# noise caculate about amplifier!

Hellow,  ENGINEER;

I  have  caculate the following   structure  noise   ,but   I   dong't  know  how  to   caculate  the   inveresting  current  noise    that  flow  through   the  gain   resitor   ,can  the  current  flow  through  the  feed back  network of   R1  and   R2   or  RG? I   WANT  to  know  the  path of  the  the current noise?thank  you

• Lei,

The best way to analyze this amplifier is to split it through the middle of Rg and analyze each half separately.  After doing this you have two separate amplifiers that can be analyzed as non-inverting amplifiers with gain = 1 + RF / (0.5Rg) .

For a standard inverting amplifier the current noise can be converted to a noise voltage referred to the input.  The equation for this noise voltage is Vn = In x [ Rf || (0.5Rg) ].  Based on th size of the resistors in your example I do not expect the total noise to be significant compared to the op amp voltage noise.

The attached article describes details about noise analysis on a three amplifier instrumentation amplifier.  The front end of this amplifier is the same as your circuit.

Art Kay

Art Kay Senior Applications Engineer High Performance Linear

INA-noise10-Mar 2010.pdf