Linear amplifiers/transmitters
At the heart of many of today's high
performance digital wireless products is a transmitter whose linearity specification is
becoming more and more exacting. Not only are these specifications calling for excellent
linearity, but the close-in and wideband noise performance is also becoming more stringent
and the anticipated/expected efficiency of transmitters is rising.
Techniques for linearizing amplifiers or
transmitters fall largely into two categories feedback or feedforward topologies. A
short description of each of these techniques is given here, highlighting the applications
of each.
Cartesian Loop

Cartesian Loop Block Diagram
One of the most widely used linearization techniques
in the market-place is based on Cartesian feedback which controls the linearity of an
entire transmit chain, including the up-converter and amplifier stages. This technique is
well suited to integration with a few Cartesian ICs already available in the market-place.

Output of Cartesian loop
amplifier with TETRA modulation input
The nature of the feedback process means that the
amount of correction afforded by the control system decreases with increasing modulation
bandwidth. Typically, intermodulation improvement in the order of 30 dB is possible over a
25 kHz modulation bandwidth, and more than 10 dB improvement has been demonstrated for
CDMA-type bandwidths in excess of 1 MHz. Solutions that meet the ETSI TETRA specifications
have been designed. Efficiencies of Cartesian loop transmitters can be as high as 70%.
Further information can be found in Wilkinson and Bateman (1989).
RF synthesis/CALLUM

Callum transmitter block diagram
The RF synthesis approach to transmitter
linearization is a relatively new method which has the potential to achieve levels of
efficiency approaching 80% without sacrificing intermodulation suppression or noise
performance. CALLUM (Combined Analogue Locked Loop Universal Modulator) is a recent
invention allowing closed loop control of the RF synthesis process. Simulation of this new
technique suggests that this method can exceed the performance of Cartesian loop in all
respects. Further information can be found in Bateman (1992).

Implementation of CALLUM transmitter
Pre-distortion

Pre-distortion amplifier block
diagram
Fixed and controlled pre-distortion of a signal
prior to amplification is a well established technique for improving amplifier performance
and recently a range of sophisticated adaptive pre-distortion designs have been published
which aim to extend the capability of pre-distortion significantly.
Baseband adaptive pre-distortion techniques operate
by attempting to generate a transfer function in the pre-distorter block that is
complementary to the transfer function of the amplifier to be linearized, such that the
combination has a linear input-output power characteristic. This approach is very similar
to the channel equalization methods for overcoming gain and phase distortion on telephone
channels or mobile radio channels. Further information can be found in Mansell and
Bateman (1997.)
Feedforward

Block diagram of feedforward
amplifier
A rapidly expanding application of linear amplifiers
is in multi-carrier applications, where flexibility of channel allocation,
cost/performance benefit and modulation independence make this approach much more
attractive than several single carrier amplifiers feeding a high power lossy tuned
combiner network. The specification of linearity for GSM or PCS multi-carrier amplifiers
is in excess of 75 dBc which requires very accurate intermodulation cancellation
techniques.

Implementation of feedforward
amplifier
Single channel feedforward amplifiers are also of
importance for wideband modulation formats such as IS-95 CDMA, where the challenge is to
achieve linearity for minimal cost and high reliability. Further information can be found
in Parsons and Kenington (1994.)

Feedforward
amplifier output for 75 dBc performance
Other techniques

Block diagram of envelope elimination
and restoration technique
Less well known methods of amplifier and transmitter
linearization include, envelope elimination and restoration, polar loop correction, and
LINK vector feedback. Further information can be found in Petrovic (1983.)