13
RF
OUTPUT
AMPLIFIER
BYPASS MODE
RF
INPUT
Application Information: Designing with the
MGA-725M4 RFIC Ampli er/Bypass Switch
Description
The MGA-725M4 is a single stage GaAs RFIC ampli er
with an integrated bypass switch. A functional diagram
of the MGA-725M4 is shown in Figure 1.
Without external matching, the input return loss for
the MGA-725M4 is approximately 5 dB at 1900 MHz. If
desired, a small amount of NF can be traded o for a
signi cant improvement in input match. For example,
the addition of a series inductance of 2.7 to 3.9 nH at the
input of the MGA-725M4 will improve the input return
loss to grater than 10 dB with a sacri ce in NF of only
0.1 dB.
The output of the MGA-725M4 is internally matched to
provide an output SWR of approximately 2:1 at 1900
MHz. Input and output matches both improve at higher
frequencies.
Driver Ampli er Applications
The exibility of the adjustable current feature makes the
MGA-725M4 suitable for use in transmitter driver stages.
Biasing the ampli er at 40–50 mA enables it to deliver
an output power at 1 dB gain compression of up to +16
dBm. Power e ciency in the unsaturated driver mode is
on the order of 30%. If operated as a saturated ampli er,
both output power and e ciency will increase.
Since the MGA-725M4 is internally matched for low noise
gure, it may be desirable to add external impedance
matching at the input to improve the power match for
driver applications. Since the reactive part of the input of
the device impedance is capacitive, a series inductor at
the input is often all that is needed to provide a suitable
match for many applications. For 1900 MHz circuits, a
series inductance of 3.9 nH will match the input to return
loss of approximately 13 dB. As in the case of low noise
bias levels, the output of the MGA-725M4 is already well
matched to 50Ω and no additional matching is needed
for most applications.
When used for driver stage applications, the bypass
switch feature of the MGA-725M4 can be used to shut
down the ampli er to conserve supply current during
non-transmit period. Supply current in the bypass stage
is nominally 2 mA.
Biasing
Biasing the MGA-725M4 is similar to biasing a discrete
GaAs FET. Passive biasing of the MGA-725M4 may be
accomplished by either of two conventional methods,
either by biasing the gate or by using a source resistor.
Gate Bias
Using this method, Pins 1 and 3 of the ampli er are DC
grounded and a negative bias voltage is applied to Pin 2
as shown in gure 2. This method has the advantage of
not only DC, but also RF grounding both of the ground
pins of the MGA-725M4. Direct RF grounding of device’s
ground pins results in slightly improved performance
while decreasing potential instabilities, especially at
higher frequencies. The disadvantage is that a negative
supply voltage is required.
Figure 1. MGA-725M4 Functional Diagram.
The MGA-725M4 is designed for receivers and transmit-
ters operating from 100 MHz to 6 GHz with an emphasis
on 800 MHz and 1.9 GHz CDMA applications. The MGA-
725M4 combines low noise performance with high
linearity to make it especially advantageous for use in
receiver front-ends.
The purpose of the switch feature is to prevent distor-
tion of high signal levels in receiver applications by
bypassing the ampli er altogether. The bypass switch
can be thought of as a 1-bit digital AGC circuit that not
only prevents distortion by bypassing the MGA-725M4
amplifier, but also reduces front-end system gain by
approximately 16 dB to avoid overdriving subsequent
stages in the receiver such as the mixer.
An additional feature of the MGA-725M4 is the ability to
externally set device current to balance output power ca-
pability and high linearity with low DC power consump-
tion. The adjustable current feature of the MGA-725M4
allows it to deliver output power levels in excess of +15
dBm (P
1dB
), thus extending its use to other system appli-
cation such as transmitter driver stages.
The MGA-725M4 is designed to operate from a +3-volt
power supply and is contained in miniature Minipak 1412
package to minimize printed circuit board space.
LNA Application
For low noise ampli er applications, the MGA-725M4 is
typically biased in the 10–20 mA range. Minimum NF
occurs at 20 mA as noted in the performance curve of
NF
min
vs I
d
. Biasing at currents signi cantly less than 10
mA is not recommended since the characteristics of the
device begin to change very rapidly at lower currents.
The MGA-725M4 is matched internally for low NF. Over a
current range of 10–30 mA, the magnitude of G
opt
at 1900
MHz is typically less than 0.25 and additional impedance
matching would only net about 0.1 dB improvement in
noise gure.
