[讨论] Quenching the thirst of RF power amps and extending the life

State-of-The-Art in Efficient Linear PAs
There have been several techniques proposed to improve the average efficiency of linear PAs, and most notable are the dual PA (Doherty), envelope following, and power tracking schemes [5-6]. As justified earlier, linearizing switching PAs with negative feedback is discounted in this foregoing discussion because of its inherent bandwidth (speed) limitations. Feedforward schemes are also discounted because the forward-bypass signals must synchronize with the main transmitting RF or phase signal and the delays must therefore match to maintain linearity, which present practical difficulties in high performance applications and is why the envelope elimination and restoration (EER) method [7] is not further discussed in this article.

Dual PA Power-Sharing Approach (Doherty Technique)
In the dual PA scheme, one amplifier sources the low-to-moderate load power levels while the other sources the above-average power range (Figure 2). Only one PA is therefore active and optimally designed (i.e., near its gain-compression point [8]) for the critical light-to-moderate load region, where efficiency is normally lost. At higher power levels, both PAs can continue to operate in their gain-compression regions, thereby maintaining peak power efficiency. Unfortunately, the power divider and combiner required are lossy, especially when integrated on-chip [9], and designing two PAs may not be entirely attractive, in terms of cost.

Figure 2. Dual PA power-sharing approach

Envelope Following Supply
Avoiding the dual PA paradigm implies adding intelligence to the supply voltage, and in the case of the envelope follower, forcing the supply to follow the envelope of the transmitted RF signal, as shown in Figure 3. The envelope-following supply is therefore kept slightly above the actual peak of the signal to allow the PA to fully process the RF signal, envelop and phase, which is how linearity is maintained. Since the difference in supply and signal peak voltage is kept low, minimal power losses are incurred by the PA.

Figure 3. Envelope-following supply scheme

To add the intelligence to the supply is to design a dynamically adaptive supply circuit, which unfortunately also incurs power losses, but hopefully less than the PA drain losses just saved by the envelope-following scheme. Switching power supplies are therefore viable solutions, given their propensity for high efficiency. Their efficiency performance, however, is ultimately limited by their switching frequencies (higher frequencies incur more power losses), which is why RF signals with low envelope bandwidths like CDMA and WCDMA signals benefit the most from this scheme, unlike the higher spectral-density signals like the 802.11 a/b/g signals.

Power Tracking Supply
As envelope bandwidths increase, another dynamic scheme must therefore be implemented, which is how average power tracking finds its niche (Figure 4). The bandwidth required to track the average power of high bandwidth envelopes is significantly lower, given the nature of the averaging function, which is attractive for switching power supply circuits, within the context of power efficiency, of course. Since the supply is now at an average level, the PA incurs the power losses the envelop-following scheme saves for low output power levels, but significant savings are still achieved over the conventional fixed supply scheme.

Figure 4. Power tracking supply