Structure of DTV Networks and Transmitting Stations - Digital Video and Audio Broadcasting Technology - page 532

31.3.1.1.2 Single Power Amplifier Stage (appr. 50 W)
Firstly, the basic principle of a single power amplifier stage for
DVB-T/ATSC/Analog TV of the 50 W averaged power will be explained.
Such power amplifiers also form the high-power transmitter for
DVB/ATSC/Analog TV. The input signal is split into two signals of -3dB
power each and 90 degrees phase difference by means of a 3 dB coupler.
Each amplifier path consists of a class AB amplifier which, in turn, con-
sists of a dual transistor operated in push-pull mode. The operating point is
set in such a way that similar conditions are achieved for all transistors in
class AB mode and the transfer distortion is minimal. In contrast to FM
transmitters, the amplifiers used here are very linear transmitters but must
still be pre-corrected. In FM transmitters, class C amplifiers are used
which are quite nonlinear but have much greater efficiency.
Vdc=approx. 30 V DC
Vdc
Vdc
50 Ohm
Push pull
MOS FET
See above
50 Ohm
Fig. 31.7. Principle of a single power amplifier stage (50 W), used to build up a
power amplifier module
In the FM transmitter, the required power can even be chosen by con-
trolling the supply voltage of the class C amplifiers. In television transmit-
ters, where very good linearity is required, power is controlled via the am-
plifier feed power. This applies especially to analog television, but also to
digital television (DVB-T, ATSC, ISDB-T). These amplifiers operate with
VMOS transistors (VHF band), or LDMOS transistors (UHF band) and are