# Op amp problem

1. Feb 22, 2009

### hani114

1. The problem statement, all variables and given/known data

everything is attached in the image below

2. Relevant equations

Ip=In=0 Vp=Vn

3. The attempt at a solution
i used the two rules that vp=vn & ip=in=0

so i tink no current flows in Vc & Vd and the 60kohm resistor.. as for Va & Vb we apply kcl on the input voltage node and solve for Vo

current in Va is Ia and so forth.. and current in 375kohm(Re) resistor is Ie

so Ia + Ib + Ie = 0
Va/Ra + Vb/Rb + Vo/Re = 0
so we calculate Vo from that

for the second part we need to calculate the range for Vb so that -20<Vo<20
so i think we have to put Vo in terms of Vb and replace that in the range so that we get Vb

can anyone help me with this??? it's late now and that's the best i can do..

#### Attached Files:

• ###### prob.jpg
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2. Feb 23, 2009

### Redbelly98

Staff Emeritus
Since the 60k is connected to ground, current can flow in from the Vc and Vd sources without going through the + pin on the op-amp.