TDA7374

Output stage

11/19

3

Output stage

The fully complementary output stage was made possible by the development of a new

component: the ST exclusive power ICV PNP.

A novel design based upon the connection shown in Figure 17 has then allowed the full

exploitation of its possibilities.

Figure 17.

The new output stage

The clear advantages this new approach has over classical output stages are as follows:

3.1

Rail-to-rail output voltage swing with no need of bootstrap

capacitors

of 0.6

Ω each.

Classical solutions adopting composite PNP-NPN for the upper output stage have higher

saturation loss on the top side of the waveform. This unbalanced saturation causes a

significant power reduction. The only way to recover power consists of the addition of

expensive bootstrap capacitors.

3.2

Absolute stability without any external compensation

By controlling the amount of this local feedback it is possible to force the loop gain (A *

β) to

less than unity at frequency for which the phase shift is 180°C. This means that the output

buffer is intrinsically stable and not prone to oscillation.

Most remarkably, the above feature has been achieved in spite of the very low closed loop

gain of the amplifier.

In contrast, with the classical PNP-NPN stage, the solution adopted for reducing the gain at

high frequencies makes use of external RC networks, namely the Boucherot cells.