Investigating and optimising linearity in load modulated balanced amplifier

Three of the most important factors to consider in the design of power amplifiers (PA) are the spectral efficient, linearity and power efficiency. Usually, a compromise must be struck between these three parameters, with spectral efficiency and linearity on one side and power efficiency on the other. When trying to operate PAs with high power output, the integrity of the original signal must be sacrificed. The Doherty amplifier is a commonly used PA architecture which provides good power efficiency without distorting the original signal too much. However, this architecture lacks tuneability and thus lacks large broadband application.

Sheppard et al. have developed a broadband reconfigurable PA known as the Load Modulated Balanced Amplifier (LMBA) [1]. This device utilises a control signal on an isolated port in order to modulate the matching of the device which allows the amplifier's frequency of operation of be dynamic thus allowing greater bandwidth. The control signal is recombined with the output RF signal so as to not impact the efficiency of the PA. This technique allows for efficiencies of around 50% with greater bandwidth than standard Doherty PAs [2]. The LMBA has been further elaborated upon Collins et al. in the form of the Orthogonal LMBA which required lower power in the control signal than the standard LMBA [3].

Where this device lacks, however, is linearity. This parameter is crucial in order to keep the output signal faithful to the input. By applying a control signal into a balanced amplifier - as with the LMBA - it is hoped that linearity can be improved upon while conserving the LMBA's wideband power efficiency.

Initially, balanced Doherty amplifiers can be embedded into the OLMBA, replacing the balanced transistors. Through simulation and prototyping this design can be characterised, with a focus on capturing distortion and linearity - although power efficiency will remain an important criterion. From this, the benefits and drawbacks of thedesign can be identified and new architectures can be explored to maximise the performance of the amplifier.

[1] D. Shepphard, J. Powell, S. Cripps, "An Efficient Broadband Reconfigurable Power Amplifier Using Active Load Modulation," IEEE microwave and wireless components letters, Vol 26, Issue 6, pp 443-445, Jun 2016
[2] R. Quagila, S. Cripps, "A Load Modulated Balanced Amplifier for Telecom Applications", IEEE transactions on microwave theory and techniques, vol. 66, issue 3, pp. 1328-1338, Mar 2018
[3] D. Collins, R. Quagila, J. Powell, S. Cripps, "The Orthogonal LMBA: A Novel RFPA Architecture With Broadband Reconfigurability", IEEE microwave and wireless components letters, vol. 30, issue 9, pp. 888-891, Sep 2020