# Try to Solve this electronics circuit based problem!!

Discussion in 'Homework Help' started by abeyravi, Sep 23, 2009.

1. ### abeyravi Thread Starter New Member

Jan 19, 2009
1
0
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?"

326x319 : FBZA9HUFZQ1GUGX (0)
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2. ### mik3 Senior Member

Feb 4, 2008
4,846
70
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.

3. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
Just to re-assure you, I believe Mik3 has given the correct method.

4. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
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.

5. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,724
496
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.

6. ### t_n_k AAC Fanatic!

Mar 6, 2009
5,448
790
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]

7. ### The Electrician AAC Fanatic!

Oct 9, 2007
2,724
496
You are correct, sir!

Exactly the same result I got.

8. ### ELECTRONERD AAC Fanatic!

May 26, 2009
1,146
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I think he was bribing everyone to get information! Those who knew at least...