Definitions for op amps?

[24Volts]

Hello,

I have done some op amp calculations, but I am a little confused about equation definitions. In reference to attachment, can someone please confirm the following definitions in respect to gains:

Inverting configuration:
G1 = -rf/ra

Non-Inverting configuration:
G1 = -rf/ra
G2= (r2/(r1+r2)) (1+(rf/ra))

QUESTIONS:
1) Isn't this a gain on its own as well (and what do we call it):
(r2/(r1+r2))

2) And isn't this a gain on its own as well (and what do we call it):
(1+(rf/ra))

Coonfused

 

[Laplace]

Attached image extracted from Texas Instruments Application Report SLAA068A - April 2000 "Understanding Basic Analog – Ideal Op Amps"

It explains how to analyze the op-amp differential amplifier circuit shown in your image.

 

[24Volts]

Hi LaPlace, I would like to ask you a question... if I may!

In reference to another attachment in this post, I know it is possible to make an input ramp to the non-inverting side from 4 to 6Vdc and an output ramp of 2 to 3Vdc.... with r1 = 7500 and r2 = 2500 ohms and with ra and rf being 10K.

But why is it we can't do an input ramp of 4 to 6Vdc with an output ramp of 2 to 4Vdc


G2 = (4-2)/(6-4) = 1

So now how do we figure out the resistors???

 

[BobK]

removed, incorrect solution.

Bob

 

[24Volts]

Hi Bob,

okay, but what I meant to ask is.... is there any values of resistors
that would satisfy that circuit for that I/o ramp without modifying or adding
anything to the circuit.

and if not, then why? we seem to be able to do it with ramp output of 2 to 3 Vdc
why not 2 to 4 Vdc?

 

[BobK]

Actually, my solution above would not work.

You cannot do it because a non-inverting amplifier cannot have a gain of 1.

There should be a way to make a difference ampliifer to subtract 2 from the input, but I am failing at coming up with the right configuration.

Bob

 

[BobK]

Okay, I have got it.

Use the differential configuration (here)

http://en.wikipedia.org/wiki/File:Op-Amp_Differential_Amplifier.svg

And set all R the same.

V2 is your input, V1 is set to 2V and you get your input (4-6) - 2 = (2-4)V out.

Bob

 

[24Volts]

okay...

You cannot do it because a non-inverting amplifier cannot have a gain of 1.

is this because of:

(1 + rf/ra) can't be equal to 1 ???

thanks BobK I appreciate your help

 

[Laplace]

But why is it we can't do an input ramp of 4 to 6Vdc with an output ramp of 2 to 4Vdc ???

Recall from Algebra I that a straight line is defined by a slope and zero intercept. Your circuit, regardless of the resistor values, has a zero intercept of zero, i.e., zero at the input produces zero at the output. So instead of one of the circuit inputs being grounded, it would need to be adjusted to provide the zero offset for your defined transfer characteristic (which is non-zero).

Also, a non-inverting amplifier with a gain of 1 is called a voltage follower.

 

[24Volts]

thanks all for your help.