A Planar Balanced Approach to Increase Transmitter Linearity

ABSTRACT
The TI TRF1123 MMIC transmitter chip is a highly linear power amplifier with digital
gain control, mute control, and output power detection for use in MMDS/MDS/WCS
applications. With a typical P1dB of 31 dBm and a third-order intercept point of 46 dBm,
the TRF1123 makes an excellent output stage for communication systems, particularly
those that employ high-order modulation schemes such as 64 QAM and OFDM.
In applications with higher output power and/or higher linearity requirements, two
TRF1123s can be combined in a balanced configuration to increase the P1dB and
third-order intercept points. An ideal balanced configuration provides low input and
output VSWR in addition to a 3 dB improvement of the power and linearity
performance. Typical microstrip balanced power combiners use a Lange type combiner,
which, although effective, has the disadvantage of a non-planar implementation.
This application note documents a novel design approach for a planar, balanced power
combiner and presents results using the TRF1123 transmitter chip.