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H. Ertl, F. Zach, J.W. Kolar:
"Dimensioning and Control of a Switch-Mode Power Amplifier Employing a Capacitive Coupled Linear-Mode Ripple Suppression Stage";
Talk: International Conference on Power Electronics and Intelligent Motion (PCIM), Nürnberg, Germany; 2005-06-07 - 2005-06-09; in: "Proceedings of the International Conference Power Electronics Intelligent Motion Power Quality", (2005), ISBN: 3-928643-41-x; 277 - 284.

A new power amplifier system focused mainly
on EMC testing applications is analyzed. The proposed
topology consists of a three-level switching power amplifier
connected in parallel with a capacitive coupled linear stage
which absorbs the output current ripple of the switch-mode
stage resulting in an almost ripple-free output voltage of the
total amplifier.
This concept alternatively can also be interpreted as a
switch-mode power amplifier with a hybrid output filter
consisting of a passive LC-circuit and of a linear amplifier
which compensates the voltage ripple appearing across the
filter capacitor (resulting from the ripple current of the
switch-mode stage).
The design of the linear amplifier has to be performed
only regarding the ripple voltage and the ripple current of
the passive filter part. Consequently, the rated power of the
amplifier and the appearing losses become low as compared
to the output power of the total system. A further advantage
of the concept is that the output impedance of the total
system will be determined mainly by the linear stage. Therefore,
contrary to conventional switch-mode amplifiers
where the output filter significantly worsens the output
impedance, the presented system concerning this property
(which especially is important for EMC testing applications)
almost is equivalent to a pure linear power amplifier
which, however, would show a much lower efficiency.
The paper describes the operating principle of the system,
specifies the fundamental relationships being relevant to the
circuit design and analyzes the control behavior of the
switch-mode stage as well as of the linear circuit part.

http://aleph.ub.tuwien.ac.at/F?base=tuw01&func=find-c&ccl_term=AC05935464