STA540SA

General structure

Doc ID 16066 Rev 2

19/30

8.6

Output stage

The fully complementary output stage is possible with the power ICV PNP component.

This novel design is based on the connection shown in Figure 28 and allows the full

exploitation of its capabilities. The clear advantages this new approach has over classical

output stages are described in the following sections.

8.6.1

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

The output swing is limited only by the VCEsat of the output transistors, which are in the

range of 0.3

Ω (Rsat) 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 includes the addition of expensive bootstrap capacitors.

8.6.2

Absolute stability without any external compensation

With reference to the circuit shown in Figure 28, the gain Vout/Vin is greater than unity, that

is, approximately 1+R2/R1. The DC output (VCC/2) is fixed by an auxiliary amplifier common

to all the channels.

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

β) to

less than unity at a frequency where the phase shift is 180 °. This means that the output

buffer is intrinsically stable and not prone to oscillation.

The above feature has been achieved even though there is very low closed-loop gain of the

amplifier.

This is in contrast with the classical PNP-NPN stage where the solution adopted for

reducing the gain at high frequencies makes use of external RC networks, namely the

Boucherot cells.

Figure 28.

The new output stage

Obsolete

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- Obsolete

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