Datasheet

Practical information STA540
18/25 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 V
CEsat
of the output transistors, which are in the
range of 0.3 Ω (R
sat
) 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 V
out
/V
in
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