[SOLVED] op amp with both positive and negative feedback resistances

[srimatresri]

what is the approach to solve a problem like this?

 

[zorro]

First observe that it must be R3<R2 (otherwise it is unstable).
Pose 3 equations considering that input currents are 0 and the amplifier has finite gain A:

1) V+ in function of V1 and Vo (resistive divider)
2) V- in function of V2 and Vo
3) Vo = A(V+ - V-)

Solve for Vo and make A -> infinity.
Regards

Z

 

[user_hostile]

What's the instability due to (i. e., "R3<R2")? Divide-by-zero?

I created an Excel spreadsheet implementing the circuit, but two questions:

1) What would it be used for? Offsetting an input? Level-shifting?

2) I can't quite grasp 3) "Vo = A(V+ - V-)". How does 'A' fit into this? I just solved for Vo without
using any gain (KCL).

 

[ic98]

Use the principles of circuit feedback theory from Gray & Meyer's book on "Analysis And Design Of Analog Integrated Circuits"

 

[user_hostile]

Not exactly the answers I'm looking for. I'm currently broke and can't afford purchasing Gray and Meyer's book.