Philips TDA1562Q Application Note page 4

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Philips Semiconductors

The class H amplifier uses an internal circuit to create it's own high supply voltage. The

only additional components necessary to create this high supply voltage are two

electrolytic capacitors with a sufficient capacitance to store energy for low frequency

operation.

The schematic in fig.1 shows how the class H amplifier is built up.

Principally, the output stage is a normal class AB BTL output stage. The special feature

of the class H amplifier lies in the way the supply is connected to the output stage.

In a normal class AB amplifier the collectors of output transistors T1 and T3 would be

directly connected to the positive supply rail. The output stage of the class H amplifier

however, is connected to the power supply (by means of power diodes D1 and D2) and

the + terminals of the lifter capacitors C1 and C2.

When the output power that is required is below 10W, transistors T7 and T8 will be

driven, connecting the negative terminal of the lifter capacitors C1 and C2 to ground. This

allows the lifter capacitors to be charged to near the supply voltage level.

When more output power is required, transistors T7 and T8 will be shut down, and

transistors T5 and T6 will start conducting, lifting the negative terminal of the lifter

capacitors to a higher voltage. The negative terminals of the lifter capacitors can be lifted

to approximately the supply voltage.

When the negative terminal of the lifter capacitors is lifted, the voltage at the positive

terminal of the lifter capacitors will be lifted above the supply voltage, so the lifter

capacitors will start acting as power supply for the power transistors. By this mechanism,

the supply voltage of the output stage can effectively be lifted to nearly twice the supply

voltage coming from the battery.

While the supply voltage of the output stage is lifted above the actual supply voltage, the

amplifier relies on the energy stored in the capacitors. This means that especially for low

frequencies the capacitors must be able to store much energy. For proper operation at

low frequencies it is therefore necessary that large lifter capacitors are used.

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Application note

Fig 1. Basic schematic of a class H amplifier

Rev. 01.02 — 05 May 2006

LIFT/

RECHARGE

CONTROL

Rload

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TDA1562Q application note

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