Please help!!! Op-amps

Thread Starter


Joined Jan 9, 2012
Going back to college next week to do electronics exam after 6 year away and really need help with the question below. Any help would be great. Thanks

In the case of a non inverting op am determine
-the attenuation factor B
-the closed loop input impedance
-closed loop output impedance
-closed loop voltage gain


Input impedance (Zin) 4MΩ
Output impednace (Zout) 60Ω
Open loop gain (Aol) 200,000
Input resistor 15KΩ
Feedback resistor 180KΩ

So i think the folrmulas

attenuation factor = B = Ri / (Ri + Rf)
closed loop input impedance Zin(NI) = (1 + Aol B) Zin
closed loop output impedance Zout(NI) = Zout / (1 + Aol B)
closed loop voltage gain = Acl(NI) = 1 + (Rf/Ri)

But have am not sure what figures to sub in where
I presume
Ri = 4MΩ
Rf = 180KΩ

B = 4,000,000 / (4,000,000 + 180,000) = 0.9569
Acl(NI) = 1 + (180,000 / 4,000,000) = 1.045

and that is about as far as i get... any help would be great. Thanks


Joined Jan 29, 2011
> Ri is the input resistance and not the input impedance as you assumed.
> substitute the values for Ri and Rf in your formulas to get correct result for B and Acl.