Philips Semiconductors
AN<nnnnn>
TDA1562Q application note
<12NC>
© Koninklijke Philips Electronics N.V. 2004. All rights reserved.
Application note
Rev. 01.02 — 05 May 2006
37 of 62
6. Typical waveforms
Since the class H principle is based on the principle that the supply voltage is raised at
moments that the output power may exceed 20W into 4
Ω
, the waveforms that can be
observed at the outputs are different from waveforms which can normally be observed at
the output of an amplifier.
Usually, the output signal of an amplifier is a symmetrical sine wave signal (see fig. 22).
In the TDA1562 this is only the case up to the point where the supply voltage starts
lifting. When the supply voltage starts lifting, the shape of the signal at each output will no
longer be symmetrical. The following figures show that the sine wave is expanded
upwards, so that the top half of the sine wave will become “sharper” than the bottom half
of the sine wave. However, when the signal is measured across the load (between the
two outputs). The distortion will be compensated and the signal between the two outputs
will still be a sine wave.
Fig 22. Output signals before lifting
When the lift circuits are not yet active, the signals at both outputs are ordinary
sinewaves. Across the load the two outputs are added, resulting in a sine wave with
twice the amplitude of the two output signals (BTL principle).
When the input voltage is increased to a level which would cause a normal class AB
amplifier to start clipping, the lift circuits will activate and the supply voltage of the output
stages will be lifted. The result of this is that the sinewave output signal will expand
upwards.
OUT 2
OUT 1
Vload
