Broadcom Design Tip: 2.4 GHz application using MGA-71543, a Low Noise Amplifier User guide
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Revision 19/07/01 Page 1
2.4 GHz application using MGA-71543, a Low
Noise Amplifier with Bypass Switch.
_________________________
INTRODUCTION
:
The MGA-71543 is a single stage GaAs RFIC
low noise amplifier with an integrated bypass
switch (Figure 1).
Figure 1. MGA-71543. Functional Diagram
The purpose of the switch feature is to prevent
distortion of high signal levels in receiver
applications by bypassing the amplifier.
Furthermore it saves current thus improving
battery life.
One of the two ground leads of the amplifier is
tied to the control leads of the switch to allow
simple control of the bias. Therefore, using one
of the ground leads, it is possible to vary the bias
current (including completely shutting down the
amplifier) by varying the resistor to ground that
is attached to it.
In short, the MGA-71543 is a small LNA/Bypass
Switch MMIC that provides a low Noise figure,
a high gain and high third order input intercept
point (IIP3) especially suitable for the LNA stage
in applications.
This application note describes a 2.4GHz Low
noise amplifier design using Agilent
Technologies’ MGA-71543.
Design overview and summary:
The board used for this design is shown in
figure2. The schematic of this board is shown
in figure 3. The number of components used
can be reduced in a real application circuit
because some jumpers (0 Ω resistors), and
bypass capacitors can be eliminated without
performance degradation (some are only used
for ease, or if tuning is necessary and also as a
precaution to avoid any possible oscillation).
Matching and biasing
A simple shunt L of 1.3nH at the input provides
the noise matching and the necessary DC
grounding of the input pin. Series L is shown as
0R link, but a small amount of inductance here
of typically 0.5nH does improve return loss and
NF slightly. Such a small lumped element
becomes impractical although individual PCB
designs should be ale to incorporate this into
tracking. Because the main active device is a
depletion mode FET, source-biasing technique is
used so that only one positive DC supply is
required at the output. The input (gate) is DC
grounded and the source is RF bypassed, while
current setting resistors (R20 or R21) are
attached to ground.
A simple shunt-series network at the output does
the required matching for linearity at lower
frequencies but at 2.4GHz it was found that the
addition of a shunt L element vastly improved
output and input return loss. The arrangement
therefore consists of a 3.9nH shunt element
which also provides the DC feed / RF choke,
with a series 3.3pF cap. The shunt element is
placed directly across the output connector. The
output-matching network does not affect the
noise figure, which was already set by the input
matching.
R38 and R25 provide some resistor loading at
low frequencies. R37 improves stability at lower
frequencies but may be omitted depending on
each application. Omission will increase gain
slightly. The rest of the components used,
including bypass capacitors (C8, C36, C37, C44,
C47), current setting resistors (R20 or R21),
voltage drop resistor (R24), DC block capacitors,
are all listed in table 1.
Performance Results
It is possible to achieve 3.7dBm of input Third
order intercept Point (IIP3) at 2.4GHz with a
device current of 11mA.
RF IN
RF OUT
SW & Bias Control
2
Conclusion
The MGA-71543 can be used for 2.4GHz
applications with the following typical
performance summary data:
NF Gain IIP3 S11 S22
1.2 16 3.7 20 15
Current = 11mA
Frequency: 2.1GHz
Description4 layer Board
Designation B
WCDMA-2100
Part Number Package
U2 or 71
DUT
*
MGA-71543 SOT-343 (SC-70) / Switch LNA
U4 or 03 Switch b/n Gnd resistors FDG6303N Dual N-channel, Digital FET
C12 3.3pF
Size 0402/Output match, DC block
C8
0.033 µ
µµ
µF
Size 0402 / Bypass capacitor
C9 6.8 pF
Size 0402 / DC block capacitor
C38 Not used
C36, C37, C44 82, 82, 27 pF
Size 0402 / Bypass capacitors
C47
0.033 µ
µµ
µF
Size 0402 / Bypass capacitor
L5 3.9 nH
TOK LL1005 0402/Out match/RF choke,DC feed
L6 1.3 nH
TOK LL1005 Size 0402 / input noise matching
L7
0 Ω
ΩΩ
Ω
0402 Jumper see notes
Lshunt 2.7nH
Improves rloss, fit to connector
L25 Not used For tuning / Not used here
R38
51 Ω
ΩΩ
Ω
Size low frequency loading
R20
36 Ω
Size 0402 / set Id for 16mA Bias
R21
56 Ω
ΩΩ
Ω
Size 0402 / set Id for 10.5mA Bias
R24, R25
6.2 Ω
ΩΩ
Ω
R24Drop voltage,R25resistive load
R16,R17
0 Ω
ΩΩ
Ω
Size 0402 (Jumper)
R37
10Ω
ΩΩ
Ω
Size 0402 improves k, omit w care
R18,R28 Not Used Used for other current setting
Table 1. Component Values for 2400 MHz amplifier on smaller board
Software
controlling
the switch
Manual switch
control
3
Figure 2. Test board layout
U4 = FDG6303N
Dual N-channel, Digital
FET
Figure 3. Test board layout and
schematic
!
L7
L6
R38
C8
C37
C9
R20
R21
R37
R25
R24
L5
C47
C44
C12
R16 (0O Jumper)
R17 (0O Jumper)
OUTIN
MGA-71543
FDG6303N
Vd=3 Volt
Selects current
set by R20
Selects current
set by R21
L
Lshunt
R=
L=2.7 nH
4
ADS Model results from the above test PCB.
m1
freq=2.400E9Hz
S(1,1)=0.106 / -51.986
impedance = Z0 * (1.123 - j0.19
m2
freq=2.400E9Hz
S(2,2)=0.172 / -140.345
impedance = Z0 * (0.749 - j0.17
0
m1
freq=2.400E9Hz
S(1,1)=0.106 / -51.986
impedance = Z0 * (1.123 - j0.19
m2
freq=2.400E9Hz
S(2,2)=0.172 / -140.345
impedance = Z0 * (0.749 - j0.17
0
freq (1.000GHz to 3.000GHz)
S(1,1)
m1
S(2,2)
m2
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0.90
0.95
1.00
1.05
1.10
1.15
1.20
freq, GHz
our_k
NFmin
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
-25
-20
-15
-10
-5
0
freq, GHz
dB(S(1,1))
dB(S(2,2))
1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0
0
2
4
6
8
10
12
14
16
18
20
freq, GHz
dB(S(2,1))
-
1
-
2
-
3
-
4
Broadcom Design Tip: 2.4 GHz application using MGA-71543, a Low Noise Amplifier User guide
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