TS4994 View Datasheet(PDF) - STMicroelectronics
Part Name
Description
MFG CO.
TS4994
STMicroelectronics
'TS4994' PDF : 31 Pages View PDF
Application Information
TS4994
4 Application Information
4.1 Differential configuration principle
The TS4994 is a monolithic full-differential input/ output power amplifier. The TS4994 also includes a
common mode feedback loop that controls the output bias value to average it at Vcc/2 for any DC
common mode input voltage. This allows the device to always have a maximum output voltage swing,
and by consequence, maximize the output power. Moreover, as the load is connected differentially
compared to a single-ended topology, the output is four times higher for the same power supply voltage.
The advantages of a full-differential amplifier are:
l Very high PSRR (Power Supply Rejection Ratio).
l High common mode noise rejection.
l Virtually zero pop without additional circuitry, giving an faster start-up time compared to conventional
single-ended input amplifiers.
l Easier interfacing with differential output audio DAC.
l No input coupling capacitors required thanks to common mode feedback loop.
l In theory, the filtering of the internal bias by an external bypass capacitor is not necessary. But, to
reach maximal performances in all tolerance situations, it’s better to keep this option.
The main disadvantage is:
l As the differential function is directly linked to external resistors mismatching, in order to reach
maximal performances of the amplifier paying particular attention to this mismatching is mandatory.
4.2 Gain in typical application schematic
Typical differential applications are shown on the figures on page 2.
In the flat region of the frequency-response curve (no Cin effect), the differential gain is expressed by the
relation:
Avdiff
=
VO+ − VO−
Diff.Input + −Diff.Input−
=
Rfeed
Rin
where Rin = Rin1 = Rin2 and Rfeed = Rfeed1 = Rfeed2 .
Note: For the rest of this chapter, Avdiff will be called Av to simplify the expression.
4.3 Common mode feedback loop limitations
As explained previously, the common mode feedback loop allows the output DC bias voltage to be
averaged at Vcc/2 for any DC common mode bias input voltage.
However, due to VICM limitation of the input stage (see Electrical Characteristics on page 4), the
common mode feedback loop can ensure its role only within a defined range. This range depends upon
the values of Vcc, Rin and Rfeed (Av). To have a good estimation of the VICM value, we can apply this
formula:
VICM
=
Vcc × Rin + 2 × VIC × Rfeed
2 × (Rin + Rfeed )
(V)
with
VIC
=
Diff.Input +
+ Diff.Input−
2
(V)
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