TDA7350A Datasheet, PDF (13/22 Pages) STMicroelectronics – 22W BRIDGE-STEREO AMPLIFIER FOR CAR RADIO

Figure 28: ICV - PNP Gain vs. I_C
Figure 29: ICV - PNP VCE(sat) vs. IC
TDA7350A
OUTPUT STAGE
Poor current capability and low cutoff frequency
are well known limits of the standard lateral PNP.
Composite PNP-NPN power output stages have
been widely used, regardless their high saturation
drop. This drop can be overcome only at the ex-
pense of external components, namely, the boot-
strap capacitors. The availability of 4A isolated
collector PNP (ICV PNP) adds versatility to the
design. The performance of this component, in
terms of gain, VCEsat and cut-off frequency, is
shown in fig. 28, 29, 30 respectively. It is realized
in a new bipolar technology, characterized by top-
bottom isolation techniques, allowing the imple-
mentation of low leakage diodes, too. It guaran-
tees BVCEO > 20V and BVCBO > 50V both for
NPN and PNP transistors. Basically, the connec-
tion shown in fig. 31 has been chosen. First of all
because its voltage swing is rail-to-rail, limited
only by the VCEsat of the output transistors,
which are in the range of 0.3Ω each. Then, the
gain VOUT/VIN is greater than unity, approxi-
mately 1+R2/R1. (VCC/2 is fixed by an auxiliary
amplifier common to both channel). It is possible,
controlling the amount of this local feedback, to
force the loop gain (A . β) to less than unity at fre-
quencies for which the phase shift is 180°. This
means that the output buffer is intrinsically stable
and not prone to oscillation.
Figure 31: The New Output Stage
Figure 30: ICV - PNP cut-off frequency vs. IC
In contrast, with the circuit of fig. 32, the solution
adopted to reduce the gain at high frequencies is
the use of an external RC network.
AMPLIFIER BLOCK DIAGRAM
The block diagram of each voltage amplifier is
shown in fig. 33. Regardless of production
spread, the current in each final stage is kept low,
with enough margin on the minimum, below which
cross-over distortion would appear.
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