OUTPUT STAGE

The fully complementary output stage was made

possible by the development of a new compo-

nent: the ST exclusive power ICV PNP.

A novel design based upon the connection shown

in fig. 19 has then allowed the full exploitation of

its possibilities.

The clear advantages this new approach has over

classical output stages are as follows:

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

Classical solutions adopting composite PNP-NPN

for the upper output stage have higher saturation

loss on the top side of the waveform. This unbal-

anced saturation causes a significant power re-

duction. The only way to recover power consists

of the addition of expensive bootstrap capacitors.

2 - Absolute Stability Without Any External

Compensation.

Referring to the circuit of Fig. 19

the gain

iliary 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 for which the phase shift

is 180 Deg. 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 (20 dB).

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.

OTHER OUTSTANDING CHARACTERISTICS:

Clipping Detector Output

The TDA7370 is equipped with an internal circuit

able to detect the output stage saturation provid-

ing a current sinking into a open collector output

(pin 10) when a certain distortion level is reached

at each output.

This particular function allows gain compression

facility whenever the amplifier is overdriven, thus

obtaining high quality sound at all listening levels.

Figure 19: The new Output Stage

Figure 20: Clipping Detection Waveforms

TDA7370

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