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
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 Senior Applications Engineer High Performance Linear
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