Opamp Doesn't Satisfy Ohm's law

[jaus tail]

Homework Statement

Homework Equations

No current enters of leaves Opamp Terminals

The Attempt at a Solution

Red line is current path.
Inverting terminal of Opamp -1 is at ground due to virtual ground concept, so Current = 2/1 = 2mA.
This current goes through feedback path and then right toward inverting terminal of opamp-2
And then it goes up as shown in arrow.
But at output of Opamp -1, let the V be Vo.
So Vo / 5 (feedback resistor of opamp 1) is not same as Vo / 1 (inverting terminal resistor of opamp 2).
Why is this so?
I got answer by: Vo = -Rf/ R1 for opamo 1. And then I found current as Vo1/1k. And then using KCL at inverting terminal of Opamo-2
But why is the underlined part wrong?

Using KCL at inverting terminal of Opamp-2
2mA = (0-2)/1k + (0 -Vo)/1k
Which gives Vo = 4V.

 

[mfb]

No current enters of leaves Opamp Terminals

Not at the inputs. The output can have a current. "Infinite" current for an ideal opamp.

 

[jaus tail]

But I solved this other example using the assumption no current flows out of opamo 1 output and i got correct answer of 6V
When to take Vo has no current and Vo has current output?

 

[mfb]

It can happen that you have zero output current, but that is a special case.

 

[berkeman]

When to take Vo has no current and Vo has current output?

Never make that assumption, it is totally false in general.

Homework Equations

No current enters of leaves Opamp Terminals

No, that is not accurate for the Relevant Equations for opamp circuit problems.

Instead, the Relevant Equations / Assumptions should be more like these:

• The input impedance of the + and - inputs to an opamp is infinite (no current flows in or out of the inputs)
• The output impedance of the opamp is zero (you can draw lots of current out of the output)
• The negative feedback of an opamp's external circuit keeps the - input at the same voltage as whatever the + input's voltage is

Now, using those rules, please write the KCL equations for each of the nodes in your opamp circuit, and solve for the unknown voltages. Please show your work. Thanks!

 

[donpacino]

When you are working with op amps it can also help to think of it as the below model. The output is simply a voltage source dependent on the input. In an ideal op amp, Rin is inf and Rout is zero.

So this voltage source will act like any other voltage source, it will produce whatever current it has to in order to maintain it's voltage. Therefore you do not want to use the op amp current to solve the circuit, rather you want to use the voltage.

If you have used nodal analysis, you are told to never use the opposite side of a voltage supply as a node, the same principal applies here, for the same reason.