INDUSTRY MOBILES
from the addition of ET will help to unlock an entirely new avenue for smartphone manufacturers and for future developments within the RF front-end. Now that CMOS PAs are demonstrating the capabilities to serve high-end 3G and 4G applications, the time has come when smartphone developers can start to explore the full benefits of an integrated CMOS system.
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Figure 3). By defining amplifier behaviour with a digital look-up table, it is possible to optimise PA performance dynamically using software control. Thanks to this ET approach, non-linear devices are linearised with no computational overhead. When ET tracking, including the use of an IsoGain shaping table, is applied to a CMOS PA, this amplifier can be linearised to such an extent that it can meet both the stringent in-band performance requirements, such as error vector magnitude, and thetough out- of-band specifications, such as adjacent channel interference.
What is not clear, however, is whether the boost in linearity generated by ET is sufficient to enable CMOS to match – or exceed – the performance of traditional GaAs PAs. To answer that question, we have recently performed a series of measurements.
Assessing performance To test the viability of CMOS for 4G waveforms, our team of engineers connected a prototype CMOS PA operating in the 700 MHz band (Band 13) to our ET modulator. By characterising the PA with our ET Surface Explorer toolset, we generated an IsoGain shaping table.
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Efficiency measurements involved using a high-performance current probe to sample the instantaneous PA supply voltage and current. Armed with this information, the instantaneous power being supplied from the high bandwidth ET supply can be calculated.
We also sampled the RF output power at high bandwidth, which enabled calculations of the collector efficiency (see Figure 4). These measurements revealed an average PA efficiency of 57 percent. This is a significant increase over a traditional GaAs PA without ET, where an efficiency of 30-35 percent would be expected for the same waveform.
To assess the maximum linear power that could be achieved using IsoGain ET, compared to a fixed 3.4 V DC supply voltage, we fed the amplifier with a variety of different waveforms (note that for both forms of power supply, conditions were ACLR compliant at -38 dBc).
These measurements revealed that the output power of the CMOS device quadrupled when driven with ET (see Figure 5). Meanwhile, when driven with a fixed supply voltage, the PA suffered significant nonlinearity with high PAPR waveforms such as LTE QAM.
Figure 4. The measured efficiency plot of CMOS PA from dynamic supply measurements. Note that each point on this curve represents a measurement of instantaneous efficiency captured at around 100 MSPS under dynamic ET supply conditions using an LTE waveform.
Improvements in the linearity of a PA that is powered with a fixed voltage are possible by optimising the bias settings of the device (see Figure 6). However, even then the results are inferior to those produced with ET IsoGain, which delivers twice the output power.
Historically, ET has been viewed as an energy saving technology, and in our hands it can realise this. However, the main strengths of our patented IsoGain technology are the
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www.compoundsemiconductor.net Issue VI 2014 Copyright Compound Semiconductor
The very impressive performance gains resulting
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