Hi,
Is the following equation correct for calculating a simple j-fet amp noise?
The positive battery terminal is going to an R, which then goes to the N-channel J-FET, which then goes to ground. The total gain is 9.4.
Vnoise = (4 * K * T * (Rd + Rs) * dF)^0.5 + (8/3 * K * T * 1/gm * dF)^0.5 + (KF * IDaf * dF / f)^0.5 + shot noise
I have a circuit in LTspice using a 2N5434, which has Rd and Rs at 1. Beta, which I believe is gm, is 18E-3 and AF is 1. LTspice doesn't have KF for this fet, so I don't know the 1/f noise. Hopefully 1/f noise will not dominate at 500 Hz, but I'm sure it will not at say 10+ KHz. According to LTspice this basic fet amp works best with an ~80 ohm resister since this fet has such high gm.
According to my calculations at 290 Kelvin and dF = 1Hz I get 1.9 nV +
1/f noise. Also there's shot noise, but I read it's usually neglected.Does 1.9 nV output noise sound close for this gain of 10 fet amp? I'll be using this amp for extremely weak ac signals, like 10 nV input, from
500 Hz to 700 KHz and highly inductive input source, 100 mH. Amp linearity is of very little importance. The main goal is to just detect very weak signals. LTspice shows just over 66 mA DC through the fet & resister with an 8V battery.Thanks, Paul
The LTspince .asc file:
Version 4 SHEET 1 880 680 WIRE -16 288 -32 288 WIRE 128 288 64 288 WIRE 176 32 176 0 WIRE 176 144 176 112 WIRE 176 336 176 320 WIRE 288 224 176 224 FLAG 176 336 0 FLAG -32 288 0 FLAG 288 224 out FLAG 176 0 0 SYMBOL voltage 80 288 R90 WINDOW 3 24 104 Invisible 0 WINDOW 39 0 0 Left 0 SYMATTR InstName V1 SYMATTR Value SINE(0 10e-9 723 0) SYMATTR Value2 AC 0 0 SYMBOL njf 128 224 R0 SYMATTR InstName J1 SYMATTR Value 2N5434 SYMBOL res 160 128 R0 SYMATTR InstName R1 SYMATTR Value 80 SYMBOL Misc\\\\battery 176 128 R180 WINDOW 0 24 104 Left 0 WINDOW 3 24 16 Left 0 SYMATTR InstName V2 SYMATTR Value 8 SYMATTR Value2 AC 0 0 TEXT 270 74 Left 0 !.tran 0 10e-3 0 1e-7