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Old 10-22-2012, 05:59 AM   #9
LongGone
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Join Date: Jul 2008
Location: Malaysia
Age: 33
Posts: 1,679
Default Re: Circuit Problem "2"

Do you have the answers to doublecheck? I might be wrong at this but..

For questions like these, important things to remember about an ideal op amp
-Infinite gain
-Infinite input resistance, thus no current flows into the op amp inputs
-Zero output resistance

Since the Op Amp is ideal, Vout=12V, the limit of the linear range of the power rails supplied (+-12V). An Ideal Op Amp has infinite gain; thus any difference in values between the two inputs will cause an output. This value is usually taken to be positive (the output is denoted as a + in this example)



Now we have to find Vi. Since an ideal op amp has infinite input resistance, we can write a simple equation to find Vi using Kirchoff's Current Law(KCL)

(12-Vi)/Rf = (Vi-2)/Rin
Vin = 4V

With that found, we can now find the output current, i(out). Again, by KCL, lets look at the output

i(out) = i1+i2 = (4-12)/100000 + (0-12)/20000 = -0.68mA, i.e. current of 0.68mA flows out of the Op Amp
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Last edited by LongGone; 10-23-2012 at 03:27 PM..
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