Ultra-Low Cost +30dBm PA for GSM and 900MHz-ISM Applications
Abstract: This application note, based on the MAX2235 power amplifier (PA) shows the operation in GSM and 900MHz ISM band applications. +30dBm of output power is achieved with 40 percent efficiency. The device provides 24dB gain, and operates with a 2.7V to 5.5V power supply. A schematic, the circuit board layout, and a bill of material are provided.
The MAX2235 is a low-cost, non-linear power amplifier, intended for constant-envelope applications. Originally characterized for US AMPS, this application note demonstrates the use of the device for European GSM, 868MHz European-ISM and US 900MHz-ISM band applications.
At 900MHz, the device delivers over +30dBm of output power at 40% efficiency, with 24dB of gain from a +5V supply. The device can be operated from a single +2.7V to +5.5V voltage supply. A gain-control pin allows the output power to be varied over a 35dB range. A low-power shutdown mode reduces supply current consumption to less than 1µA. A key feature of this PA is output power autoramping, which minimizes transient noise and spectral splatter.
Figure 1 demonstrates the output power performance of the MAX2235 over the 880MHz to 915MHz frequency band. Figures 2 through 4, demonstrate the output power, efficiency and supply current performance of the MAX2235 versus input power and frequency. Figure 5 demonstrates the output power of the MAX2235 versus gain control voltage.
Figure 1. The MAX2235 output power vs frequency.
Figure 2. The MAX2235 output power vs input power.
Figure 3. The MAX2235 efficiency vs input power.
Figure 4. The MAX2235 supply current vs input power.
Figure 5. The MAX2235 output power vs GC voltage.
Figures 6 and 7 demonstrate the component changes and placement required
to tune the MAX2235 for operation over the 880MHz to 915MHz frequency
band.
Figure 6. Schematic for the MAX2235 PA for GSM and 900MHz.
Figure 7. Component Placement Guide for the MAX2235 PA for GSM and 900MHz.
Table 1 lists the component values.
Table 1. Component list for the MAX2235 PA for GSM and 900MHz
SMA connectors (PC edge mount)
E.F. Johnson 142-0701-801
J3, J4
2
Test points
JU1
1
3-pin header (0.1" centers)
R1
1
0Ω resistor (0603)
Kamaya RMC16-000T
VCTRL
1
1-pin header
U1
1
MAX2235EUP (TSSOP-20)
None
1
MAX2235 EV kit PC board
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