MGA-725M4 Datasheet, PDF (15/19 Pages) Agilent(Hewlett-Packard) – Low Noise Amplifier with Bypass Switch In Miniature Leadless Package

control, electronic switches can be used to control the
value of the source resistor in discrete increments. Both
methods of adaptive biasing are depicted in Figure 6.
Applying the Device Voltage
Common to all methods of biasing, voltage V_d is applied
to the MGA-725M4 through the RF Output connection
(Pin 4). A RF choke is used to isolate the RF signal from the
DC supply. The bias line is capacitively bypassed to keep
RF from the DC supply lines and prevent resonant dips or
peaks in the response of the amplifier. Where practical, it
may be cost effective to use a length of high impedance
transmission line (Preferably λ /4) in place of the RFC.
When using the gate bias method, the overall device
voltage is equal to the sum of V_ref at Pin 2 and voltage V_d
at Pin 4. As an example, to bias the device at the typical
operating voltage of 3 volts, V_d would be set to 2.5 volts
for a V_ref of -0.5 volts. Figure 7 shows a DC schematic of a
gate bias circuit.
Just as for the gate bias method, the overall device
voltage for source resistor biasing is equal to V_ref + V_d.
Since V_ref is zero when using a source resistor, V_d is the
same as the device operating voltage, typically 3 volts. A
source resistor bias circuit is shown in Figure 8.
V_d = +2.5 V
RFC
RF
Output
RF
Input
Vref = -0.5 V
Figure 7. DC Schematic for Gate Bias.
V_d = +2.5 V
RFC
RF
Input
R_bias
RF
Output
Figure 8. DC Schematic of Source Resistor ­Biasing.
A DC blocking capacitor at the output of the RFIC isolates
the supply voltage from succeeding circuits. If the source
resistor method of biasing is used, the RF input terminal
of the MGA-725M4 is at DC ground potential and a
blocking capacitor is not required unless the input is
connected directly to a preceding stage that has a DC
voltage present.
Biasing for Higher Linearity or Output Power
While the MGA-725M4 is designed primarily for use up
to 50 mA in 3 volt applications, the output power can be
increased by using higher currents and/or higher supply
voltages. If higher bias levels are used, appropriate
caution should be observed for both the thermal limits
and the Absolute Maximum Ratings.
As a guideline for operation at higher bias levels, the
Maximum Operating conditions shown in the data sheet
table of Absolute Maximum Ratings should be followed.
This set of conditions is the maximum combination of
bias voltage, bias current, and device temperature that is
recommended for reliable operation. Note: In contrast to
Absolute Maximum Ratings, in which exceeding may one
parameter may result in damage to the device, all of the
Maximum Operating conditions may reliably be applied
to the MGA-725M4 simultaneously.
Input
2
Output
4
& V_d
1
3
Analog
Control
V_ref
(a) Analog
Figure 6. Adaptive Bias Control.
15
Input
2
Output
2
& V_d
13
Analog Control
V_ref
(b) Digital