Helmut Sennewald wrote:
Just a typical op-amp circuit:
Make R4 open, and then make R3, R1, and R2 1 uOhm and you get fV/Hz1/2 noise. There seems to be no lower limit. Then make both R3 & R4 open while leaving R1 & R2 shorted and you get the opposite-- to much noise.
At 780 Hz my LTspice gave a gain of about 6000 for the NTE937M, but I'm sure open loop gains can greatly vary between parts. I just wanted to see a ballpark figure, so I didn't take the time to measure my NTE937M exact gain, but it seems the noise is definitely in the low two digit nV range. I was just surprised to see such high noise from an open loop LT1028N.
Now that I think about it, I guess an air gap would generate noise, even a vacuum gap. If you have potential then the electrons will accelerate in the vacuum, so the energy has to go somewhere. Perhaps an ideal noiseless resister is one that directs the energy somewhere besides current. I know radiation resistance is noise free. Here's another thought. Take a circuit with a DC voltage source and two resisters. Lets say there's 1 volt across resister B. That resister is generating noise. Now replace that resister with a 1 volt battery. Essentially at DC it's the equivalent circuit except less noise. :) I believe magnetic viscosity is another example of a noiseless resistance.
BTW, I'm now trying to measure power mosfet noise at 20+ pulsed amps. Am I wasting my time trying to get an accurate figure with LTspice? I tried adding KF and AF to the IRF530, but it doesn't seem to change the noise in any way. Also for some reason the input voltage to the mosfet doesn't seem to change the noise. Increasing the mosfet's input voltage equates to more output current. In real life I'd expect more current to equal more noise.