AN3247| Application Note
Maxim/Dallas > App Notes > POWER-SUPPLY CIRCUITS
WIRELESS, RF, AND CABLE
Keywords: PA, PA efficiency, PAE, power added effiency, DC-DC, switching supply
May 26, 2004
APPLICATION NOTE 3247
RF Power Amplifier Efficiency Improves with Varied Vcc, from DCDC Supply
Power amplifiers are usually optimized for ACPR and efficiency at full output power. However, most of the time cellular handsets operate at medium/low power range, where the power amplifier's efficiency drops dramatically. With a variable power supply from a switching regulator such as the MAX8506, the power amplifier efficiency can be improved more than two-fold at +16dBm output. At +10dBm, efficiency is increased from 2.8% to 8.3%.
Introduction
The MAX2291 chip-scale-packaged, linear, RF, power amplifier was designed for use in N-CDMA handsets using the PCS band. It also performs well in the 1920-1980MHz WCDMA band by changing the input and output matching circuits. It includes a high-power path for full power transmission and a low-power path for mediumand low-power transmission with higher efficiency. In cellular handset applications, the average transmission power is +12 to +16dBm. Therefore, the power amplifier efficiency at these "medium" output power levels is critical for longer battery talk time. The MAX2291 provides 37% of added power efficiency (PAE) at +27dBm output from the high-power path and 12% at +16dBm output by using low-power path. An undesired side-effect is that the signal phase shifts over 80 degrees when changing between the two paths. This causes concerns in base-band processing and correction. Varying the MAX2291 supply voltage (while using only the high-power path) is another good way to enhance efficiency at medium and lo
w output power. The MAX8506 switching regulator can be used to supply the required voltage. From bench test results, the MAX2291 can achieve 18.9% PAE at +16dBm with -38dBC ACPR, and without any phase shift. PA gain decreases with lower Vcc as expected.
Circuit Description
The MAX8506 is a 600mA switching regulator to supply variable Vcc to the MAX2291 PA based on the desired output power. There are two control pins for the MAX8506: BP and REF_IN. BP is used to bypass the regulator to reduce voltage drop at full output power, and REF_IN controls the output voltage. The MAX8506 output is taken as the main Vcc source for the MAX2291 PA as shown in Figure 1.
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