INDUSTRY MOBILES
undergoing a continual decline. What’s more, working with CMOS technology opens the door to partnering with a wide choice of foundries that are capable of producing chips in far larger quantities; this, in turn, can help handset manufacturers regain control over their supply chains.
From a design perspective, the most attractive attribute of turning to CMOS is that it unlocks additional levels of integration. As a result, non-PA components, such as controllers, antenna switches and tuners, can all be readily integrated within the PA. Similarly, CMOS also offers the integration of on-chip calibration, plus performance enhancing functionality, such as complex bias circuitry.
What’s needed is a way to draw on these benefits, while not suffering from the consequences of the inherent non-linearity of CMOS, which makes it very challenging for this technology to play a role in the 4G marketplace.
Fortunately, this is possible by turning to external performance enhancement technologies that allow silicon CMOS PAs to
Figure 1. Power amplifiers built from GaAs deliver a linear gain over a wide range, while those based on silicon are significantly non-linear.
4G LTE, the CMOS device would need to be backed off to a low power, where the characteristic is relatively linear. This approach is highly undesirable, because it results in extremely poor efficiency.
Looking at the positives
Given these performance issues, why are CMOS PAs even under consideration for 4G handsets? Well, while GaAs is broadly synonymous with both RF design and HBTs, CMOS brings a number of attractive attributes to the PA marketplace.
One of the benefits of switching to silicon is lower production costs. While CMOS development costs can be higher at the product level, in large volume production the wafer costs for base CMOS are not just lower than GaAs − they are also
Figure 3.Envelope Tracking signal processing chain (courtesy of OpenET Alliance)
compete with their GaAs cousins. At Nujira of Cambridge, UK, we have employed a technology that can do just that: Envelope Tracking (ET).
The primary benefit of ET is the hike it gives to the energy efficiency of a RF PA. This improvement results from the replacement of a traditional DC:DC converter with a highly agile, ET power supply modulator. Thanks to this modification, at any instant in time the PA is operating in a highly efficient compressed state, where the power supply voltage is just sufficient to enable the PA to transmit the instantaneous output power required (see Figure 2).
To do this, the ET power supply modulator has to dynamically modulate the power supply pin of the RF PA with a high bandwidth, low-noise waveform, synchronised to the instantaneous envelope (amplitude) of the signal being transmitted.
Figure 2. Envelope tracking leads to far more efficient operation of a PA, because the supply voltage is varied to give exactly what is needed at every moment in time.
A key feature of ET systems is the digital control of the mapping of the instantaneous RF signal amplitude to the supply voltage using a programmable Shaping Table (see
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
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