Help with EE homework

[tontod]

I just started taking a EE class in op amps. Its been a while since I have last used this material, and I have gotten rusty. If any resident EE's can help me out, that would be great. I need help on the 1st and 3rd problem :

Text

 

[dighn]

just remember that: v- ~= v+ and input z ~= infinity and the problems shouldn't be too hard to solve

 

[tontod]

Ok, vp=vn, but not zero, right? And the direction of current flow is towards Vo?

 

[dighn]

Originally posted by: tontod
Ok, vp=vn, but not zero, right? And the direction of current flow is towards Vo?

well since the op amps here do not have gain, i guess you should assume ideal op amp, so vp - vn = 0

yes current towards vo

 

[tontod]

Originally posted by: dighn

Originally posted by: tontod
Ok, vp=vn, but not zero, right? And the direction of current flow is towards Vo?

well since the op amps here do not have gain, i guess you should assume ideal op amp, so vp - vn = 0

yes current towards vo

Thanks. Going by those assumptions, I am getting -1V for Vp and Vn, and 3V for Vo for the 1st problem. Does that look right?

 

[dighn]

well actually i get -5v for vout becaues current is going from vn -> vo so it's vn - vo = 4v

 

[tontod]

Originally posted by: dighn
well actually i get -5v for vout becaues current is going from vn -> vo so it's vn - vo = 4v

Oh, you get -5 for Vo, by adding -1 with -4? With -1 being Vn and -4 being the drop across the 4k ohm resistor?

 

[dighn]

well the current is going from vn to vo, so vn is + and vo is -. so using ohms law you get

(vn - vo)/4k = 1ma =>
vn - vo = 4v =>
vo = vn - 4v | vn = -1 so vo = -5

 

[tontod]

Originally posted by: dighn
well the current is going from vn to vo, so vn is + and vo is -. so using ohms law you get

(vn - vo)/4k = 1ma =>
vn - vo = 4v =>
vo = vn - 4v | vn = -1 so vo = -5

Ah ok, I made a mistake in that I thought vn-vo=-4V, I just had my signs switched, I did get vn=-1 correctly then. Thanks for your help dighn.

 

[tontod]

Going by the same logic, for the last problem, vn-vo=3v, vn=vp=vo*(2/5)? Substituting and solving I get vo=-5V. Does that look right?