14
Noise Parameter Applications Information
F
min
values at 2 GHz and higher are based on measure-
ments while the F
mins
below 2 GHz have been extrapo-
lated. The F
min
values are based on a set of 16 noise
gure measurements made at 16 di erent impedances
using an ATN NP5 test system. From these measure-
ments, a true F
min
is calculated. F
min
represents the true
minimum noise gure of the device when the device is
presented with an impedance matching network that
transforms the source impedance, typically 50ý, to an
impedance represented by the re ection coe cient G
o
.
The designer must design a matching network that will
present G
o
to the device with minimal associated circuit
losses. The noise gure of the completed ampli er is
equal to the noise gure of the device plus the losses
of the matching network preceding the device. The
noise gure of the device is equal to F
min
only when
the device is presented with G
o
. If the re ection coef-
cient of the matching network is other than G
o
, then
the noise gure of the device will be greater than F
min
based on the following equation.
NF = F
min
+ 4 R
n
|
s
–
o
|
2
Zo (|1 +
o
|
2
)(1 - |
s
|
2
)
Where R
n
/Z
o
is the normalized noise resistance, G
o
is
the optimum re ection coe cient required to produce
F
min
and G
s
is the re ection coe cient of the source
impedance actually presented to the device. The losses
of the matching networks are non-zero and they will
also add to the noise gure of the device creating a
higher ampli er noise gure. The losses of the matching
networks are related to the Q of the components and
associated printed circuit board loss. G
o
is typically fairly
low at higher frequencies and increases as frequency is
lowered. Larger gate width devices will typically have a
lower G
o
as compared to narrower gate width devices.
Typically for FETs, the higher G
o
usually infers that an
impedance much higher than 50ý is required for the
device to produce F
min
. At VHF frequencies and even
lower L Band frequencies, the required impedance can
be in the vicinity of several thousand ohms. Matching
to such a high impedance requires very hi-Q compo-
nents in order to minimize circuit losses. As an example
at 900 MHz, when airwwound coils (Q>100) are used for
matching networks, the loss can still be up to 0.25 dB
which will add directly to the noise gure of the device.
Using muiltilayer molded inductors with Qs in the 30
to 50 range results in additional loss over the airwound
coil. Losses as high as 0.5 dB or greater add to the
typical 0.15 dB F
min
of the device creating an ampli er
noise gure of nearly 0.65 dB. A discussion concerning
calculated and measured circuit losses and their e ect
on ampli er noise gure is covered in Avago Technolo-
gies Application 1085.
