
11
from the circuit, a typical value
for R3 is 1K
Ω. Rc is a feedback
element that keeps I
d constant.
The value of Rc is approximated
by assuming a 0.5V drop across it;
see equation below. For 27 mA I
d,
5Volt Vcc bias, a typical value for
R1 is 680
Ω and R2 is 180Ω. A CAD
program such as Avago Tech-
nologies ADS is recommended
to determine the values of R1 and
R2 at other bias levels. The value
of the RF choke should be large
compared to 50
Ω, typical value
for a 1.9 GHz amplifier would be
22 nH. The DC blocking capacitors
are calculated as described above.
A typical value for C3 would be
1.0 uF.
Rc = 0.5
Ω
I
d
The active bias solution will only
require about a 1.3V difference
between Vcc and V
d for good bias
stability over temperature. For
more details on the active bias
circuit please refer to application
note AN-A003 Biasing MODAMP
MMICs.
RFC
Vd
C2
C3
Vcc
26x
C1
Rc
R3
R2
R1
Figure 4. Active Bias Circuit.
1.9 GHz Design
To illustrate the simplicity of using
the MSA-2643, a 1.9 GHz ampli-
fier for PCS type applications is
presented. The amplifier uses a
5V, 27 mA supply. The input and
output of the MSA-2643 is already
well matched to 50
Ω and no ad-
ditional matching is needed.
RFC=
22 nH
C2=18 pF
C3=330 pF
Vcc=5V
26x
C1=18 pf
Rc=150
Ω
Figure 5. Schematic of 1.9 GHz Circuit.
A schematic diagram of the
complete 1.9 GHz circuit with DC
biasing is shown in Figure 5. DC
bias is applied to the MSA-2643
through the RFC at the RF Output
pin. The power supply connection
is bypassed to ground with capaci-
tor C3. Provision is made for an
additional bypass capacitor, C4,
to be added to the bias line near
the +5 volt connection. C4 will not
normally be needed unless several
stages are cascaded using a com-
mon power supply.
The input terminal of the
MSA-2643 is not at ground
potential, an input DC blocking
capacitor is needed.
The values of the DC blocking and
RF bypass capacitors should be
chosen to provide a small reac-
tance (typically < 5 ohms) at the
lowest operating frequency. For
this 1.9 GHz design example, 18 pF
capacitors with a reactance of 4.5
ohms are adequate. The reactance
of the RF choke (RFC) should be
high (i.e., several hundred ohms)
at the lowest frequency of op-
eration. A 22 nH inductor with a
reactance of 262 ohms at 1.9 GHz
is sufficiently high to minimize the
loss from circuit loading.
Table 2. Component Parts List for the
MSA-2643 Amplifier at 1.9 GHz.
R1
56
Ωchipresistor
RFC
nHLL1608-FHN
C1,C
18pFchipcapacitor
C3
330pFchipcapacitor
The completed 1.9 GHz amplifier
for this example with all compo-
nents and SMA connectors as-
sembled is shown in Figure 6.
IN
OUT
Vcc
MSA-2X43
IP 4/00
Agilent Technologies
26x
Figure 6. Complete 1.9 GHz Amplifier.
Performance of MSA-2643 1.9 GHz
Amplifier
The amplifier is biased at a Vcc of
5 volts, I
d of 27 mA. The measured
gain, noise figure, input and output
return loss of the completed am-
plifier is shown in Figure 7. Noise
figure is a nominal 3.8 to 4.0 dB
from 1800 through 2000 MHz. Gain
is a minimum of 15.4 dB from
1800 MHz through 2000 MHz. The
amplifier output intercept point
(OIP3) was measured at a nomi-
nal +20.7 dBm. P
-1dB measured
+8.8 dBm.
FREQUENCY (GHz)
Figure 7. Gain, Noise Figure, Input
and Output Return Loss Results.
GAIN,
NOISE
FIGURE,
INPUT
and
OUTPUT
RETURN
LOSS
(dB)
Output RL
Input RL
Noise
Gain
24
16
8
0
-8
-16
-24
1.5
2.3
1.9
1.7
2.1
Figure 7. Gain, Noise Figure, Input and Output
Return Loss Results.