Low Noise Amplifier Design
Amplifier Specification:
Center Frequency
2.4GHz
Bandwidth
±5%
Noise Figure
< 3 dB
Gain
As High as Possible
Source Impedance
50 ohms
Load Impedance
Relative Permittivity of
Substrate (ε)
Thickness of the Substrate
(h)
50 ohms
Thickness of the conductor
1.2 mil
Transistor
Fujitsu FHX35LG HEMT
2.31
31.5 mil
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
Go
input
matching
circuit
GL
output
matching
circuit
transistor
[S]
Γs
Γin
Γout
Zo
ΓL
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
2
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 SParameter. 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 Ґin plane.
e) For a unilateral case use the constant gain circles for
the desired or maximum gain. For a bilateral case use
the available power gain circle for the desired or
maximum gain. Plot any of these circles accordingly
in the Ґin plane.
f) Choose a ҐS value which is in the stable region as
well as with in the Noise circle, the corresponding
circle gain circle. We don’t use operating power gain
circles as it has to be plotted in Ґout plane. So it is
difficult to correlate with the noise circles in Ґin plane.
g) Calculate the corresponding ҐL. From these Ґ values
obtain the corresponding impedance values ZS and 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 the
input side, the matching network must transform Z0
to ZS. This can be done using lumped elements like
LC based network or distributed elements like open
or short circuited stubs combined with a length of a
transmission line.
j) Provide DC bias for the Q point taken from the data
sheets and take care that it is transparent with the RF
operation.
Calculated parameters and design:
The S-Parameter file FHX35LG.s2p for the given transistor
is downloaded from Fujitsu website. This file is given as an
input 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 as
NFmin = 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
6
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 transform
50 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 > 1
NFmin = 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
9
The corresponding impedance values are
ZS = 5.3 + j6.643
ZL = 33.815 + j6.211
The second matching networks are as given below
DC Bias Network:
The operating conditions for FHX35LG are read from the
data sheet as:
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Ids = 10 mA
Vgs = -0.4V
Vds = 3.0V
The 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 is
shown in the next page.
The initial simulations to test the RF performance were
done with the S-Parameter file of the transistor. This model
doesn’t need biasing circuits. But later the transistor model
with a foot print was used, needs inclusion of biasing
network.
The transmission line dimensions for the Impedance
Matching Networks and DC Bias Circuit was calculated
using Linecalc – a utility of ADS.
After initial Simulations the schematic was transferred in to
a layout specific schematic – means some modifications to
ensure 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 the
performance. This is shown in figure 5.2 below
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13
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The Layout of the amplifier was done using the layout
conversion tool of ADS and it is as shown below
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Simulation and results
The circuit was simulated for S-parameter simulation using
ADS and various parameters and their performances were
noted 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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