Ultra low cost + 30dBm PA for GSM and 900MHz-ISM band
Abstract: This application note shows the operation of the MAX2235 power amplifier (PA) in GSM and 900MHz ISM band applications. This device can achieve an output power of + 30dBm while having an efficiency of 40%. It uses a 2.7V to 5.5V power supply and can provide a gain of 24dB. The application circuit schematic diagram, circuit board layout and bill of materials are also given.
The MAX2235 is a low-cost nonlinear power amplifier suitable for constant envelope applications. Its basic operating characteristics are designed for US AMPS. This application note shows how this device works in European GSM, 868MHz European ISM, and US 900MHz-ISM bands.
At 900MHz, the MAX2235 uses a + 5V power supply to provide an output power of more than + 30dBm with 40% efficiency and 24dB gain. This device uses a single power supply from + 2.7V to + 5.5V. Its gain control pin makes the output power change range up to 35dB. Low-power shutdown mode can reduce current consumption to less than 1μA. A key feature of this PA is that its output power can automatically jump, which can reduce instantaneous noise and adjacent channel spectrum interference.
Figure 1 shows the output power characteristics of the MAX2235 in the 880MHz to 915MHz band. Figures 2 to 4 show the relationship between the output power, efficiency, and supply current characteristics of the MAX2235 and the input power and frequency. Figure 5 shows the relationship between the output power of the MAX2235 and the gain control voltage.
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| Figure 1. MAX2235 output power as a function of frequency | Figure 2. MAX2235 output power as a function of input power | |
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| Figure 3. MAX2235 operating efficiency versus input power | Figure 4. Relationship between MAX2235 supply current and input power | |
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| Figure 5. MAX2235 output power as a function of GC voltage | ||
Figures 6 and 7 are the changes to the circuit components and the placement of the components in order to tune the MAX2235 to the 880MHz to 915MHz frequency band.
Figure 6. Schematic diagram of the MAX2235 PA used in the GSM and 900MHz bands
Figure 7. MAX2235 PA application circuit component placement guidelines for GSM and 900MHz bands
Table 1 lists the component values.
Table 1. List of components when the MAX2235 PA works in the GSM and 900MHz bands
| DesignaTIon | Qty | DescripTIon |
| C1 | 1 | 100pF, 5% ceramic capacitor (0603) Murata GRM39COG101J050V |
| C2 | 1 | 68pF, 5% ceramic capacitor (0603) Murata GRM39COG680J050V |
| C3, C4 | 2 | 1000pF, 10% ceramic capacitors (0603) Murata GRM39X7R102K050V |
| C5, C6 | 2 | 100pF, 5% ceramic capacitors (0402) Murata GRM36COG101J050V |
| C7 | 1 | 10pF, 5% ceramic capacitor (0603) Murata GRM39COG100J050V |
| C8 | 1 | 0.068µF, 10% Murata GRM39X7R683K016V |
| C9, C10 | 2 | 470pF, 10% ceramic capacitors (0603) Murata GRM39X7R471K050V |
| C11 | 1 | 100pF, 5% ceramic capacitor (0603) Murata GRM39COG101J050V |
| C12 | 1 | 1500pF, 10% ceramic capacitor (0603) Murata GRM39X7R152K0504 |
| C13 | 1 | 47pF, 5% ceramic capacitor ATC 100A470JW150X |
| C14 | 1 | 9.1pF, 5% ceramic capacitor ATC 100A9R1JW150X |
| C15 | 1 | 0.01µF, 10% ceramic capacitor (0805) Murata GRM40X7R103K050V |
| C16 | 1 | 1µF, + 80%, -20% ceramic capacitor (1206) Murata GRM42-6Y5V105Z025V |
| C17, C18 | 2 | 1000pF, 10% ceramic capacitors (0805) Murata GRM40X7R102K050V |
| L1 | 1 | 8.2nH (0603) inductor Toko LL1608-FH8N2K |
| L3 | 1 | 30-gauge wire short |
| J1, J2 | 2 | SMA connectors (PC edge mount) EF 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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