TS4994 Application information
4.2
Note:
4.3
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
V
CC
/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:
●
●
●
Very high PSRR (power supply rejection ratio).
High common mode noise rejection.
Virtually zero pop without additional circuitry, giving a faster start-up time compared with conventional single-ended input amplifiers.
Easier interfacing with differential output audio DAC.
No input coupling capacitors required due to common mode feedback loop.
In theory, the filtering of the internal bias by an external bypass capacitor is not necessary. But, to reach maximum performance in all tolerance situations, it is better to keep this option.
The main disadvantage is:
● As the differential function is directly linked to the mismatch between external resistors, paying particular attention to this mismatch is mandatory in order to get the best performance from the amplifier.
Gain in typical application schematic
Typical differential applications are shown in
Figure 1
and Figure 2 on page 4 .
In the flat region of the frequency-response curve (no C in expressed by the relation:
effect), the differential gain is
A
V diff
=
V – V
------------------------------------------------------
Diff input+
– Diff input-
=
R
--------------
R in where R in
= R in1
= R in2
and R feed
= R feed1
= R feed2
.
For the rest of this section, Av diff
will be called A
V
to simplify the expression.
Common mode feedback loop limitations
As explained previously, the common mode feedback loop allows the output DC bias voltage to be averaged at V
CC
/2 for any DC common mode bias input voltage.
However, due to V
ICM
limitation of the input stage (see Table 3 on page 6
), the common mode feedback loop can play its role only within a defined range. This range depends upon
21/35
