mga-725m4 Avago Technologies, mga-725m4 Datasheet - Page 18

• Manufacturer Part Number: mga-725m4
• Description: Low Noise Amplifer With Bypass Switch In Miniature Leadless Package
• Manufacturer: Avago Technologies

Figure 14. Schematic Diagram of 1900 MHz Evaluation Amplifier.
A complete evaluation amplifier optimized for use at
1900 MHz is shown with all related components and
SMA connectors in Figure 13. A schematic diagram of the
evaluation circuit is shown in Figure 14 with component
values in Table 1.
Table 1. Component Values for 1900 MHz Amplifier.
The on-board resistors R3 and R4 form the equivalent
source bias resistor R
diagram in Figure 14. In this example, resistor values of
R3=10Ω and R4=24Ω were chosen to set the nominal
device current for the four states: (1) bypass mode, 0 mA,
(2) LNA mode, 20 mA, (3) driver, 35 mA, and (4) driver,
40 mA.
Other currents can be set by positioning the DIP switch
to the bypass state and adding an external bias resistor to
V
the V
provided for the both the input and output.
The 2-pin, 0.100" centerline single row headers attached
to the V
venient means of making connections to the board using
either a mating connector or clip leads.
18
Input
RF�
R1
R2
R3
R4
L1
RFC
SW1, SW2
SC
con
. Unless an external resistor is used to set the current,
R2
con
V
d
d
terminal is left open. DC blocking capacitors are
and V
C1
C3
C0
con
R1
=5.1 kΩ
=5.1kΩ
=10Ω
=24Ω
=3.9nH
=22nH
DIP switch
Short
connections on the PCB provide a con-
L1
C
C0
bias
C4
as indicated in the schematic
C5
SW1
SW2
C
C0
C1
C2
C3
C4
C5
C6
C
R
R3
R4
bias
C6
RFC
=100pF
=1000pF
=100pF
=47pF
=30pF
=22pF
=22pF
=30pF
Vcon
C0
C2
C
Output
RF�
A Note on Performance
Actual performance of the MGA-725M4 as measured in
an evaluation circuit may not exactly match the datasheet
specifications. The circuit board material, passive compo-
nents, RF bypasses and connectors all introduce losses
and parasitics that degrade device performance.
For the evaluation circuit above, fabricated on 0.031-
inch thick GETEK G200D (e
circuit losses of about 0.3 dB would be expected at both
the input and output sides of the RFIC at 1900 MHz.
Measured noise figure (3 volts, 20 mA bias) would then
be approximately 1.8 dB and gain 13.8 dB.
Hints and Troubleshooting
Preventing Oscillation
Stability of the MGA-725M4 is dependent on having very
good RF grounding. Inadequate device grounding or
poor PCB layout techniques could cause the device to be
potentially unstable.
Even though a design may be unconditionally stable (K>1
and B1>0) over its full frequency range, other possibili-
ties exist that may cause an amplifier circuit to oscillate.
One condition to check for is feedback in the bias circuit.
It is important to capacitively bypass the connections to
active bias circuits to ensure stable operation. In multi-
stage circuits, feedback through bias lines can also lead
to oscillation.
Components of insufficient quality for the frequency
range of the amplifier can sometimes lead to instabil-
ity. Also, component values that are chosen to be much
higher in value than is appropriate for the application can
present a problem. In both of these cases, the compo-
nents may have reactive parasitics that make their imped-
ances very different than expected. Chip capacitors may
have excessive inductance or chip inductors can exhibit
resonances at unexpected frequencies.
A Note on Supply Line Bypassing
Multiple bypass capacitors are normally used throughout
the power distribution within a wireless system. Consider-
ation should be given to potential resonances formed by
the combination of these capacitors and the inductance
of the DC distribution lines. The addition of a small value
resistor in the bias supply line between bypass capacitors
will often de-Q the bias circuit and eliminate resonance
effects.
r
=4.2) dielectric material,