Philips 37PF9631D/37 Service Manual page 107

Circuit Descriptions, Abbreviation List, and IC Data Sheets EJ2.0U LA 9. EN 107
9.2 Audio Amplifier
In the Jaguar platforms of this generation, a so-called "Class-
D" audio-amplifier is used.
Conventional audio amplifiers operate in class AB, which
means that their output stages operate linearly. Class-D
amplifiers use switching techniques -- like in a switch-mode
power supply -- so the power stage is a simple switch. The
advantage of the class-D technique is that the circuit is more
efficient and less power is wasted.
+V
-V
G_15940_043.eps
240506
Figure 9-3 Principle Class-D ampifier
The class-D amplifier works by varying the duty cycle of a Pulse
Width Modulated (PWM) signal.
By comparing the input voltage to a triangle wave, the amplifier
increases duty cycle to increase output voltage, and decreases
duty cycle to decrease output voltage.
The output transistors (item 7D18 on diagram B08A) of a Class
D amplifier switch from 'full off' to 'full on' (saturated) and then
back again, spending very little time in the linear region in
between. Therefore, very little power is lost to heat.
In this chassis, discrete components are used for the
implementation of the audio class-D amplifiers. The amplifiers
are situated on-board the SSB.