Unity Gain Buffer: Why R1 Must be Infinity

[seang]

all of the electronics textbooks say that you can make a unity gain buffer out of a non inverting amplifier by making R2 = 0 and R1 = infinity. Why does R1 have to be infinite? The TF for a non inverting amp is 1 + r2/r1, so as long as r2 = 0, and R1 should make a unity gain buffer, right?

Is this just a practical consideration, IE why waste a resistor, if you don't need it?

 

[mgb_phys]

You don't actaully use r1 or (r2) in an actual unity gain circuit you just connect Vout to -ve in.
It is set to infinity in the maths just so you don't have to consider the potential divider, you can't really of course have an infinite resistance just like you can't have r2=0.

 

[Corneo]

They do have 0 Ohm resistors. Why? I don't know.

 

[mgb_phys]

They do have 0 Ohm resistors. Why? I don't know.

A few reasons,
Sometimes you need to connect a link on some versions of a board, you would normally just use a link but for surface mount the pick-place machines can handle resistors. Alternatively they are useful for confusing students!

 

[stewartcs]

A few reasons,
Sometimes you need to connect a link on some versions of a board, you would normally just use a link but for surface mount the pick-place machines can handle resistors.

I always called these jumpers!

 

[mgb_phys]

I always called these jumpers!

On surface mount they are the same package as resistors and called 0 ohm.
I assume because it simplifies the pick-system rather than having a separate class of jumper parts.

 

[mheslep]

http://en.wikipedia.org/wiki/Image:Opampvoltagefollower.svg" the usual configuration on wiki (inverting)