Possible Answer to ECE Finals Questions

[KillaChaos]

I just took my final exam in Introduction to Electrical and Computer Engineering. I could go on describing how horrible/well it went, but a certain question has caused major disagreement.

We had to solve Io (the current leaving the Op Amp) and Vo (the voltage at the output of the op amp)

[PLAIN]http://img130.imageshack.us/img130/4702/probleme.gif

Theres a lot of disagreement on Io, whether its 0 or not (I say no because KCL), and Vo (On the exam I said 0, but after a lot of arguing I think its 30 (if you remove the op amp, then do source transformation you come across this)).

I need some confirmation because I don't have another final till friday.

Thanks

 

[berkeman]

I just took my final exam in Introduction to Electrical and Computer Engineering. I could go on describing how horrible/well it went, but a certain question has caused major disagreement.

We had to solve Io (the current leaving the Op Amp) and Vo (the voltage at the output of the op amp)

[PLAIN]http://img130.imageshack.us/img130/4702/probleme.gif

Theres a lot of disagreement on Io, whether its 0 or not (I say no because KCL), and Vo (On the exam I said 0, but after a lot of arguing I think its 30 (if you remove the op amp, then do source transformation you come across this)).

I need some confirmation because I don't have another final till friday.

Thanks

Please show us the equation that you should use to find Io. And what is the value of the current sink shown on the left?

 

[KillaChaos]

Please show us the equation that you should use to find Io. And what is the value of the current sink shown on the left?

The value is 2 milliAmps. For equation to Find Io, I guess you would use a KCL.

 

[berkeman]

The value is 2 milliAmps. For equation to Find Io, I guess you would use a KCL.

Yep. Show us.

 

[KillaChaos]

Yep. Show us.

Okay, it isn't so much as an equation, as it just reasoning it out.

In class we were told to assume the current entering either terminal of the op amp had to be zero. That means 2 milliamps HAS to be going through the 5k and 10k resistors.

Now if any current enters or leaves the Io, that means that current has to go somewhere else, but there is no where else for the current to go. So Io, must be 0.

 

[╔(σ_σ)╝]

The current has to be zero. If the circuit has negative feedback the negative ternimal tracks the positive terminal. Hence, the negative and positive terminal are virtual ground.

Vo/5k = Vo/10k

Thus, Vo=0.

 

[berkeman]

The current has to be zero. If the circuit has negative feedback the negative ternimal tracks the positive terminal. Hence, the negative and positive terminal are virtual ground.

Vo/5k = Vo/10k

Thus, Vo=0.

I don't see any ground reference shown in the circuit. How can you assign a value to Vo? (Oh, and don't post answers, remember?)

 

[╔(σ_σ)╝]

I assumed there is supposed to be a ground reference otherwise Vo can taken on "any" value. Regardless, writing down node equations at the output reveals that Io is 0.

 

[KillaChaos]

There is a ground on the negative terminal. Its written in the problem. Vo is 10V and Io is 3mA according to the solutions. Could someone explain why KCL seemingly doesn't apply here?

 

[╔(σ_σ)╝]

There is a ground on the negative terminal. Its written in the problem. Vo is 10V and Io is 3mA according to the solutions. Could someone explain why KCL seemingly doesn't apply here?

It applies.

Problem is a bit tricky and I feel for the trap.

Since there is a ground reference at the negative terminal and a current sink there, Vo has to supply the 2mA current through the 5k resistor to the current source ( KCL ).

Negative feedback ensures that the negative terminal tracks the positive and since the negative terminal is virtual gound so is the positive terminal.

In order to ensure Vo stays at 10V the opamp supplies $\frac{10V}{5k} + \frac{10V}{10k} = 3mA$ ( By KCL).