Unity Gain Buffer: Why R1 Must be Infinity

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Discussion Overview

The discussion revolves around the concept of a unity gain buffer using a non-inverting amplifier configuration, specifically focusing on the requirement for R1 to be infinite and R2 to be zero. Participants explore the implications of these resistor values in both theoretical and practical contexts.

Discussion Character

  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant questions the necessity of R1 being infinite, suggesting that with R2 at zero, a unity gain buffer should theoretically be achievable.
  • Another participant clarifies that in practice, R1 and R2 are not used in a unity gain circuit, as the output is directly connected to the inverting input, and the infinite resistance is a mathematical simplification.
  • Some participants mention the existence of 0 Ohm resistors, discussing their practical applications in circuit design and board configurations.
  • There is a suggestion that 0 Ohm resistors are used for convenience in surface mount technology, as they can be handled by pick-and-place machines similarly to standard resistors.

Areas of Agreement / Disagreement

Participants express differing views on the necessity and implications of R1 being infinite and R2 being zero, indicating that there is no consensus on the practical versus theoretical aspects of these resistor values.

Contextual Notes

Participants acknowledge that while infinite resistance and zero resistance are used in theoretical discussions, they cannot be physically realized, which may lead to confusion in practical applications.

Who May Find This Useful

This discussion may be of interest to electronics students, engineers working with operational amplifiers, and those involved in circuit design and analysis.

seang
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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?
 
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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.
 
They do have 0 Ohm resistors. Why? I don't know.
 
Corneo said:
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!
 
mgb_phys said:
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!
 
stewartcs said:
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.
 
http://en.wikipedia.org/wiki/Image:Opampvoltagefollower.svg" the usual configuration on wiki (inverting)
 
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