of device’s ground pins results in
slightly improved performance
while decreasing potential insta-
bilities, especially at higher
frequencies. The disadvantage is
that a negative supply voltage is
required.
The gate bias method would not
normally be used unless a negative
supply voltage was readily avail-
able. For reference, this is the
method used in the characteriza-
tion test circuits shown in Figures
1 and 2 of the MGA-725M4 data
sheet.
60
50
40
30
20
OUTPUT
INPUT
& V
d
10
0
Source Resistor Bias
The source resistor method is the
simplest way of biasing the
MGA-725M4 using a single,
positive supply voltage. This
method, shown in Figure 4, places
the RF input at DC ground and
requires both of the device
grounds to be RF bypassed.
Device current, Id, is determined
by the value of the source resis-
tance, Rbias, between either Pin 1
and Pin 3 of the MGA-725M4 and
DC ground. Pin 1 and Pin 3 are
connected internally in the RFIC.
Maximum device current (ap-
proximately 65 mA) occurs for
Rbias= 0Ω.
0
20
40
60
80 100 120 140
R
(Ω)
bias
Figure 5. Device Current vs. Rbias
.
V
ref
Figure 2. Gate Bias Method.
The approximate value of the
external resistor, Rbias, may also
be calculated from:
DC access to the input terminal
for applying the gate bias voltage
can be made through either a RF
or high impedance transmission
line as indicated in Figure 2.
964
R
=
(1 – 0.112 I )
d
√
bias
I
d
where Rbias is in ohms and Id is the
desired device current in mA. The
source resistor technique is the
preferred and most common
The device current, Id, is deter-
mined by the voltage at Vref (Pin 2)
with respect to ground. A plot of
typical Id vs Vref is shown in
Figure 3. Maximum device current
(approximately 65 mA) occurs at
Vref = 0.
method of biasing the MGA-725M4.
Adaptive Biasing
OUTPUT
& V
INPUT
d
2
4
For applications in which input
power levels vary over a wide
range, it may be useful to dynami-
cally adapt the bias of the
MGA-725M4 to match the signal
level. This involves sensing the
signal level at some point in the
system and automatically adjust-
ing the bias current of the ampli-
fier accordingly. The advantage of
adaptive biasing is conservation of
supply current (longer battery life)
by using only the amount of
1
3
The device current may also be
estimated from the following
equation:
R
bias
Vref = 0.11√ Id – 0.96
Figure 4. Source Resistor Bias.
where Id is in mA and Vref is in
volts.
A simple method recommended
for DC grounding the input
terminal is to merely add a
50
40
30
20
resistor from Pin 2 to ground, as
shown in Figure 4. The value of
the shunt R can be comparatively
high since the only voltage drop
across it is due to minute leakage
currents that in the mA range. A
value of 1kΩ would adequately DC
ground the input while loading the
RF signal by only 0.2 dB loss. A
plot of typical Id vs Rbias is shown
in Figure 5.
current necessary to handle the
input signal without distortion.
Adaptive biasing of the
MGA-725M4 can be accomplished
by either analog or digital means.
For the analog control case, an
active current source (discrete
device or IC) is used in lieu of the
source bias resistor. For simple
digital control, electronic switches
10
0
-0.8 -0.7
-0.6
-0.5
(V)
-0.4
-0.3
-0.2
V
ref
Figure 3. Device Current vs. Vref
.
14
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