9
Step 4 (Optional) —
Input Impedance Match
As previously noted, the internal
input impedance match to the
MGA-83563 is already reasonably
good (return loss is typically
8 dB) and may be adequate for
many applications as is. The
design of the MGA-83563 is such
that the second stage will enter
into compression before the first
stage. The isolation provided by
the first stage therefore results in
a minimal impact on the input
matching as the amplifier
becomes saturated.
If an improved input return loss is
needed, an input circuit is de-
signed to match 50
Ω
to
Γ
ms
(the
reflection coefficient of the
source impedance for a conjugate
match at the input of the
MGA-83563). The value of
Γ
ms
is
calculated from the S-parameters
in Table 1 in the same way as was
done for
Γ
ml
. Since the real part
of the input impedance to the
MMIC is near 50
Ω
and the
reactive part is capacitive, the
addition of a simple series
inductor is often all that is needed
if a better input match is required.
Figure 21. Basic PCB Layout.
PCB Materials
FR-4 or G-10 printed circuit board
type of material is a good choice
for most low cost wireless
applications for frequencies
through 3 GHz. Typical single-
layer board thickness is 0.020 to
0.031 inches. Multi-layer boards
generally use a dielectric layer
thickness in the 0.005 to
0.010 inch range.
For higher frequency applica-
tions, e.g., 5.8 GHz, circuit boards
made with PTFE/glass dielectric
materials are suggested.
0.026
This layout is a microstripline
design (solid groundplane on the
backside of the circuit board)
with a 50
Ω
input and output and
provision for inductor L2 with its
bypass capacitor.
Adequate RF grounding is critical
to obtain maximum performance
and to maintain device stability.
All of the ground pins of the RFIC
should be connected to the RF
groundplane on the backside of
the PCB by means of plated
through holes (vias) that are
placed very near the package
terminals. As a minimum, one via
should be located next to each
ground pin to ensure good RF
grounding. It is a good practice to
use multiple vias as in Figure 21
to further minimize ground path
inductance.
While it might be considered an
effective RF practice, it is recom-
mended that the PCB pads for the
ground pins
not
be connected
together underneath the body of
the package for two reasons. The
first reason is that connecting the
ground pins of multi-stage
amplifiers together can some-
times result in undesirable
feedback between stages. Each
ground pin should have its own
independent path to ground. The
second reason is that PCB traces
hidden under the package cannot
be adequately inspected for
solder quality.
0.075
PCB Layout
Recommendations
When laying out a printed circuit
board for the MGA-83563, several
points should be taken into
account. The PCB layout will be a
balance of electrical, thermal, and
assembly considerations.
0.035
0.016
Figure 20. PCB Pad Layout for
MGA-83563 Package (dimensions in
inches).
Package Footprint
A recommended printed circuit
board footprint for the miniature
SOT-363 (SC-70) package that is
used by the MGA-83563 is shown
in Figure 20.
RF Considerations
Starting with the package
padprint of Figure 20, the nucleus
of a PCB layout is shown in
Figure 21. This layout is a good
general purpose starting point for
designs using the MGA-83563
amplifier.
83
This package footprint provides
ample allowance for package
placement by automated assem-
bly equipment without adding
parasitics that could impair the
high frequency performance of
the MGA-83563. (The padprint in
Figure 20 is shown with the
footprint of a SOT-363 package
superimposed on the PCB pads
for reference.)
V
d1
Bypass
Capacitor
RF
Input
L2
RF
Output
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
