LNA Report
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Transcript of LNA Report
Low Noise Amplifier Design
Amplifier Specification:
Center Frequency
Bandwidth
Noise Figure
Gain
Source Impedance
Load ImpedanceRelative Permittivity of
Substrate (ε)Thickness of the Substrate
(h)
Thickness of the conductor
Transistor
2.4GHz
±5%
< 3 dB
As High as Possible
50 ohms
50 ohms
2.31
31.5 mil
1.2 mil
Fujitsu FHX35LG HEMT
1
Design procedure for a given Noise Figure
A single stage transistor amplifier with matching networks
at the input and output terminals of the transistor are shown
below:
Zo Gs
inputmatchingcircuit
Γs Γin
Go
transistor[S]
ΓoutΓL
GL
outputmatchingcircuit
Zo
Figure 4.1
a) Check stability performance by calculating Rolette
Stability Factor K using the S-parameter of the
transistor at the given frequency and plot respective
stability circles to determine the potentially unstable
region in the Smith Chart for ҐS and ҐL.b) If the transistor is potentially unstable, it can be
stabilized by adding a ballast resistor at the drain or a
feedback resistor from drain to the source. But this
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would add to the noise figure. Otherwise we can go
for a conditionally stable design.c) Calculate unilateral figure of merit for choosing
unilateral or bilateral design. d) Calculate NFmin, Ґopt of the transistor from the S-
Parameter. Then choose a desirable noise figure
above the NFmin. Calculate the Centre and radius of
the Noise circle. Plot it in the smith chart for Ґ in plane.
e) For a unilateral case use the constant gain circles forthe desired or maximum gain. For a bilateral case use
the available power gain circle for the desired ormaximum gain. Plot any of these circles accordingly
in the Ґin plane.
f) Choose a ҐS value which is in the stable region aswell as with in the Noise circle, the corresponding
circle gain circle. We don’t use operating power gaincircles as it has to be plotted in Ґout plane. So it is
difficult to correlate with the noise circles in Ґ in plane.g) Calculate the corresponding ҐL. From these Ґ values
obtain the corresponding impedance values ZSand ZL.
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h) These are the impedance values for a given noise
figure, gain etc for the transistor. Now these have to
be matched with their corresponding Source and load
impedance Z0 (usually 50 ohms).
i) The impedance matching networks (Input and Output)
are designed using the smith chart. For e.g. in theinput side, the matching network must transform Z0
to ZS. This can be done using lumped elements likeLC based network or distributed elements like open
or short circuited stubs combined with a length of atransmission line.
j) Provide DC bias for the Q point taken from the data
sheets and take care that it is transparent with the RFoperation.
Calculated parameters and design:
The S-Parameter file FHX35LG.s2p for the given transistor
is downloaded from Fujitsu website. This file is given as aninput to a RF utility software called Appcad. Using Appcad
the following parameters are calculated at 2.4 GHz as:
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S11 = 0.925 ∟ -53.0°
S21 = 4.200 ∟ 132.5°
S12 = 0.050 ∟ 52.5°S22 = 0.500 ∟ -45.0°
K = 0.26 < 1
Stability:The transistor is potentially unstable.
The stability circles are as given below
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The unilateral figure of merit is found to be
-5.53dB < (Gt/Gtu) < 19.7dB
The error range is 25dB which is very high. So we should
use bilateral approach.Noise and Gain circles:
The Noise parameters at 2.4 GHz are calculated asNFmin = 0.42dB
Ґopt = 0.8 ∟38°Rn = 27.4
The desired Noise Figure and the Gain is selected as
1.5 dB and 12 dB
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The corresponding noise and available gain circles are
plotted as given below
Figure 4.4
The corresponding ҐS, ҐL, ZS, ZL are found as given
below
Figure 4.5
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Impedance Matching networks:
The Source impedance matching network should transform50 ohms to ZS and it is done as given below.
The load impedance matching network is as given below
The circuit is still not stable at certain frequencies. So a
ballast resistor of value 150 ohms was added at the drain. This value was obtained by tuning and optimization in
ADS.
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But this increases the noise figure to 3.128 dB which is not
acceptable. Therefore the entire circuit was treated as a 2
port network and the corresponding parameters were
calculated once again. The circuit was matched once again
at the source and load. The performance in terms of noise
figure, gain and stability was very good after the second
matching network.
Calculations for second matching:S11 = 0.806 ∟ -128.4°
S21 = 4.569 ∟ -140° S12 = 0.038 ∟ 146.3°
S22 = 0.031 ∟ 66.3°
K = 1.085 > 1NFmin = 1.177dB
Ґopt = 0.761 ∟163.86°Rn = 2.017
Then the ҐS, ҐL values are chosen for a noise figure of 2dB
and gain 11.5dB
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The corresponding impedance values are
ZS = 5.3 + j6.643
ZL = 33.815 + j6.211The second matching networks are as given below
DC Bias Network:
The operating conditions for FHX35LG are read from thedata sheet as:
Ids = 10 mA
Vgs = -0.4V
Vds = 3.0VThe DC biasing network is designed as given below
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Amplifier Simulation and Results
Schematics and Layout:
The Schematics for the amplifier drawn using ADS isshown in the next page.
The initial simulations to test the RF performance weredone with the S-Parameter file of the transistor. This modeldoesn’t need biasing circuits. But later the transistor modelwith a foot print was used, needs inclusion of biasingnetwork.
The transmission line dimensions for the ImpedanceMatching Networks and DC Bias Circuit was calculatedusing Linecalc – a utility of ADS.
After initial Simulations the schematic was transferred in toa layout specific schematic – means some modifications toensure a proper layout were done. These were
1) Adding Tee networks for branching in the layout.2) Adding via’s for grounding.3) Adding a small length of transmission line to connect
the pads of devices to other transmission lines.
All these modifications were done without affecting theperformance. This is shown in figure 5.2 below
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The Layout of the amplifier was done using the layoutconversion tool of ADS and it is as shown below
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Simulation and results
The circuit was simulated for S-parameter simulation usingADS and various parameters and their performances werenoted as given below.
S21 and gain at 2.4 GHz
Figure 5.4
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S11 and S22 (return losses):
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Noise Performance:
Stability:
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Measured results:
S11 and gain at 2.4 GHz
Figure 5.10
The measured gain at 2.4 GHz was 13.986 dB
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Return loss S11:
Figure 5.11
At 2.4 GHz -13.306dB
Figure 5.12
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