OP amp circuit doubt???????

[malli_1729]

hi!

Here i am a circuit and some text from application note, in which i have doubt.
First one 3 circuits 3.26A , B and C.Next 2 are matters regarding the circuits attached.

in 1.jpg, given as

"Noise gain is increased, due to that sensitivity to input offset voltage and Noise are increased

"
what is noise gain and How these input offset voltage depend on it.

in 2.jpg, Given as

3.36A is sensitive to impedance of Vref. this is due to signal current flows into/from Vref, due to Vin operating the op amp in its CM Range.where for other circuits CM range is fixed

Generally CM range for an opamp will be fixed and Known from the datasheet.But why here it is given CM range is fixed in B and C and not for A.
one more doubt
signal current flows into/from Vref means current from Vi or Vout which will flow through Vref

I don't know whether i can clear or not...i am brushing my basics...it will be great help to if any body explain these things...

thankQ

 

[malli_1729]

Sharing any thing which you people know regarding this, will be very helpful to me.

thank you...

 

[LvW]

Quote: what is noise gain and How these input offset voltage depend on it.


The formula for noise gain is given with figure 3.26.
Noise gain is the gain which applies to all internal signals within the opamp - referenced to its input nodes. In particular, this applies to all noise sources and the input offset voltage. With other words: Noise and input offset voltage appear at the opamp output multiplied with a gain factor called "noise gain".

 

[malli_1729]

Thank you Lvw,

How we can calculate it....
why they are calculating noise gain as a Non inverting amplifier, why not inverting configuration..
in 3.26A Noise gain = 1+R2/R1; why not -R2/R1
3.26B Noise gain = 1+R2/(R1||R3) why not -R2/(R1||R3)
same as in 3.26C also..

thanking you,

 

[FvM]

The OP configuration "seen" by the noise and offset sources is always non-inverting. A positive input offset is always generating a positive offset at the output, independant of the feedback factor.

 

[LvW]

The OP configuration "seen" by the noise and offset sources is always non-inverting. A positive input offset is always generating a positive offset at the output, independant of the feedback factor.

Yes, you can check it by yourself: Introduce a small voltage source between the inverting input terminal and the common node of the feedback resistors.
As a result of the calculation - this voltage appears at the output multiplied by (1+R2/R1).
This is no surprise at all, since the noise gain is independent on external sources.

 

[malli_1729]

Yes, You Both are Right. I have calculated it .

thank you.
I am just posting the top post again..here

Circuit 3.36A is sensitive to impedance of Vref. this is due to signal current flows into/from Vref, due to Vin operating the op amp in its CM Range.where for other circuits CM range is fixed

Generally CM range for an opamp will be fixed and Known from the datasheet.But why here it is given CM range is fixed in B and C and not for A.

in 3.26A,B and C Vref is connected to any one of the inputs of op amp through resistors then why then mentioned circuit 3.26A only sensitive to Vref and others are not


thank you,

 

[FvM]

Part of your problem is due to unexact phrasing of the said application note, I think. There is no fixed CM range with circuit B and C rather than a constant CM voltage, independant of Vin.

I'm sure, you'll find the answer yourself by playing around with the circuits using pencil and paper (or a circuit simulator, if you prefer). Simply apply variable Vin and calculate the resulting node voltages.