op amp problem, the book gives a different answer

[huub8]

I'm trying to solve a question about an op amp ciruit, but my answer differs from the given answer.



I did this:

because the output of the two left most op amps are directly connected to their respective inputs the output of the op amp is the same as the input on their positive terminals right?

So for the left most lower op amp, the output would be 7V, which would mean:
40 / (20 + 40) * 7 = 4.666V applied to the positive terminal of the right op amp.

Now for the negative terminal of the right most op amp:
(6.98 - 4.666) / 20,000 = (4.666 - O) / 40,000

where O is the output of the right most op amp. This would mean that O = 0.04V

0.04 / 50,000 = 8.0000000×10^-7

Which is different from the negative answer that the book provides.

 

[crutschow]

I think that's just a dash, not a negative sign.

Also the current is 800 nA as you found, not 800 µA as stated in the given answer.

 

[zorro]

Yes, i0 is 800 nA in the sense of the arrow.
The other possibility is that the sense of the arrow in the drawing reversed. This would be consistent with the convention that positive currents enter the device.
regards

Z