Pushing the envelope

11 February 2013

Toshiba Research Europe report a power efficient wideband envelope modulator power amplifier for LTE systems including 4G. Although relatively low-cost the amplifier could facilitate full 20MHz bandwidth LTE systems at substantial output powers.

Meeting demand

The need for faster, higher-capacity mobile communications is driving the rollout of LTE and 4G systems. As a result, in these early generation systems power efficiency can take a backseat, but it is a problem that will need to be addressed.

The peak amplitude of an LTE signal can be many times larger than the average value. Conventionally a fixed high supply voltage is used to accommodate the peaks. As the average amplitude is substantially smaller than the peak, the average efficiency of the RF amplifier will be low. Reducing the supply voltage when the transmitted signal is at the average amplitude value can dramatically improve efficiency.

One answer to this problem is envelope modulation. Envelope modulators increase the efficiency of RF amplifiers by dynamically controlling the supply voltage to the amplifier in harmony with the amplitude envelope of the transmitted signal. The amplitude of the transmitted signal is constantly tracked so the RF amplifier always operates in an efficient region.

Competing demands

In addition to the need for better efficiency LTE base station transmitters require both a large bandwidth and high peak power which, together, is beyond the capability of commercially available power amplifiers. Although integrated operational amplifiers can achieve the bandwidth their output powers are very limited and envelope modulators have struggled to achieve wide bandwidths at high transmit powers. The closest commercial product has insufficient bandwidth and is very expensive, with a significant impact on overall system cost.

For narrow bandwidth applications, a switched mode power supply (SMPS) can be used as the envelope modulator. In wideband applications a split frequency architecture, splitting the envelope signal into a DC and an AC component is used. The DC component is amplified efficiently by a SMPS and the AC by a low frequency power amplifier. For LTE signals approximately 80% of the envelope power is DC and 20% AC. However, because the peak amplitude of the signal applied to the linear amplifier is many times the average amplitude, it generally operates inefficiently. Typically the SMPS and power amplifier consume the same amount of DC power, even though the SMPS’s output is four times greater. It has been shown that improving the power amplifier’s efficiency has the largest effect on overall system efficiency.

Totem of power

The power amplifier reported in this issue achieves Watt level output power over a three decade bandwidth (17kHz to 23MHz). With envelope modulated RF amplifiers on the cusp of commercial deployment, this work will facilitate full 20MHz bandwidth LTE systems at substantial output powers.

The amplifier uses a ‘totem pole’ architecture, which is not novel in itself as author Dr Gavin Watkins explained, “The totem pole architecture used here is not new, one of my references dates from 1962, but it is one which has fallen out of vogue over the past 20 years. However, the need for power efficient LTE transmission has required us to look for solutions outside the normal areas.”

The Toshiba team found that with the correct component choice the totem pole architecture provides excellent performance. At low frequencies, power amplifiers tend to be based on matched complementary pairs of transistors. These are not available for high frequencies. The only high frequency alternative is the transformer coupled push-pull amplifier, but its bandwidth is limited to two decades. The totem pole amplifier is capable of high frequency operation without transformers and only uses one polarity of output transistor.

Finding the right output transistor was a major challenge. In the end RF LDMOS N-channel FETs were selected through a combination of simulation and practical experimentation. They are currently operating outside the region covered in their datasheet.

Short term long term

As well as allowing higher transmit power wideband LTE transmitters this design has the advantage of being low cost and compact, lending itself to integration. It is also applicable for transmitter applications with high PAPR wide bandwidth signals such as digital video broadcast and applications needing broadband efficient amplifiers like software defined radios operating in short wave radio bands.

As far as LTE application is concerned, although there are some linearity issues to iron out and significant phase shift at the top of the band, the Toshiba team are confident these can be dealt with and expect that, as efficiency becomes an issue in 4G networks over the next few years, their work will very soon find its way into LTE base stations.

The next phase will be incorporating an RF amplifier to produce a complete system. Beyond this work the team are looking further ahead at the next generation of efficient RF amplifier technologies and plan to publish work on both of these later this year.

Further reading

This article is based on the Letter: High bandwidth class B totem pole power amplifier for envelope modulators (new window).

A PDF version (new window) of this feature article is also available.

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Cover of Electronics Letters, volume 50, issue 08

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