Improving RF Power Amplifier Efficiency Using a Varying Vcc from a DC-DC Power Supply

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Improving RF Power Amplifier Efficiency Using a Varying Vcc from a DC-DC Power Supply [Copy link]

Power amplifiers are usually optimized for ACPR and efficiency at maximum output power, but cellular phones operate in the medium/low power range in most cases, during which the efficiency of the power amplifier will be significantly reduced. Using the MAX8506 switching regulator to provide a variable supply voltage can significantly improve the efficiency of the power amplifier at +16dBm output power. At +10dBm output power, the efficiency is improved from 2.8% to 8.3%.

Introduction
The MAX2291 chip-scale packaged linear RF power amplifier (PA) is designed for N-CDMA mobile phones operating in the PCS band. By modifying the input and output matching circuits, it can also work well in the 1920-1980MHz WCDMA band. The chip contains a high-power channel for strong transmit power and a low-power channel for medium/low power transmission to achieve higher efficiency.

In cellular handset applications, the average transmit power is +12 to +16dBm, so the efficiency of the power amplifier at "medium" power is critical to extend battery life. The MAX2291 has a +37% power efficiency (PAE) when providing +27dBm output power from the high-power channel and 12% power efficiency when providing +16dBm output power from the low-power channel. When switching between the two channels, the negative impact is that the phase deviation of the signal exceeds 80°, which needs to be processed and corrected in the baseband section.

Changing the MAX2291 supply voltage (using only the high-power channel) is another way to improve power efficiency at medium or low power. The MAX8506 switching regulator can provide the required supply voltage. From the test results, the MAX2291 can improve power efficiency to 18.9% at +16dBm output power, with an ACPR of -38dBc and no phase shift. The PA gain decreases as Vcc decreases.

Circuit Description
MAX8506 is a 600mA switching regulator that can provide a variable power supply voltage Vcc for MAX2291 PA according to the required output power. MAX8506 has two control pins: BP and REF_IN. BP is used to bypass the regulator to reduce the voltage drop at full power output, and REF_IN is used to control the output voltage. The output of MAX8506 can be used as the main Vcc power supply of MAX2291 PA, as shown in Figure 1.

Figure 1. Using the MAX8506 switching regulator to provide a varying Vcc for the MAX2291 PA.

The WCDMA development board was used for testing, and the required output power and desired ACPR of -38dBc were achieved by adjusting the PA's Vcc and RF input power. The current consumption was then recorded for efficiency calculation.

Test equipment:

Agilent E4433B signal generator

Agilent E4406A

Power supply, RF coupler, 20dB attenuation head

RF Power Meter

The E4433B selects 3GPP modulation, uplink, DPCCH + 1 DPDCH, and the Agilent E4406A is used to measure ACPR.

Test data is shown in Figure 2. When the output power of MAX2291 is +27dBm, because MAX8506 is in bypass state, the input power is directly connected to the output without passing through the switching regulator. At this time, the efficiency of MAX2291 is 34.6% in both cases. When the output power is +24dBm, if a fixed 3.5V power supply is used, the efficiency is 25.7%; if a switching regulator is used for power supply, the efficiency can be improved to 30.5%. When the output power is only +16dBm, the efficiency is improved from 8.3% to 18.9%, and the ACPR is maintained at -38dBc. When the output power is +10dBm, the PAE is improved from 2.8% to 8.3%.

Figure 2. PAE and gain measured at different power levels