Doug Lung /
09.16.2011 12:00AM
LDMOS Transistor Provides High Efficiency in Doherty Config

I have more information on a breakthrough in high power UHF solid-state amplifier efficiency. Freescale announced last Friday that its MRFE6VP8600H LDMOS RF power transistor can use up to 15 percent less power than previous generations of LDMOS power transistors and can handle high peak to average power ratios present in OFDM. It is extremely rugged, capable of handing VSWR of greater than 5:1 at all phase angles.

The most interesting part of the announcement was this statement was that the new transistor could deliver 125 watts of linear power (more than 600 watts of peak envelope power) throughout the entire broadcast band. It noted that there was "exceptional efficiency," with "typically" 30 percent at 860 MHz, and as much as 45 percent "when employed in a Doherty configuration."

What's a Doherty configuration?

You may remember that Doherty "high-efficiency amplifier" technology was first used in AM transmitter, beginning in the 1930s. I did a little research and found this old technique is finding application in digital transmitter systems that require high peak-to-average ratios, and has been in use at microwave frequencies for several years. For details on one application of Doherty amplifier technology for digital systems see A High Efficiency Doherty Amplifier with Digital Predistortion for WiMAX from High Frequency Electronics.

This is probably the breakthrough that I was hearing about prior to IBC, although I have yet to see any reports from manufacturers describing a UHF transmitter using a Doherty amplifier that's capable of 45 percent efficiency.

Last week I reported on Thomson's new solid-state transmitter with improved efficiency. Rohde and Schwarz also introduced a new solid-state transmitter line that's more compact and offers higher efficiency. For details on that, see the TV Technology on-line article Rohde & Schwarz THU9 Claims Most Efficient Solid State Transmitter Title.



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