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Envelope tracking as enabling tech for CMOS PAs

06 Jan 2015  | Jeremy Hendy

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The bias settings of the device were then optimised to improve the linearity with fixed supply, resulting in the performance improving as shown in figure 7. Against these optimised bias conditions, the ET IsoGain still yielded a 3dB (2x) increase in output power.

Figure 7: Maximum linear power at -38dB ACLR: optimised bias settings.

Future integration and supply chain benefits
Although envelope tracking is primarily regarded as an energy saving technology, the results described above show how the IsoGain approach to ET enables CMOS PAs to deliver the same output power, linearity and efficiency as GaAs ET PAs, even for LTE signals with high dynamic range.

As well as unlocking the potential of CMOS PAs for high-end 3G and 4G applications, this also extends the integration and supply chain benefits of CMOS further into the PA market.

Most of today's GaAs PA modules already include a separate CMOS die, as it is difficult to integrate the complex bias generation and digital control circuits onto a GaAs die. CMOS PAs integrate these functions into a single chip, enabling them to take advantage of simpler, lower-cost packaging such as flip-chip onto FR4 substrates.

ET power supply modulators have similar process requirements to those of a CMOS PA, and are today built on compatible CMOS process platforms. The future integration of ET power supplies with CMOS PAs offers the potential for reduced system cost and higher performance, opening up intriguing possibilities for next-generation products.

The market leading suppliers of mobile phone modems and application processors are all large, fabless CMOS vendors, with well-established foundry and supply chain relationships and significant buying power. As the value of the RF front-end increases, the potential to absorb the PA into CMOS, and therefore make it an integral part of the smartphone chipset, offers some potential for significant revenue growth, improved multi-sourcing ability, and greater supply chain control.

Realising the vision
In order to take advantage of the opportunity presented by CMOS PAs, a whole new approach to RF front-end design needs to be established. Next generation CMOS PAs, like Qualcomm's RF360 and Peregrine Semiconductor's UltraCMOS Global 1, are part of highly integrated front ends—an approach our team at Nujira agrees with—rather than functionally equivalent devices built on a different process.

In order to achieve maximum performance gains, new CMOS PAs need to be optimised for ET from the outset. Designers can only do this by starting from scratch and completely rethinking the architecture and operation of a CMOS PA. To demonstrate the full potential of ET and CMOS PAs, Peregrine has completed testing using the Nujira's NCT-L1300 ET modulator in conjunction with an UltraCMOS MMMB PA. Operating at 780MHz using a 25 RB QPSK LTE signal with a PAPR of 7.3 dB this combination achieved a more than 18 per cent point improvement in power added efficiency (PAE) and delivered a system efficiency of more than 50 per cent (figure 8).

Figure 8: Performance enhancements from combination of Peregrine Semiconductor's Global1 PA and Nujira's NCT-L1300 ET modulator.

These results were achieved while still delivering +29.8 dBm output power and remaining ACLR compliant at -38dBc. The results were taken using a linear output match and only modulating the final-stage supply. It would be possible to gain more performance improvement if the fixed-driver supply were modulated and if the match were optimised for saturated operation.

These results confirm our belief that CMOS PAs are ultimately the best path towards RF front end integration. As far as our team is concerned, the next step of this development path is to integrate the PA onto the ET chip itself. For high bandwidth applications like 802.11ac and LTE-A, this is the only approach that gives the performance that will be required in handsets. Regardless, the PA industry is certainly set for a significant shake up over the coming years as a result of ET unlocking the potential of CMOS PAs for high-end devices.

About the author
Jeremy Hendy, VP Sales & Marketing, at Nujira brings considerable experience of semiconductor sales and marketing across multiple technologies for wireless communication and digital video. Previous positions include Marketing Director of wireless USB start-up Artimi, VP Marketing for Aspex Semiconductor, and Strategic Technology Director of Cadence's Wireless and Multimedia business unit. He started his career with Texas Instruments, and holds a first class honours degree in Electronic Engineering from the University of Liverpool.

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