Datasheet

f =
c
1
2 Z Cp
i i
–3dB
f
c
C =
i
1
2 Z fp
i c
TPA3123D2
SLOS541C –JULY 2007–REVISED AUGUST 2010 www.ti.com
INPUT CAPACITOR, C
I
In the typical application, an input capacitor (C
i
) is required to allow the amplifier to bias the input signal to the
proper dc level for optimum operation. In this case, C
i
and the input impedance of the amplifier (Z
i
) form a
high-pass filter with the corner frequency determined in Equation 2.
(2)
The value of C
i
is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider
the example where Z
i
is 20 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 2 is
reconfigured as Equation 3.
(3)
In this example, C
i
is 0.4 mF; so, one would likely choose a value of 0.47 mF, as this value is commonly used. If
the gain is known and is constant, use Z
i
from Table 2 to calculate C
i
. A further consideration for this capacitor is
the leakage path from the input source through the input network (C
i
) and the feedback network to the load. This
leakage current creates a dc offset voltage at the input to the amplifier that reduces useful headroom, especially
in high-gain applications. For this reason, a low-leakage tantalum or ceramic capacitor is the best choice. When
polarized capacitors are used, the positive side of the capacitor should face the amplifier input in most
applications as the dc level there is held at V
BYP
(V
CC
/8), which is likely higher than the source dc level. Note that
it is important to confirm the capacitor polarity in the application. Additionally, lead-free solder can create dc
offset voltages, and it is important to ensure that boards are cleaned properly.
Single-Ended Output Capacitor, C
O
In single-ended (SE) applications, the dc blocking capacitor forms a high-pass filter with the speaker impedance.
The frequency response rolls of with decreasing frequency at a rate of 20 dB/decade. The cutoff frequency is
determined by:
f
c
= 1/2pC
O
Z
L
(4)
Table 3 shows some common component values and the associated cutoff frequencies:
Table 3. Common Filter Responses
C
SE
– DC Blocking Capacitor (mF)
Speaker Impedance (Ω)
f
c
= 60 Hz (–3 dB) f
c
= 40 Hz (–3 dB) f
c
= 20 Hz (–3 dB)
4 680 1000 2200
8 330 470 1000
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