Elektronik für Physiker - Physik-Institut

KAPITEL 1. STROM, SPANNUNG, WIDERSTAND 28
generated in the channel corresponds to the variance of an equivalent noise voltage Vinp
at the gate input, as shown in the Figure, (c), of
d < V 2
inp >
= 4 k T
df
2
3 gm
(1.51)
where gm = ∂Idrain/∂Vgate denotes the forward transconductance of the FET (see
[Laker94], 1-65a/b). Since g2 m depends linearly on the channel’s geometrical width di-
vided by the length and on its channel current, low noise input amplifier need to be
designed with wide channels and high currents in the input FET.
Pure capacitances do not generate any thermal noise.
In complex system often additional noise, whith a 1/f spectrum is observed, which
is called flicker noise. It has different physical origins. For high frequency applications
it can usually be neglected.