Note that the two terminals are the one shown and the ground reference.
I am supposed to find Req.
First I made a label on the upper terminal Vs, and labeled Vn and Vp, and Vo, and a current I from right to left across the 1 k ohm resistor.
I then made the assumption this is an approximately ideal op-amp, and set
1) Vp = Vn, and Ip = In = 0.
Then I decided that
2) Req = Vs / I
Then,
3) I = (Vo - Vp) / 2k
4) I = Vp / 1k
1, 3, 4 => Vo = 3Vp = 3Vn
Then I did a nodal analysis at the node with Voltage Vn.
(Vn - Vs) / 10k + (Vn - Vo)/10k = 0
Then subbing 3Vn for Vo,
Vs = -Vn
Then combining equ. 4 and 2,
Vs/Vp = Req K ohms
and then subbing Vn for Vp and then -Vs for Vn,
Vs/-Vs = Req K ohms
Req = -1 k ohms
But the answer is supposed to be 5 k ohms. Did I make a mistake or interpret the question incorrectly?
Attachments
-
29.9 KB Views: 79