ATF-34143 查看數據表（PDF） - Avago Technologies
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ATF-34143 Datasheet PDF : 14 Pages
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Noise Parameter Applications Information
Fmin values at 2 GHz and higher are based on measure-
ments while the Fmins below 2 GHz have been extrapo-
lated. The Fmin values are based on a set of 16 noise figure
measurements made at 16 different impedances using an
ATN NP5 test system. From these measurements, a true
Fmin is calculated. Fmin 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 50Ω, to an impedance represented
by the reflection coefficient o. The designer must design
a matching network that will present 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 Fmin
only when the device is presented with o. If the reflection
coefficient of the matching network is other than o, then
the noise figure of the device will be greater than Fmin
based on the following equation.
NF = Fmin + 4 Rn |s – o | 2
Zo (|1 + o| 2)(1–s|2)
Where Rn/Zo is the normalized noise resistance, o is the
optimum reflection coefficient required to produce Fmin
and s is the reflection coefficient of the source imped-
ance 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 ampli-
fier noise figure. The losses of the matching networks
are related to the Q of the components and associated
printed circuit board loss. o is typically fairly low at higher
frequencies and increases as frequency is lowered. Larger
gate width devices will typically have a lower o as com-
pared to narrower gate width devices.
Typically for FETs, the higher o usually infers that an im-
pedance much higher than 50Ω is required for the device
to produce Fmin. 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 components 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 Fmin 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 Application 1085.
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