Hi All,
This note concerns the flicker noise in pre-amplifiers which may be=
of=20
interest to those who have a coil resistance that is greater than 5 K ohms=
..=20
Chopper amplifiers such as the LMV2011 etc.are said to be free of fli=
cker=20
noise but the shot noise voltage is large at 35nv / root Hz. For a 5k ohm=
source=20
resistance the noise figure (N.F.) is about 12 db and for a band width of=
0.05=20
to 5 Hz gives a total noise of about 100 nv.
The LT1007 op amp has a flicker noise voltage level of 10-17 volts sq=
uared=20
/ f and a flicker noise current of 2*10-23 amps squared / f , this gives=
an=20
optimum source resistance of about 700 ohms and a N.F. of 15.5 db at 0.05=
Hz,=20
though at 5 K ohms this rises to 21 db. This particular IC has two pairs=
of=20
transistor and 6 noise current sources which give it the high current nois=
e=20
value and are unchangeable.
The Hitachi transistor 2SC2545 noise perfor=
mance=20
is very low indeed with regard to flicker noise and the data is given as=
N.F.=20
contour on a log of collector current versus log of source impedance graph=
at=20
1Kc/s and 10 c/s. It has been possible to extract the principle flic=
ker=20
coefficients from this for a simple model. The flicker noise voltage remai=
ns=20
constant but the flicker noise current depends on the collector current sq=
uared=20
and at a collector current of 0.055 ma, gives an optimum source=20
impedance equal to 5K ohms. A pair of these transistors would=
give a=20
spot noise at 0.05 Hz equal to the chopper amplifier but the total no=
ise=20
for a 0.05 to 5 c/s bandwidth of about 1/6. This bandwidth is calcul=
ated=20
for a simple RC high and low pass filter.
It may appear a small improve=
ment=20
but every bit may help.
Martin