## 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

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### 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.

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### Output of Cartesian loop amplifier with TETRA modulation input

#### (plot courtesy of Wireless Systems International Ltd)

#### 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

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### 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).

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### Implementation of CALLUM transmitter

#### (picture courtesy of Wireless Systems International Ltd)

### Pre-distortion

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### 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

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### 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.

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### Implementation of feedforward amplifier

#### (picture courtesy of Wireless Systems International Ltd)

#### 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.)

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### Feedforward amplifier output for –75 dBc performance

#### (plot courtesy of Wireless Systems International Ltd)

### Other techniques

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### 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.)