Avago Technologies MGA-725M4 - Manual (Page 3)

3
Setting Device Current
The MGA-725M4 incorporates an
adjustable bias feature. The
MGA-725M4 is typically operated
within a device current range of
10-60 mA. A current of 10-20 mA
is appropriate for LNA
applications. Biasing the amplifier
at currents of 40-60 mA allows it
to deliver higher output power
levels for use as a transmit driver
stage.
Device current is set by the
resistance between Pin 4 (or
Pin 1) of the MGA-725M4 and DC
ground. For evaluation purposes,
an external resistor is suggested
for setting the bias current. The
external resistor is connected
between the Vcon terminal and
ground. Alternatively, the current
can be fixed by installing resistor
R2 onto the PCB as indicated in
Figure 2 and in the schematic
diagram in Figure 3. The
approximate value of the
current-setting resistor is
calculated from:
Where R
bias
is in ohms and I
d
is
the desired device current in mA.
If desired, the MGA-725M4 can
also be biased by directly
grounding device Pins 1 and 4 and
applying a negative control
voltage to Pin 3. The connection
labeled V
in
is provided for this
purpose. (Note : R
bias
is not
used with this method of biasing.)
R
bias
= 1 - 0.112
964
I
d
I
d
Operation
To operate the MGA-725M4
demonstration amplifier, it is only
necessary to apply +3 volts to the
+V
d
connection. If an on-board
resistor is used to set the current,
the V
con
terminal is left open.
Switch Bypass Mode
The MGA-725M4 is placed in the
bypass mode by setting the device
current to zero. This is done by
open-circuiting the bias resistor,
R
bias
.
A Note on Performance
Actual performance of the
MGA-725M4 as measured in the
demonstration circuit may not
exactly match the data sheet
specifications. The circuit board
material, passive components, RF
bypasses and connectors all
introduce losses and parasitics
that degrade device performance.
For this demonstration circuit,
circuit losses of about 0.3 dB
would be expected at both the
input and output sides of the
RFIC at 2 GHz.