Practical information

STA540

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Doc ID 13907 Rev 6

6.6

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 22 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.

6.6.1

Rail-to-rail output voltage swing without 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 of the addition of expensive bootstrap capacitors.

6.6.2

Absolute stability without external compensation

With reference to the circuit shown in Figure 22, the low frequency gain Vout/Vin is greater

than unity, that is, approximately 1 + R2/R1. The DC output level (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 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 contrasts with the classical PNP-NPN stage which makes use of external RC networks,

namely the Boucherot cells, for reducing the gain at high frequencies.

Figure 22.

The new output stage