The demand for higher data rates is driving the evolution of mobile communication systems from 2G to 3G. This transition brings with it more stringent performance and design requirements for the RF components in mobile devices, particularly the power amplifiers. To maximize bandwidth efficiency within the allocated frequency spectrum, third-generation systems employ linear modulation techniques such as Quadrature Phase Shift Keying (QPSK), 8-Phase Shift Keying (8PSK), and Quadrature Amplitude Modulation (QAM). These methods help improve spectral efficiency, but they also require the use of linear RF power amplifiers to maintain good adjacent channel power rejection ratio (ACPR) and error vector magnitude (EVM).
In CDMA-based systems, a common solution is the use of Class A amplifiers, which are known for their linearity but suffer from low efficiency. Typically, Class A amplifiers operate at less than 1dB compression, resulting in power efficiencies that drop significantly. In systems like IS-95 and CDMA2000, RF power amplifiers often operate with a back-off of 6dB to 40dB from peak power or the 1dB compression point. As a result, these amplifiers are frequently running at very low efficiency, making them one of the most power-hungry components in a mobile phone.
Studies have shown that RF power amplifiers can consume between 20% and 40% of the total power in conventional mobile applications. This makes reducing RF power consumption a critical goal for extending battery life and improving talk time. To address this challenge, this paper introduces a simple power tracking technique that enhances the efficiency of RF power amplifiers.
The method utilizes a dB-linear RF power detector, such as the LMV225, along with a DC-DC converter switch. By dynamically adjusting the supply voltage (VCC) of the RF power amplifier based on the output power level, this technique helps optimize energy usage. Off-the-shelf CDMA2000 power amplifiers can benefit from this approach, leading to improved energy efficiency in mobile devices.
RF power amplifiers are central to these applications, and when paired with a DC-DC converter, they form an efficient power management circuit. For example, the SKY77152 is a widely used CDMA2000 power amplifier that can achieve over 40% power-added efficiency (PAE) near the 1dB compression point. These amplifiers typically have two supply voltage pins—VCC and VBIAS—as well as a reference voltage pin (VREF), which must be set to 2.85V. The VCONT pin allows users to switch between high and low power modes depending on the RF output level.
Switching between these modes can introduce phase differences between signal paths, potentially causing issues with baseband processing. However, by using power tracking, the system can maintain optimal performance while minimizing power waste. Figure 2 illustrates how reducing the DC supply voltages VCC and VBIAS can still yield acceptable RF output power levels.
Power efficiency, or power-added efficiency (PAE), is defined as the ratio of RF output power to DC input power. While manufacturers often highlight peak PAE at maximum output, real-world operation rarely reaches this level. In mobile applications, high peak power is mainly used for heat dissipation, and most of the time, the RF power amplifier operates at lower power levels, where efficiency drops significantly.
To estimate the average efficiency in a mobile system, it's essential to consider the probability distribution of the RF output power, as shown in Figure 3. For IS-95 devices, most of the time the output power is below +15dBm, so improving efficiency at low signal levels becomes crucial. Reducing the supply voltage VCC can lower DC power consumption, but this must be done carefully to maintain ACPR, EVM, and fast switching between voltage levels.
ACPR measures the ratio of power in adjacent channels to the main channel. Table 1 outlines the ACPR requirements for CDMA2000, highlighting the importance of maintaining linearity in the transmission path. Although earlier systems like IS-95 may not have formal ACPR specifications, designers should still aim to meet these standards to ensure reliable communication and minimize interference. A poor ACPR value indicates non-linear behavior, which can distort signals and degrade reception quality.
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