Hello,
Please help me to solve this circuit based problem using Op-amp. I tried to do this problem but i didn't get the result. I am preparing for an exam, this question is one of the previous question. Please suggest the method to solve this question as soon as possible. The question and circuit diagram is shown below.
Note: This question is not simple, it is coming under analog teaser section.Please try to give correct answer/method.otherwise don't comment!

Question :
"Assuming the open-loop gain & bandwidth of the op-amp to be infinity, what will be the current through the 100 ohm resistor?"

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326x319 : FBZA9HUFZQ1GUGX (0)
i

 

This is not very hard.

1) Because of infinite gain the voltage at the inverting input is the same as the non inverting input (0.5V).

2) Then, assuming infinite input impedance at the inverting input, the 1A current passes through the 1R resistor and drops 1V across it. Because of this and (1) the voltage at the right side of 100R is -0.5V.

Find the current through the 2R and 4R. Finally, apply KCL at the node at the right of the 100R to find the current through the 100R.

 

Hello,

Note: This question is not simple, it is coming under analog teaser section.Please try to give correct answer/method.otherwise don't comment!

Just to re-assure you, I believe Mik3 has given the correct method.

 

And to make it into a more challenging problem I'd ask a supplementary question (as "The Electrician" often does) in which the open loop gain of the amplifier is not infinite - let's say it's Av=5000.

 

And to make it into a more challenging problem I'd ask a supplementary question (as "The Electrician" often does) in which the open loop gain of the amplifier is not infinite - let's say it's Av=5000.

Yes, indeed, that is more interesting.

Take it even further. Find a general expression for the opamp output voltage and the current in the 100Ω resistor in terms of Av.

 

Yes, indeed, that is more interesting.

Take it even further. Find a general expression for the opamp output voltage and the current in the 100Ω resistor in terms of Av.

As a check I have

Vout = -163Av/(76+Av)

and

|I_100Ω|=(1.25+1.625Av)/(76+Av)

which seem reasonable since when Av = ∞,
opamp Vout=-163V
&
|I_100Ω|=1.625A

[i.e. the values I got for abeyravi's original question]

 

You are correct, sir!

Exactly the same result I got.

 

I think he was bribing everyone to get information! Those who knew at least...