Quenching the thirst of RF power amps and extending the life

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Quenching the thirst of RF power amps and extending the life [复制链接]

Battery life or rather runtime is crucial in portable wireless systems such as cell phones, PDAs, laptop computers, and so on because it ultimately defines the device''s mobility. With decreasing form factors and increasing functional densities, energy, which is another way of saying battery life, is in short supply, decreasing the practical nature of these mobile electronic solutions. Within a wireless system, the radio frequency (RF) power amplifier (PA), in fact, consumes most of the energy because it must drive the antenna with sufficient power to transmit signals across space to remote receivers. As a result, the battery life of a wireless system is strongly dependent on its PA and the efficiency of the PA should therefore be as high as possible, but not at the cost of linearity.

To achieve both high efficiency and linearity in RF PAs, two approaches can be undertaken: (1) linearize an efficient, non-linear (switching) PA with feedback control and (2) increase the efficiency of an inherently linear PA. In the former, the bandwidth of the control loop must be significantly higher than the linearizing signal, be it the envelope or the complete RF signal, to process the information fast enough to truly follow the signal. Understanding that technology is already being pushed to its limits and high frequency operation necessarily implies higher switching losses, linearizing a PA inevitably decreases the maximum bandwidth of the PA to well within the limits and not at the limit of a given process technology, which is especially detrimental in high bandwidth, highly linear applications like 802.11 a/b/g applications (see Table 1). Consequently, increasing the efficiency of linear PAs may be a more tenable approach for improving battery life without significantly degrading linearity.

Several wireless technologies have been developed and widely used nowadays, such as global system for mobile (GSM) communication, CDMA IS-95, WCDMA, IEEE 802.11a/b/g, etc. The demand for local wireless applications has grown and 802.11 a/g signals have therefore gained popularity. Unfortunately, higher spectral density and consequently higher linearity are the side-effects of this trend, as seen by the high bandwidth, high peak-to-average power ratios (PAPR), and high propensity for above-average power levels required by 802.11 a/g signals (Table 1). The average efficiency of conventional and state-of-the-art PAs in this environment degrades, as a result, because not only are linearity requirements increased but also signal and envelop bandwidths, all of which incur efficiency trade-off losses.

Table 1. Key RF signal parameters for various state-of-the-art wireless technologies

Wireless Technology   Typical Carrier Frequency [GHz]  Application   PAPR [dB]  Above-Average Power Levels [dB] with roughly 1% Probability of Occurrence   Envelope Bandwidth [MHz]  802.11a   5  Wireless LAN   8.2  6.6  20  802.11g   2.4  Wireless LAN   8.2  6.6   20   802.11b   2.4  Wireless LAN   1.8  1.3  20  CDMA IS-95  1.95  Mobile phone   5.1  3.7  1.23  WCDMA   1.95  Mobile phone   3.2  2.4  3.84  GSM   0.9/1.8/1.9  Mobile phone   0  0  0

The efficiency of a conventional, fixed-supplied linear PA is highest at the maximum output power and drops quickly as the output power decreases [1-2]. The difference of the supply voltage and the RF signal''s peak voltage is a measure of power not delivered to the load (power losses). Unfortunately, the supply voltage must be high enough to supply the worst-case output power, in other words, the highest peak voltage of the RF signal, and when the output power is below that level, power is lost. Consequently, dynamically adjusting the supply voltage as a function of RF power (e.g., envelop of transmitted signal) is an attractive means of increasing PA drain power efficiency.

Before blindly working on designing adjustable supplies, the power probability distribution of the RF signal must be considered because it defines how battery life can best be improved. Figure 1 shows an example of the power output probability distributions for 802.11g signals [1], illustrating the tendency for portable systems to mostly operate in light-to-moderate power-level conditions, not high power modes. The maximum output power may be 25 dBm but the most probable value is approximately 16 dBm, which is where the PA''s efficiency is significantly lower, and this is similar for 802.11 a/g, CDMA IS-95, and WCDMA signals [1, 3-4]. Consequently, the efficiency of PAs in light-to-moderate loading conditions is critical for operation life.

Figure 1. Power output probability distribution for 802.11 g signals [1]