APPLICATION NOTE 4010

MAX2831 Power Amplifier (PA) Output Power and Bias-Current Optimization


Abstract: This article presents data and register settings for optimized power amplifier (PA) efficiency at various power levels for the MAX2831.

Introduction

The MAX2831 is a fully integrated transceiver with power amplifier (PA) for 802.11b/g applications. The bias current and output-matching network for the two-stage PA have been optimized to nominally deliver +18.5dBm while consuming 209mA from a 3.3V supply (at less than 5.6% EVM in 54Mbps (64-QAM) mode) while meeting 802.11g spectral mask requirements. At lower output-power levels, reprogramming the PA Bias Current Registers allows the bias current and overall PA current to be reduced for improved Tx efficiency while still meeting 802.11g requirements. When the PA bias current is optimized for +17dBm transmit power, a current savings of 26mA is achieved as compared with the nominal register setting. When operating at +3dBm transmit power, a current savings of 62mA is achieved as compared with nominal register setting. Further current savings can be achieved for a specific output-power level through optimization of the simple two-capacitor output-matching network.

Programming

Register 10 (A3:A0 = 1010) allows programming of the two-stage PA bias current. Bits D2:D0 allow optimization of the first-stage bias current, and bits D6:D3 allow programming of the second-stage bias current. Table 1 summarizes the register settings and the equivalent MAX2831 EV kit bias-current settings.

Table 1. MAX2831 PA Bias-Current Register Settings
Power Amplifier First-Stage Bias Current Power Amplifier Second-Stage Bias Current
Register 10
(A3:A0 = 1010)
Bit D2:D0
Equivalent Bias Current
(µA)
Register 10
(A3:A0 = 1010)
Bit D6:D3
Equivalent Bias Current
(µA)
100 125 0100 160
010 75 0011 120
001 50 0011 120
001 50 0010 80


Results

Figure 1 and Table 2 summarize the current savings that can be achieved by optimizing the PA bias current. Table 3 and Figure 2 summarize the performance for nominal register settings. Figures 3, 4, and 5 summarize performance for register settings optimized for +17dBm and +15dBm and their output spectrums, respectively. Figure 6 summarizes performance for register settings optimized for +12dBm and +9dBm. Figure 7 provides the measurement setup.

Figure 1. MAX2831 supply current vs. output power for nominal PA bias and PA bias current optimized for specific output power levels.
Figure 1. MAX2831 supply current vs. output power for nominal PA bias and PA bias current optimized for specific output-power levels.

Table 2. Register Settings and Measured Results with Bias Current Optimized for Output Power
(VCCTX = 2.85V, VCCPA = 3.3V, Signal Source = 90mVRMS 802.11g 64-QAM OFDM (54Mbps), f = 2437MHz, ICCTX = 81mA)
Measured Results Register Settings
(See MAX2831 EV Kit Control Software)
POUT (dBm) EVM (%) PA Current (mA) PA Current Savings (mA) Tx VGA Gain (SPI™)
[Register 12
(A3:A0 = 1100)
Bit D5:D0]
PA Driver Output Current (µA)
[Register 10
(A3:A0 = 1010)
Bit D2:D0]
PA Output-Stage Bias (µA)
[Register10
(A3:A0 = 1010)
Bit D6:D3]
2.9 3.5 73 61 35 (100011) 50 (001) 80 (0010)
6.1 5.1 79 59 41 (101001) 50 (001) 80 (0010)
9.2 4.4 98 45 44 (101100) 50 (001) 120 (0011)
12.1 5.6 111 41 50 (110010) 50 (001) 120 (0011)
15.2 5.0 140 28 53 (110101) 75 (010) 120 (0011)
17.0 5.3 158 26 57 (111001) 75 (010) 120 (0011)
18.7 5.5 204 57 (111001) 125 (100) 160 (0100)


Table 3. MAX2831 Output Power, Supply Current, EVM and Register Settings Optimized for +18.5dBm (Nominal)
(VCCTX = 2.85V, VCCPA = 3.3V, Signal Source = 90mVRMS 802.11g 64-QAM OFDM (54Mbps), f = 2437MHz, ICCTX = 81mA)
Measured Results Register Settings
(See MAX2831 EV Kit Control Software)
POUT
(dBm)
EVM (%) PA Current (mA) Tx VGA Gain (SPI)
[Register 12
(A3:A0 = 1100)
Bit D5:D0]
PA Driver Output Current (µA)
[Register 10
(A3:A0 = 1010)
Bit D2:D0]
PA Output-Stage Bias (µA)
[Register10
(A3:A0 = 1010)
Bit D6:D3]
3.3 2.1 135 24 (011000) 125 (100) 160 (0100)
6.3 2.3 138 30 (011110) 125 (100) 160 (0100)
9.2 2.6 143 36 (100100) 125 (100) 160 (0100)
12.2 2.9 152 43 (101011) 125 (100) 160 (0100)
15.1 3.4 168 49 (110001) 125 (100) 160 (0100)
17.1 4.1 184 53 (110101) 125 (100) 160 (0100)
18.7 5.5 204 57 (111001) 125 (100) 160 (0100)


Figure 2. MAX2831 output power, supply current, and EVM optimized for +18.5dBm. (A3:A0 = 1010, D6:D3 = 0100, D2:D0 = 100)
Figure 2. MAX2831 output power, supply current, and EVM optimized for +18.5dBm. (A3:A0 = 1010, D6:D3 = 0100, D2:D0 = 100)

Figure 3. MAX2831 output power, supply current, and EVM with register settings optimized for +17dBm and +15dBm. (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)
Figure 3. MAX2831 output power, supply current, and EVM with register settings optimized for +17dBm and +15dBm. (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)

Figure 4. Output spectrum at +17dBm (red line denotes 11g spectral mask requirements). (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)
Figure 4. Output spectrum at +17dBm (red line denotes 11g spectral mask requirements). (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)

Figure 5. Output spectrum at +15dBm (red line denotes 11g spectral mask requirements). (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)
Figure 5. Output spectrum at +15dBm (red line denotes 11g spectral mask requirements). (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 010)

Figure 6. MAX2831 output power, supply current, and EVM with register settings optimized for +12dBm and +9dBm. (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 001)
Figure 6. MAX2831 output power, supply current, and EVM with register settings optimized for +12dBm and +9dBm. (A3:A0 = 1010, D6:D3 = 0011, D2:D0 = 001)

Figure 7. Test setup diagram for the MAX2831 wireless LAN transceiver.
Figure 7. Test setup diagram for the MAX2831 wireless LAN transceiver.


Related Parts
MAX2831 Free Sample
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APP 4010:
APPLICATION NOTE 4010,AN4010, AN 4010, APP4010, Appnote4010, Appnote 4010