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MRFIC1818 Datasheet PDF : 8 Pages
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APPLICATIONS INFORMATION
Design Philosophy
The MRFIC1818 is a 3–stage Integrated Power Amplifier
designed for use in cellular phones, especially for those used
in DCS1800 (PCN) 4.8 V operation. With matching circuit
modifications, it is also applicable for use in DCS1900 (PCS)
equipment. Due to the fact that the input, output and some of
the interstage matching is accomplished off chip, the device
can be tuned to operate anywhere within the 1500 to
2000 MHz frequency range. Typical performance at different
battery voltages is:
• 36 dBm @ 6.0 V
• 34.5 dBm @ 4.8 V
• 32.0 dBm @ 3.6 V
This capability makes the MRFIC1818 suitable for portable
cellular applications such as:
• 6V and 4.8 V DCS1800 Class I
• 6V and 4.8 V PCS tag5
• 3.6 V DCS1800 Class II
RF Circuit Considerations
The MRFIC1818 can be tuned by changing the values
and/or positions of the appropriate external components. Re-
fer to Figure 2, a typical DCS1800 Class I applications circuit.
The input match is a shunt–L, series–C, High–pass structure
and can be retuned as desired with the only limitation being
the on–chip 6 pF blocking capacitor. For saturated applica-
tions such as DCS1800 and DCS1900, the input match
should be optimized at the rated RF input power. Interstage
matching can be optimized by changing the value and/or
position of the decoupling capacitor on the VD1 and VD2 sup-
ply lines. Moving the capacitor closer to the device or reduc-
ing the value increases the frequency of resonance with the
in–ductance of the device’s wirebonds and leadframe pin.
Output matching is accomplished with a one–stage low–
pass network as a compromise between bandwidth and har-
monic rejection. Implementation is through chip capacitors
W mounted along a 30 or 50 microstrip transmission line. Val-
ues and positions are chosen to present a 2.5 W loadline to
the device while conjugating the device output parasitics.
The network must also properly terminate the second and
third harmonics to optimize efficiency and reduce harmonic
output. Low–Q commercial chip capacitors are used for the
shunt capacitors, as shown in Figure 2. Loss in circuit traces
must also be considered. The output transmission line and
the bias supply lines should be at least 0.6 mm in width to
accommodate the peak circulating currents which can be as
high as 2 amperes under worst case conditions. The bias
supply line which supplies the output should include an RF
choke of at least 18 nH, surface mount solenoid inductors or
quarter wave microstrip lines. Discrete inductors will usually
give better efficiency and conserve board space. The DC
blocking capacitor required at the output of the device is best
W mounted at the 50 impedance point in the circuit where the
RF current is at a minimum and the capacitor loss will have
less effect.
Biasing Considerations
Gate bias lines are tied together and connected to the VSS
voltage, allowing gate biasing through use of external resis-
tors or positive voltages. This allows setting the quiescent
current of all stage in the same time while saving some board
space. For applications where the amplifier is operated close
to saturation, such as TDMA amplifiers, the gate bias can be
set with resistors. Variations in process and tempera–ture
will not affect amplifier performance significantly in these ap-
plications. The values shown in the Figure 1 will set quies–
cent currents of 20 to 40 mA for the first stage, 150 to 300 for
the second stage and 400 to 800 mA for the final stage. For
linear modes of operation which are required for CDMA am-
plifiers, the quiescent current must be more carefully con-
trolled. For these applications, the VG pins can be referenced
to some tunable voltage which is set at the time of radio
manufacturing. Less than 1 mA is required in the divider net-
work so a DAC can be used as the voltage source.
Power Control Using the MC33169
The MC33169 is a dedicated GaAs power amplifier sup-
port IC which provides the –4 V required for VSS, an N–MOS
drain switch interface and driver and power supply sequenc-
ing. The MC33169 can be used for power control in applica-
tions where the amplifier is operated in saturation since the
output power in non–linear operation is proportional to VD2.
This provides a very linear and repeatable power control
transfer function. This technique can be used open loop to
achieve 40–45 dB dynamic range over process and temper-
ature variation. With careful design and selection of calibra-
tion points, this technique can be used for DCS1800 control
where 30 dB dynamic range is required, eliminating the need
for the complexity and cost of closed–loop control. The trans-
mit waveform ramping function required for systems such as
DCS1800 can be implemented with a simple Sallen and Key
filter on the MC33169 control loop. The amplifier is then
ramped on as the VRAMP pin is taken from 0 V to 3 V. To im-
plement the different power steps required for DCS1800, the
VRAMP pin is ramped between 0 V and the appropriate volt-
age between 0 V and 3 V for the desired output power. For
closed–loop configurations using the MC33169,
MMSF4N01HD N–MOS switch and the MRFIC1818 provide
a typical 1 MHz 3 dB loop bandwidth. The STANDBY pin
must be enabled (3 V) at least 800 µs before the VRAMP pin
goes high and disabled (0 V) at least 20 µs before the VRAMP
pin goes low. This STANDBY function allows for the enabling
of the MC33169 one burst before the active burst thus reduc-
ing power consumption.
Conclusion
The MRFIC1818 offers the flexibility in matching circuitry
and gate biasing required for portable cellular applications.
Together with the MC33169 support IC, the device offers an
efficient system solution for TDMA applications such as
DCS1800 where saturated amplifier operation is used.
For more information about the power control using the
MC33169, refer to application note AN1599, “Power Control
with the MRFIC0913 GaAs Integrated Power Amplifier and
MC33169 Support IC.”
Evaluation Boards
Two versions of the MRFIC1818 evaluation board are
available. Order MRFIC1818DCSTF for the 1.8 GHz version
and order MRFIC1818PCSTF for the 1.9 GHz version. For a
complete list of currently available boards and ones in devel-
opment for newly introduced product, please contact your lo-
cal Motorola Distributor or Sales Office.
MOTOROLA RF DEVICE DATA
MRFIC1818
7
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