DEMO-ATF-5X143 Datasheet, PDF (14/16 Pages) Broadcom Corporation. – Low Noise Enhancement Mode Pseudomorphic HEMT in a Surface Mount Plastic Package

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
figure measurements made at 16 different impedances
using an ATN NP5 test system. From these measure-
ments, a true F_min is calculated. F_min represents the true
minimum noise figure 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 reflection coefficient G_o.
The designer must design a matching network that will
present G_o to the device with minimal associated circuit
losses. The noise figure of the completed amplifier is
equal to the noise figure of the device plus the losses
of the matching network preceding the device. The
noise figure of the device is equal to F_min only when
the device is presented with G_o. If the reflection coef-
ficient of the matching network is other than G_o, then
the noise figure of the device will be greater than F_min
based on the following equation.
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 figure 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 amplifier
noise figure of nearly 0.65 dB. A discussion concerning
calculated and measured circuit losses and their effect
on amplifier noise figure is covered in Avago Technolo-
gies Application 1085.
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 reflection coefficient required to produce
F_min and G_s is the reflection coefficient 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 figure of the device creating a
higher amplifier noise figure. 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.
14