Quenching the thirst of RF power amps and extending the life

JasonYoo

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

In tracking the average power, high peak-to-average power ratio (PAPR) signals are clipped and therefore distorted (i.e., linearity is compromised), which is why this method is most attractive for low PAPR and infrequent above-average power level applications, like CDMA, WCDMA, and 802.11 b, not 802.11 a/g applications. The few and low power signals that clip ultimately translate to acceptable bit-error rates, and in the case of 802.1 a/g signals, bit-error rates would increase significantly.

Table 2 shows a qualitative comparison of the techniques just discussed. Because there are numerous system tradeoffs, no single solution is best suited for all applications. Therefore, assuming the number one concern is a practical portable solution, we picked chip integration, efficiency, linearity of spectrally dense signals, and complexity as the most important parameters to consider and assigned them weighing factors of three, three, two, and one, respectively. We then qualitatively rated the schemes from one for the poorest performance to four, the best rating.

Table 2. Qualitative evaluation of PA efficiency enhancement schemes.

Priority Weight	Performance Parameter		Dual PAs	Follow Envelope	Track Power
3	Integration Capability (Rating)		Low⁽¹⁾	High⁽³⁾	Very High⁽⁴⁾
3	Efficiency (Rating)	Main PA	High	Moderate	Low
		Supply or 2^nd PA	High	Moderate	High
		Total	High⁽⁴⁾	Moderate⁽²⁾	Moderate⁽²⁾
2	Output signal quality (Rating)		High⁽⁴⁾	Moderate⁽³⁾	Low⁽²⁾
1	System Complexity (Rating)		Very High⁽¹⁾	Moderate⁽³⁾	Low⁽⁴⁾
	Scores (Cumulative Rating = ΣRating x Weight)		21	24	26

(x) is the rating, where a high rating (e.g., x = 4) implies better performance

Following the envelope is not the most attractive solution because of its bandwidth requirements and therefore supply efficiency limitations. Tracking average power, on the other hand, seems to be the best solution, scoring high on integration and complexity. In the case of highly probable, high PAR, spectrally-dense signals like 802.11 a/g, the dual PA approach may be the only viable option, in spite of its relative complexity and integration limits. It is noted, at this point, that completely adjusting the biasing conditions of any dynamically adaptive supply scheme (i.e., also adjusting bias current) improves efficiency, be it an envelope or power tracking scheme.

Considering the growing demand for 802.11 a/g signals and the less-than-ideally suited techniques discussed, we are focusing on increasing the ability of the power-tracking scheme to capture PAR events and therefore improve linearity and bit-error rate performance. In short, we are seeking to reduce the slow-moving PA supply voltage to increase PA efficiency and maintain high supply efficiency by adding a nonlinear but continuous circuit to track frequent PAR events " all this while keeping complexity and integration in check. The ultimate gauges, of course, will be the PA linearity performance of spectrally dense signals and battery life.

To comment on this article and/or ask questions, contact us, the Georgia Tech Analog and Power IC Design Lab, at gtap@ece.gatech.edu. Information about our other research projects can be found at www.ricon-mora.com/research.

References:
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