipulating the equation A(v2-v1)=V_out and applying an open loop gain, but I always end up with R2/R1.


please help (pic of circuit)

 

[Gibson486]

Do I imaginally hook up therminal with the 1k resistor to the Vo ouput, thus making an non inverting opamp?

 

[Zeeky Boogy Doog]

if i had any idea what the hell you were talking about i would totally help you

 

[Demon-Xanth]

You have no feedback resistor. You'll rail the opamp positive every time (assuming a single ended input). The gain depends on the opamp itself and is usually around 100,000 or so.

 

[Colt45]

if input is 4v and output is 4v, gain is unity.. simple as that, no?

edit: looked at pic.

the 1M and the 1k are a voltage divider, so input is 4v x 1000/1001000 = .003996v = 3.996mV

Vo / Vin = 4v/.003996v = 1001

 

[Gibson486]

fvck, that is what i didin't do! i kept 3.996 as mV, but i didn't convet back to volts. All that pain for nothing!!!!!!!!

Colt45, please shoot me now......

 

[Colt45]

lol, I do stupid stuff like that sometimes too.

What's the point of this though? with no feedback it's pretty well useless, no?

 

[Gibson486]

it was a dumb question. Especially with no feadback. I guess my prof wanted us to know it since Vo/Vi=gain, which is something that was needed for the next question. Even then, that's something we learned in circuits.

 

[Slvrtg277]

Originally posted by: Colt45
if input is 4v and output is 4v, gain is unity.. simple as that, no?

edit: looked at pic.

the 1M and the 1k are a voltage divider, so input is 4v x 1000/1001000 = .003996v = 3.996mV

Vo / Vin = 4v/.003996v = 1001

Good job man. I haven't seen this stuff since 96, and I had no clue what to do with this.

 

[ConwayJim]

Out of all the classes i had to take, i've always hated electronics; i found the subject matter extreamly boring. That being said, i would always work out problems such as the one indicated in the question using a program called "Electronic Workbench". It was usually always bundled in all of my electronic text books (you know the cd at the back of the book). Try getting a hold of that program, and wiring up the circuit, and take all the measurements.

 

[FrustratedUser]

Originally posted by: Gibson486
I hate asking for homework help on these forums, but this is my only hope now...


On this problem, I have an ideal Op Amp, whose input voltage is measured to be 4V while it's outpout voltage is measured to be 4V also. the question asks, what is the Op Amp Gain A? Well....ideally, gain A is Vo/Vi, so that is easily 1. That answer however, is wrong. It is supposed to be 1001 v/v. Well...the only way 2 ways i can get this is if I divide Resistor 2 by Resistor 1 then add 1. That doesn't work because you cannot just do that with no explanation. The other way is taking the sum of the two resistors and dividing it by resistor 2, but why would you do that. I tried manipulating the equation A(v2-v1)=V_out and applying an open loop gain, but I always end up with R2/R1.


please help (pic of circuit)

This schematic shows a non-inverting amplifier and the Gain of the circuit is:

1. G = (1Mohm + 1kohm)/1kohm = 1001

The input voltage to the positive input of the OP is:

1. V = Vin * (1kohm/(1Mohm + 1kohm)) = 4 * 9.99000999e-4 = 3.996004e-3 Volts

Now to get Vout, multiply 'G' with 'V':

1. Vout = G * V = 1001 * 3.996004e-3 = 4

:beer:

 

[Demon-Xanth]

I still say the question of gain cannot be solved without the datasheet as the circuit is drawn. Gain doesn't stop being gain just because the output is railed.

Edit:
Correct non-inverting opamp configuration.

 

[Gibson486]

I am a little confused here. I know there is a relation equation between open loop gain and closed loop gain, but what do both gains represent? In my book, it tells me that if i have a finite open loop gain, i must make a virtual ground between the two inputs of the opamp. But what is this supposed to tell me about open loop gain?