•  
• EDN Europe MAGAZINE
•  
• Free SUBSCRIPTION
• Subscription Renewal
• View Media Kit

• EDN
• EDN Australia
• EDN Asia
• EDN China
• EDN Japan

Envelope tracking modulator boosts RF PA performance

Constantly adjusting supply voltage saves power in linear output stages

Envelope tracking as a means of improving the efficiency of an RF amplifier is, the founders of Cambridge, UK-based Nujira acknowledge, a proposal that has been around for some time. With its HAT product, (for “high accuracy tracking”) the company says it has realised that proposal, and can greatly increase the efficiency of RF output stages at, potentially, all power levels. The concept is simple; linear amplifiers have high standing currents and constant power dissipation in their output stage. If you supply the output stage of a linear RF amplifier with a constant-voltage power rail, then when the modulated waveform is at low amplitude, most of the power in the output stage is wasted. If you could arrange for the power rail to exactly track the waveform envelope, then at any given time the output stage is only capable of delivering just the power needed – while still operating linearly – and less power is wasted. The HAT RF power modulator does exactly that, Nujira says; it is a hybrid module that a basestation designer will build into his RF amplifier. The module is effectively a DC/DC converter with 100 MHz bandwidth and 80% efficiency, that delivers peak power of 650W with 5 kV/μsec and 2 kA/μsec slew rates. It operates in a closed loop in the transmitter architecture, but is independent of the RF frequency – the HAT device only needs to “know” about the waveform envelope, Nujira says. With the device, a basestation PA can run at 50% efficiency, “We will get to 60%,” the company says.
After basestatations the company has its sights set on DVB-T transmitters, where PA efficiency of 45% is feasible: and 4-G cellular handsets. In handsets, the primary motivation to adopt the technique will not be efficiency but cost – using envelope tracking will improve the performance of handset PAs to the extent that fewer output modules will cover more of the multiplicity of RF bands that 4-G will require. Whereas in larger devices Nujira will be a fabless semiconductor supplier – delivering products in the form of its hybrid module – in handsets, an IP-based business model is possible.