INDUSTRY MICROELECTRONICS
power output stage serving a broad groupings of bands within the complete frequency range that the terminal covers; and it is relatively rugged. However, they cannot be optimised, relative to operating frequency, on a per-band basis.
Global 1 not only implements an all-CMOS RF front-end, it embodies a design approach that makes the optimal use of the characteristics of UltraCMOS technology. It takes advantage of the intelligence that can be integrated on a CMOS IC, to make use of the device characteristics possible in RF SOI. Once that intelligence is added, it becomes possible to not only recognise the need for the 5000-fold increase in possible operating states of the RF front-end, but to optimise operation for every one of them. Control is implemented, via the on-chip logic, through a MIPI interface, to deliver a performance that is markedly in advance of standard CMOS (see Figure 2). Handling a WCDMA signal, UltraCMOS Global 1 has better performance than GaAs technology, while bulk CMOS lags by an uncompetitive margin.
In an LTE-era smartphone, a typical PA stage will have three paths – in effect, three distinct power-amplifiers – that each cover part of the overall frequency range. They are selected for operation according to the band in use, and other operating conditions (see Figure 3). Operation of a GaAs-based PA can be optimised, at best, for each of those three frequency segments, and one set-up has to serve across the whole of each segment.
With the architecture of Global 1 it is a very different story. In this case, there is a much more focussed optimisation operating at the level of the actual band in use; this may only be 10 MHz for the narrowest bands as opposed to over 200 MHz for the entire path. For each operating state, it is possible to set up antenna tuning, filtering and stage-to-stage matching. To efficiently achieve high delivered-power levels from CMOS devices, it is necessary to use a stacked configuration of several FETs, connected in series – this overcomes the relatively low breakdown voltage of an individual FET. This structure functions as a single device that can handle the higher supply voltage needed to reproduce the LTE waveform profile, but presents
Above: Peregrine’s state- of-the-art processes ensure uniformity and quality of high- end RF solutions.
Right: Peregrine’s UltraCMOS 10 technology addresses the unique growth requirements for mobile applications and is the foundation of Peregrine’s next-generation RF switches, tuners and power amplifiers, including UltraCMOS Global 1.
challenges in setting the bias level of each transistor in the stack. However, this is not an issue with UltraCMOS technology, which can yield consistent devices that meet this requirement. This allows several devices to operate effectively as a single high-voltage FET.
With Global 1, the PA is not limited to just competitive WCDMA performance in terms of efficiency, output power and linearity. Instead, GaAs-equivalent performance is maintained for LTE waveform allocations up to 100 Resource Blocks (RB) (see Figure 4). In mid and high bands of operation, similar competitive results have been demonstrated. The CW measurements show efficiency when the PA runs into saturation and is a good indication of the approximate efficiency that can be achieved with the addition of envelope tracking.
Figure 4. With W-CDMA, LTE5, LTE10 and LTE20 waveforms, Peregrine’s UltraCMOS Global 1 PA shows little performance roll-off across the low band.
There is a clear precedent from many other semiconductor- device domains; as soon as CMOS with high-integration-levels can match the performance of a prior technology, then it quickly becomes the preferred solution. In this case, not only is the performance being matched, it is matched in a single device that is capable of being fabricated on a standard silicon CMOS line. Moreover, by covering all the operating states required for all worldwide frequency allocations and signal configurations, it will equip handset makers with what they have been wanting for years: a single stock-keeping unit that will allow them to build one PCB for all markets.
Copyright Compound Semiconductor Issue VI 2014
www.compoundsemiconductor.net 49
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