Design of differential amplifier

[kasse]

Homework Statement

http://i36.tinypic.com/hugkyb.jpg

Homework Equations

According to my book, if R₃ = R₂ and R₄ = R₁, I can use the following formula:

V_o = (R₂/R₁)(V₂ - V₁).

The Attempt at a Solution

We want R₂/R₁ to equal 5. This means that R₃ and R₂ must be 5*3,3 = 16.5 k(ohms).

However, I don't know what to do with the inner resistance of V₂ and V_o. Any suggestions?

 

[dlgoff]

For an ideal op-amp, the input current is zero.