energies
Article
A Comprehensive Harmonic Analysis of Current-Mode
Power Amplifiers
Chiara Ramella
1,2,*
, Paolo Colantonio
2,3
and Marco Pirola
1,2
Citation: Ramella, C.; Colantonio, P.;
Pirola, M. A Comprehensive
Harmonic Analysis of Current-Mode
Power Amplifiers. Energies 2021, 14,
7042. https://doi.org/10.3390/
en14217042
Academic Editor: Davide Astolfi
Received: 21 September 2021
Accepted: 20 October 2021
Published: 28 October 2021
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1
Department of Electronics and Telecommunications, Politecnico di Torino, 10129 Turin, Italy;
marco.pirola@polito.it
2
Microwave Engineering Center for Space Applications (MECSA), 00133 Rome, Italy;
paolo.colantonio@uniroma2.it
3
Department of Electronic Engineering, Università degli Studi di Roma Tor Vergata, 00133 Rome, Italy
* Correspondence: chiara.ramella@polito.it
Abstract: This work presents a comprehensive theoretical analysis of current-mode power amplifiers
as a function of input power for different biasing classes under the common simplifying assumption
of constant transconductance and hard current cut-off/saturation. Typically, the theoretical analysis
of power amplifier performance and behavior are carried out only at maximum output power.
However, to achieve high data-rates, modern telecommunication systems adopt signals characterized
by a very high peak-to-average power ratio, thus it is useful to analyze the power amplifier behavior
as a function of power back-off. Moreover, in many cases, to enhance the efficiency and/or to apply
harmonic shaping techniques, a clipped drain-source current, which approaches a square wave, is
required. The classical analysis can be extended to low power levels only under the assumption
of power-independent conduction angle, which is true only for class-A and class-B amplifiers, and
does not take into account possible waveform clipping at maximum current. This work presents
a complete theoretical Fourier analysis of FET-based power amplifiers as a function of quiescent
drain-source current at any input power level and accounting for the clipped current case, up to the
square-wave limit, reorganizing and completing the material that can be found in classical textbooks
in the field.
Keywords: high-frequency power amplifiers; current-mode power amplifiers; harmonic analysis
1. Introduction
High-frequency power amplifiers (PAs), from RF to microwave and millimeter-wave
range, are key elements of any wireless system. Output power capability/density is the
essential feature of a PA, but, beyond this, the PA must also be highly efficient, being one
of the most power-hungry elements of a transceiver, show reasonable gain, to avoid long
amplifying chains, and keep a predefined level of linearity, to preserve information [1]. PA
design is always made challenging by non-idealities, such as device parasitic reactances,
strongly limiting achievable bandwidths, electro-magnetic crosstalk and/or thermal issues,
which all become increasingly critical at higher frequencies. This implies that practical
design must rely as much as possible on fully nonlinear device models, accounting for
most possible parasitic and high-order nonlinear effects: an approach that has been made
possible only by Computer-Aided Design (CAD). At the time where CAD tools were
not available, a big effort was put by research in the microwave field to investigate and
develop novel PA architectures based on simplified mathematical device models that allow
a fully analytical analysis though Fourier expansion of the involved signals [2–5]. However,
theoretical PA analysis is presently still fundamental to acquire a deep understanding of
the working principle standing behind the different PA architectures which is the basis of
any practical design. Moreover, it is extremely useful to rapidly estimate achievable perfor-
mance from basic physical device parameters (maximum current, threshold voltage, etc.)
Energies 2021, 14, 7042. https://doi.org/10.3390/en14217042 https://www.mdpi.com/journal/energies