Agilent Technologies
IP 4/00
MSA-2X43
IN
OUT
Vcc
Figure 3. Multi-purpose Evaluation Board.
The amplifier and related compo-
nents are assembled onto the
printed circuit board as shown in
Figure 6. The MSA-2X43 circuit
board is designed to use edge-
mounting SMA connectors such as
Johnson Components, Inc., Model
142-0701-881. These connectors
are designed to slip over the edge
of 0.031-inch thick circuit boards
and obviate the need to mount
PCBs on a metal base plate for
testing. The center conductors of
the connectors are soldered to the
input and output microstrip lines.
The ground pins are soldered to
the ground plane on the back of
the board and to the top ground
pads.
DC blocking capacitors are
required at the input and output of
the IC. The values of the blocking
capacitors are determined by the
lowest frequency of operation for
a particular application. The
capacitor’s reactance is chosen to
be 10% or less of the amplifier’s
input or output impedance at the
lowest operating frequency. For
example, an amplifier to be used
in an application covering the
900 MHz band would require an
input blocking capacitor of at least
39 pF, which is 4.5Ω of reactance
at 900 MHz. The Vcc connection to
the amplifier must be RF bypassed
by placing a capacitor to ground at
the bias pad of the board. Like the
DC blocking capacitors, the value
of the Vcc bypass capacitor is
determined by the lowest operat-
ing frequency for the amplifier.
Space is available on the circuit
board to add a bias choke, bypass
capacitors, and collector resistors.
The MSA series of ICs requires a
bias resistor to ensure thermal
stability. The bias resistor value is
calculated from the operating
current value, device voltage and
the supply voltage; see equation
below. When applying bias to the
board, start at a low voltage level
and slowly increase the voltage
until the recommended current is
reached. Both power and gain can
be adjusted by varying I
d
.
Rc =
Vcc – V
d
Ω
I
d
taken into account. The character-
ization data in section one shows
the relationship between V
d
and I
d
over temperature. At lower
temperatures the value of V
d
increases. The increase in V
d
at
low temperatures and production
variations may cause potential
problems for the amplifier perfor-
mance if it is not taken into
account. One solution would be to
increase the voltage supply to
have at least a 4V drop across the
bias resistor Rc. This will guaran-
tee good temperature stability.
Table 1 shows the effects of Rc on
the performance of the MSA-2743
over temperature.
An alternative solution to ensure
good temperature stability without
having a large voltage drop across
a resistor would be to use an
active bias circuit as shown in
Figure 4. The resistors R1 and the
PNP transistor connected to form
a diode by connecting the base
and collector together and R2
form a potential diver circuit to
set the base voltage of the bias
PNP transistor. The diode con-
nected PNP transistor is used to
compensate for the voltage
variation of the base-emitter
junction with temperature of the
bias PNP transistor. R3 provides a
bleed path for any excess bias; it
Where:
Vcc = The power supply voltage
applied to Rc (volts)
V
d
= The device voltage (volts)
I
d
= The quiescent bias current
drawn by the device
Notes on Rc Selection
The value of Rc is dependant on
V
d
, any production variation in V
d
will have an effect on I
d
. As the
gain and power performance of
the MSA-2743 may be adjusted by
varying I
d
this will have to be
Table 1. Effects of Rc on Performance over Temperature.
Device voltage = 3.9 V nominally at 25°C.
Voltage
Drop, volts
0
Resistor
Value, Ohms
0
Temperature,
°C
0
25
85
0
25
85
0
25
85
0
25
85
Bias Current,
mA
41.8
50.0
66.8
47.3
50.0
56.5
47.8
50.0
53.5
49.1
50.0
51.8
Power Gain @
2.0 GHz, dB
15.2
15.1
15.0
15.3
15.1
14.9
15.2
15.1
14.8
15.3
15.1
14.9
1.35
27
2.35
47
6.0
120
10
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