Noise voltage from t-network feedback in opamp circuit

[jrive]

I'm struggling to compute the noise voltage due to the thermal noise on the resistors of an op-amp circuit, configured as a t-network.

If it were a single feedback resistor, the noise voltage referenced to the output (RTO)would be given by sqrt(4KTRB).

I'm not sure how to combine them. I mean, do I combine the resistors into an equivalent one using Thevenin equivalent, for ex, and use that as the R value for the johnson noise?
If I'm referencing the noise to the output, I assume I'm looking from the inverting terminal out toward the output of the op-amp, true? Assuming R1 is connected between inverting input and center node of T, R2 is connected from center node to gnd, and R3 is connected from center node to output of opamp, the thevening equivalent R is R1||R2+R3. Can I use that in the calculation for the noise voltage of the resistor network?

Thanks.

 

[Baluncore]

You are correct. Rth = R1 + (R2 // R3)
The noise due to Rth appears to be applied to the inverting input, referenced to ground. The noise will be amplified by the circuit in the same way as the non-inverting input signal.

Is the op-amp input bias current really so high that R1 is needed in the circuit?