bipolar current source

[Hraesvelg]

Can anyone help me with a formula for the following circuit?

 

[Russlk]

Well, obviosly, the positive feedback will drive the output voltage to VCC unless there is a load on it, so put a load on the output then try to solve the problem.

 

[Hraesvelg]

This is strictly theoretical, I need a formula to solve this expression. Any hints?

 

[Roff]

This is probably homework, but I can't resist.

(Vp-Vi)/100k=(Vi-Vo)/100k, so
(1) Vp-Vi=Vi-Vo

Note that, assuming overall negative feedback, which you will have if Rload < infinity, that the op amp's inverting input voltage will equal Vi, the voltage on the noninverting input. Therefore,

(2) Vn-Vi=Vi-Vx

Combining (1) and (2),

Vx=2*Vi-Vn, Vo=2*Vi-Vp

Vx-V0=Vp-Vn=deltaVin

Iout=(Vx-Vo)/R, or

Iout=deltaVin/R

Q.E.D.

 

[Hraesvelg]