ATF-38143 查看數據表（PDF） - Avago Technologies
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ATF-38143 Datasheet PDF : 12 Pages
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Noise Parameter Applications Information
Fmin values at 2 GHz and higher are based on
measurements while the Fmins below 2 GHz have been
extrapolated. 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
Zo
|s – o | 2
(|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
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. 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 compared to narrower gate width devices.
Typically for FETs, the higher o usually infers that an
impedance 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 air-wound 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 muilti-layer
molded inductors with Qs in the 30 to 50 range results
in additional loss over the air-wound 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.
9
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