while simultaneously providing a
DC block to the RF signal. The
bias decoupling network shown in
Figure 8, consisting of resistor R1,
a short length of high impedance
microstripline, and bypass
capacitor C1, provides the best
overall performance in the 2 to
8 GHz frequency range.
The use of lumped inductors is
not desired since they tend to
radiate and cause undesired
feedback. Moving the bypass
capacitor, C1, down the micro-
stripline towards the V
dd
terminal,
as shown in Figure 9, will improve
the gain below 2 GHz by trading
off some high end gain. A
minimum value of 10
Ω
for R1 is
Ground (Pins 2 and 4)
Ground pins should attach
directly to the backside ground
plane by the shortest distance
possible using the design hints
suggested in the earlier section.
Liberal use of plated through vias
is recommended.
RF Input (Pin 1)
A 50
Ω
microstripline can be used
to feed RF to the device. A
blocking capacitor in the 10 pF
range will provide a suitable DC
block in the 2 to 6 GHz frequency
range. Although there is no
voltage present at pin 1, it is
highly suggested that a DC
blocking capacitor be used to
prevent accidental application of
a voltage from a previous
amplifier stage. With no further
input matching, the MGA-86576 is
capable of noise figures as low as
2 dB in the 2 to 6 GHz frequency
range. Since
Γ
o
is not 50
Ω,
it is
possible to design and implement
a very simple matching network
in order to improve noise figure
and input return loss over a
narrow frequency range. The
circuit board layout shown in
Figure 7 provides an option for
tuning for a low noise match
anywhere in the 1.5 to 4 GHz
frequency range. For optimum
noise figure performance in the
4 GHz frequency range, L1 can be
a 0.007 inch diameter wire
0.080 inches in length as shown in
Figure 9. Alternatively, L1 can be
replaced by a 0.020 inch wide
microstripline whose length can
be adjusted for minimum noise
figure in the 1.5 to 4 GHz
frequency range.
Table 1 provides the approximate
inductor length for minimum
noise figure at a given frequency
for the circuit board shown in
Figure 7.
Table 1. L1 Length vs. Frequency
for Optimum Noise Figure.
Frequency
GHz
1.5
1.8
2.1
2.4
2.5
3.0
3.7
4.0
Length
Inches
0.70
0.60
0.50
0.40
0.30
0.20
0.10
0.05
Figure 9. Complete MGA-86576
Demonstration Amplifier.
recommended to de-Q the bias
decoupling network, although
100
Ω
will provide the highest
circuit gain over the entire 1.5 to
8 GHz frequency range. V
dd
will
have to be increased accordingly
for higher values of R1. For
operation in the 2 to 6 GHz
frequency range, a 10 pF capacitor
may be used for DC blocking on
the output microstripline. A larger
value such as 27 pF is more appro-
priate for operation at 1.5 GHz.
7 Volt Bias for Operation at
Higher Temperatures
The MGA-86576 was designed
primarily for 5 volt operation over
the -25 to +50°C temperature
range. For applications requiring
use to +85°C, a 7 volt bias supply
is recommended to minimize
changes in gain and noise figure at
elevated temperature. Figure 10
shows typical gain, noise figure,
and output power performance
over temperature at 4 GHz with
7 volts applied. With a 7 volt bias
supply, output power is increased
approximately 1.5 dB. Other
parameters are relatively
unchanged from 5 volt data.
S-parameter and noise parameter
data for 7 volts are available upon
request from Hewlett-Packard.
6-232
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
