Although the input terminal of the
MGA-81563 is at ground potential,
it is not a current sink. If the input
is connected to a preceding stage
that has a voltage present, the use
of the DC blocking capacitor (C1)
is required.
C2
C4
RFC
RF
Input
C1
L1
C2
RF
Output
V
d
bias lines that could cause oscilla-
tion. C4 will not normally be
needed unless several stages are
cascaded using a common power
supply.
When multiple bypass capacitors
are used, consideration should be
given to potential resonances. It is
important to ensure that the
capacitors when combined with
additional parasitic L’s and C’s on
the circuit board do not form
resonant circuits. The addition of
a small value resistor in the bias
supply line between bypass
capacitors will often “de-Q” the
bias circuit and eliminate the
effect of a resonance.
The value of the DC blocking and
RF bypass capacitors (C1 – C3)
should be chosen to provide a
small reactance (typically
< 5 ohms) at the lowest operating
frequency. The reactance of the
RF choke (RFC) should be high
(e.g., several hundred ohms) at
the lowest frequency of operation.
The MGA-81563’s response at low
frequencies is limited to approxi-
mately 100 MHz by the size of
capacitors integrated on the
MMIC chip.
Figure15. SchematicDiagram.
DC bias is applied to the MGA-
81563 through the RF Output pin.
An inductor (RFC), or length of
high impedance transmission line
(preferably
λ/4
at the band
center), is used to isolate the RF
from the DC supply.
The power supply is bypassed to
ground with capacitor C3 to keep
RF off of the DC lines and to
prevent gain dips or peaks in the
response of the amplifier.
An additional bypass capacitor,
C4, may be added to the bias line
near the V
d
connection to elimi-
nate unwanted feedback through
The input of the MGA-81563 is
partially matched internally to
50
Ω.
Without external matching
elements, the input VSWR of the
MGA-81563 is 3.0:1 at 300 MHz
and decreases to 1.5:1 at 6 GHz.
This will be adequate for many
applications. If a better input
VSWR is required, the use of a
series inductor, L1 in the applica-
tions example, (or, alternatively a
length of high impedance trans-
mission line) is all that is needed
to improve the match. The table in
Figure 16 shows suggested values
for L1 for various wireless fre-
quency bands.
Frequency
(GHz)
0.9
1.5
1.9
2.4
4.0
5.8
Figure16. ValuesforL1.
Inductor, L1
(nH)
10
6.8
3.9
2.7
0.5
0
These values for L1 take into
account the short length of 50
Ω
transmission line between the
inductor and the input pin of the
device.
For applications requiring mini-
mum noise figure (NF
o
), some
improvement over a 50
Ω
match is
possible by matching the signal
input to the optimum noise match
impedance,
Γ
o
, as specified in the
“Typical Noise Parameters” table.
For most applications, as shown
in the example circuit, the output
of the MGA-81563 is already
sufficiently well matched to 50
Ω
and no additional matching is
needed. The nominal device
output VSWR is
≤
2.2:1 from
300 MHz through 6 GHz.
The completed application
amplifier with all components and
SMA connectors is shown in
Figure 17.
H
C1
L1
C2
IN
RFC
C3
+V
OUT
MGA-8-A
Figure 17. Complete Application Circuit.
C4
6-202
AGILENT [ AGILENT TECHNOLOGIES, LTD. ]
