Op-amp circuit analysis

Hello everyone! I need help on this problem I'm trying to solve. The ques is to find the Thevenin equivalent circuit. I was able to determine the Vth = -7.5v.
My calculation for the Rth = 20k ohms. But when look it up in the solution, it says Rth = 0 ohms

My process is to get first the Voc at the terminals and the Isc. Then Rth = Voc/Isc.
Can you explain as to why the Rth = 0 ?
Thank you!

Hi,

V_A is driven by the OPAMP, it is not driven by the resistors.
An ideal OPAMP output is consided to give a source impedance of 0 Ohms.

To me your names are confusing. In the picture the nodes are marked with "A" , "a", and so on.... , thus I'm not sure what your names Voc and Isc stand for.

Klaus

The theoretical explain that the ideal OPAmp is composed from a nullator and a norator. Its output has the norator's characteristic, which means the output current value can be anything next to any output voltage and the output voltage is determined by the resistive feedback, the nullator and the input voltage source.
Next to the output voltage the output current will be determined only by the load due to the norator, which means the output resistance has to be 0 Ohm.
So your calculation is wrong somewhere.

KlausST said:

Hi,

V_A is driven by the OPAMP, it is not driven by the resistors.
An ideal OPAMP output is consided to give a source impedance of 0 Ohms.

To me your names are confusing. In the picture the nodes are marked with "A" , "a", and so on.... , thus I'm not sure what your names Voc and Isc stand for.

Klaus

Click to expand...

Voc is the open circuit voltage at terminals A-B and Isc is the short circuit current through terminals A-B. This is the process of getting the equivalent thevenin resistance Rth at terminals A-B.

paulmdrdo said:

Voc is the open circuit voltage at terminals A-B and Isc is the short circuit current through terminals A-B. This is the process of getting the equivalent thevenin resistance Rth at terminals A-B.

Click to expand...

In such process, dependant-sources cannot be passivated.

paulmdrdo said:

Hello everyone! I need help on this problem I'm trying to solve. The ques is to find the Thevenin equivalent circuit. I was able to determine the Vth = -7.5v.
My calculation for the Rth = 20k ohms. But when look it up in the solution, it says Rth = 0 ohms

My process is to get first the Voc at the terminals and the Isc. Then Rth = Voc/Isc.
Can you explain as to why the Rth = 0 ?
Thank you!
View attachment 150118

Click to expand...

You should specify whether you want the input or output impedance. The output impedance is an easy question to answer. The open circuit voltage (Voc) going to be some finite voltage determined by the input voltage and the feedback resistors. The output short circuit current (Is) will be infinite for a perfect OPAMP, or the highest current it can supply for a practical OPAMP. From the Rth formula you gave above, this translates into either a zero output impedance for a perfect OPAMP or a very low output impedance for a practical OPAMP.

Ratch

Yeah I think its a trick question.

Impedance basically answers this question: "If I change the current how much does voltage change in response".

The answer in this case is voltage doesn't change because the output is being directly driven by the opamp.

The thevenin resistance is easily found by following the rules. Whereas dependant sources can not be nulled out, in this case it is nulled out because it depends directly on an independant voltage source which is nulled.

Then, the output therminal of an ideal op amp looks like a short circuit to ground.