Differential Pair Input Noise Issue

I suggest you look at the LSK170 if you want lower noise levels and can stand the input capacitance.

The basic rule for resistors in low noise circuits is to make them either as large as you can or as small as you can. The one on the gate of the JFET should be small or perhaps zero.

Noise current is proportional to the square root of the total current, and resistor (Johnson) noise is proportional to the square root of the resistance value; by going for a higher current and lower resistors at that point, the designer has minimised the noise contribution atthat particular point.

Small enough the op amp doesn't swing to the rails (and saturate its internal gain paths) under normal operating conditions.

-- Bill Sloman, Nijmegen

thanks, your explanation is the best even though I read your post for a couple of times. 2k ohm is a big value based on my understanding of thermal noise that's why I posted this question here. Right now I understand the author's words-he thinks 2k ohm is small enough to minimize thermal noise. In stead I think the author should pick a op-amp running under lower supply voltages then pick a smaller resistor.

** Do not top post !!!!!!!!!!

** But the voltage noise of those 2N5486 fets is much higher - and it rises sharply at low frequencies. ** Did you come here to ask questions or give lectures ?

.... Phil

rises

It's usually not as simple as that; You need enough gain in the first stage to make sure that the op amp input noise is lower than the noise from your long-tailed pair of FETs; these are two independent noise sources so the total noise is the square root of the sum of their squares, which means that it s rarely worth getting the lesser of the two noise sources below half the bigger one.

Don't mind Phil - he's obnoxious, but he does know his electronics.

-- Bill Sloman, Nijmegen