ISSCC: Class-G and H vie to track 4G RF envelopes
ISSCC 2013 – 60th Anniversary
Held every year in February in San Francisco, the IEEE’s International Solid State Circuits Conference (ISSCC) is the world showcase for innovative circuit design. This is the 60th conference, and year’s theme is ’60 years of (em)powering the future’.
Latest-generation mobile phone modulation schemes like WCDMA and LTE-Advanced produce very peaky RF signals that need transmit power amplifiers that are linear, wideband and low power.
Traditional Class-A and AB amplifiers can do the first two, at the expense of high power consumption as they are inefficient with peaky signals.
Doherty amplifiers can get the linearity at reasonable power, but not over the bandwidth required.
Pure switching (Class-D) RF amplification is currently impossible as the frequencies as it would require switching at hundreds of GHz.
One approach which can increase efficiency considerably is to use a Class-AB RF amplifier, and modulate its power rails along with the amplitude envelope of the signal it is amplifying.
This has become known as envelope tracking and, so long as the modulated power rail has sufficient voltage headroom over the desired output waveform, can be linear and save power. It is analogous to Class-H audio amplification.
The difficulty is that the variable power rail has to have multi-MHz bandwidth to follow the RF envelope: around 20MHz for LTE and 40MHz for LTE-Advanced, and has to be provided by a switching dc-dc converter as a linear one would negate any efficeincy gains.
Nujira of Cambridge has agile switching dc-dc converter envelope tracking technology, and is offering it to 4G phone makers as a single chip.
A half-way approach to full envelope tracking is to supply the RF power amplifier (PA) from multiple fixed voltage rails, actively selecting rails in harmony with the signal envelope to guarantee sufficient headroom for clean linear operation.
This is similar to Class-G audio amplifier operation – which is used to cut power waste in state-of-the-art headphone amplifiers in phones – and is also sometimes referred to as envelope tracking.
Class-G operation is simpler, but does not save as much power in the RF amplifier as Class-H operation.
The minimum requirement for such Class-G RF amplification is switching between two rails – battery volts and a single fixed rail generated by a dc-dc converter.
At ISSCC this year, Toshiba described an unusual spin on this two rail Class-G approach that does not need a dc-dc converter.
Instead, two RF power amplifiers are used, which are connected in series across the power supply when less RF amplitude is required, each receiving VDD/2 – see diagram.
When full power is needed, they are both connected in parallel across VDD. As they are sharing the power, each can be smaller than a single amplifier doing the same job.
Toshiba built a 1.8GHz PA in 65nm CMOS with a single supply voltage of 3.3V, using combining transformers and cascode topology to boost power from the CMOS.
Low-resistance transistors for the four main power switches cut voltage waste, and the whole scheme – power transistors and control circuit – occupied only 0.2mm2 (4%) of the 2×2.6mm chip.
The control circuit samples input amplitude through a differential square-law detector, and a 200MHz low-pass filter removes carrier products leaving the envelope which is fed to a comparator.
“The cut-off frequency of the low pass filter is designed to be around 200MHz to cut off the carrier frequency and not affect the envelope signal that greatly exceeds 40MHz for the maximum 20MHz bandwidth LTE-Advanced signal,” said Toshiba.
The comparator output is used to switch the four main power transistors, and re-biases the upper amplifier for correct operation in series power mode.
From end to end, the control process takes 2ns, and careful component choice throughout minimises timing differences and glitches to the point that distortion is dominated by the amplifier characteristics rather than the switching process.
“The simulated phase discontinuities at the transition point were less than 10degrees in any bias conditions,” said Toshiba.
Full envelope tracking
In another paper, from University of California, San Diego and Brown University, a hybrid form of full envelope tracking was described where the tracking VDD envelope is provided by two switching dc-dc converter in parallel with a linear power supply.
The hysteretically controlled dc-dc, which has a low-pass response, provides around 60% VDD power.
To this, the band-pass response dead-band dc-dc adds another 25%, while the linear supply tops off the 20MHz PSU response required for LTE with the remaining 15% of power.
Using a commercial two-stage GaAs HBT RF power amplifier handling a 2.535GHz signal modulated with 20MHz LTE (6.7dB PAPR), the measured
overall power added efficiency (PAE) after accounting for both the supply modulator and amplifier loss is 48% at 28.3dBm output power, while the
same amplifier working with a fixed supply shows 35% PAE.
Paper 5.5 A 1.8GHz Linear CMOS power amplifier with supply-path switching scheme for WCDMA/LTE applications
Paper 21.3 A CMOS dual-switching power-supply modulator with 8% efficiency improvement for 20MHz LTE envelope tracking RF power amplifiers
Out of 629 papers submitted to ISSCC 2013, 209 were accepted:
74 from North America (30 industry, 43 university, 1 institution/lab)
84 from the Far East (33 industry, 50 university, 1 institution/lab) and
51 from Europe (17 industry, 22 university, 12 institution/lab)