Improving Power Amplifier Efficiency Using Adaptive Power Supply

CDMA mobile phone power amplifier MAX2291

MAX2291 is a linear RF power amplifier in UCSP package. It is designed for N-CDMA mobile phones working in PCS band. By modifying the input and output matching circuits, it can also work well in WCDMA band of 1920～1980MHz. The circuit is shown in Figure 1. The chip contains output channels for strong transmission power and medium/low power transmission. Compared with other power amplifiers, it can provide higher efficiency in low power mode. Gain and adjacent channel power rejection ratio can also be adjusted through external bias control to optimize the system in a wider operating range. Increasing the bias current of the first stage power amplifier can increase gain and improve ACPR, but will lose some efficiency; increasing the bias current of the second stage power amplifier will increase gain, but efficiency and ACPR indicators will be affected to a certain extent. The best solution is to dynamically adjust the power amplifier bias according to power requirements. For example, the BIAS pin can be driven directly with a current output DAC, or a voltage output DAC can be used. Each BIAS pin generates a reference voltage of about 1.1V. A resistor connected between this pin and GND can change the current flowing into the BIAS1H pin. If another resistor is connected between the BIAS1H pin and the voltage output DAC, the DAC output can control the current flowing into the BIAS1H pin. When the DAC output voltage is lower than 1.1V, current will flow out of the BIAS1H pin. Therefore, if DAC control is used, the DAC should have the ability to source/sink 100μA current.

In cellular phone applications, the average transmit power is +12 to +16dBm, so the efficiency of the power amplifier at "medium" power is critical to extending the battery life. The MAX2291 has made many improvements in power regulation. It has a +37% efficiency when the high-power channel provides +27dBm output power, and only 12% efficiency when the low-power channel provides +16dBm output power. Moreover, when switching between the two channels, the phase deviation of the signal exceeds 80°, which needs to be processed and corrected in the baseband section.

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

Power amplifier power supply circuit

MAX8506 is a 600mA switching regulator that can provide a variable power supply voltage Vcc for MAX2291 according to the required output power. Its internal block diagram and application circuit are shown in Figure 2. MAX8506 integrates a PWM buck controller and a bypass field effect transistor with an on-resistance of 75mΩ. It can dynamically adjust the power supply voltage of the power amplifier in WCDMA or CDMAOne mobile phones within the range of 0.4 to 3.4V. When the mobile phone transmits power below its maximum value, it helps to improve the efficiency of the power amplifier. MAX8506 has two control pins, HP and REF_IN. HP is used for power converter bypass control. When high power is output, HP is set to a high level, so that the MAX8506 power output is directly connected to the battery through the internal MOSFET to reduce the voltage difference at full power output. At this time, the voltage difference between input and output is the product of the load current and the on-resistance of the MOSFET (typical value is 75mΩ) and the parallel value of the buck converter and the equivalent resistance of the inductor. REF_IN is the external reference input terminal. When connected to an external DAC, the output voltage can be dynamically controlled. The gain from VREFIN to VOUT is set to 1.76 internally. It can respond to changes in VREFIN within 30μs and establish a stable output. In Figure 2, the input capacitor must use a capacitor with low equivalent series resistance to reduce high-frequency input ripple. Ordinary aluminum electrolytic capacitors have large ESR and should be avoided as much as possible. Low-ESR tantalum capacitors and polymer capacitors are more suitable for small-size designs. Ceramic capacitors have the smallest ESR. In order to reduce the peak current and noise of the input power supply, a low-ESR capacitor of 2.2 to 10μF can be connected to the input. RC, CC, and Cf are compensation components. If the output capacitor has a lower ESR, Cf can also be omitted. Capacitor CC is used to integrate the current of the transimpedance amplifier, average the output capacitor ripple, and set the transient response speed of the device. Since the capacitor ripple after a certain phase shift will not interfere with the current regulation loop, this circuit allows the selection of small-size ceramic output capacitors. The compensation resistor RC sets the proportional gain of the output voltage error. Increasing the resistance value can increase the sensitivity of the control loop to the output ripple. According to the recommended component parameters in the figure, appropriate phase margin can be provided over the entire output voltage and load current range to ensure stable operation of the circuit. The external inductor of MAX8506 must meet the following requirements: the inductance is 4 to 6μH, the DC resistance is less than 400mΩ, and the saturation current is higher than the sum of the load current corresponding to the maximum power amplifier power and 1/2 of the inductor peak current.

The circuit connection of the MAX8506 output as the main Vcc power supply of the MAX2291 is shown in Figure 3. Using the WCDMA development board for testing, the required output power and the desired ACPR: -38dBc are achieved by adjusting the Vcc and RF input power of the power amplifier. Then the current consumption is recorded and the efficiency is calculated.

The test equipment used is Agilent E4433B signal generator, Agilent E4406A tester, power supply, RF coupler, 20dB attenuation head and RF power meter.

E4433B selects 3GPP modulation, uplink, DPCCH+1 DPDCH, and Agilent E4406A is used to measure ACPR. The test data is shown in Figure 4. 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 of -38dBc is maintained. When the output power is +10dBm, the PAE is improved from 2.8% to 8.3%.