Datasheet

C
i
IN
Z
i
Z
f
Input
Signal
f=
1
2 Z Cp
i i
f =
c
1
2 Z Cp
i i
-3dB
f
c
C =
i
1
2 Z fp
i c
TPA3106D1
www.ti.com SLOS516C –OCTOBER 2007–REVISED AUGUST 2010
INPUT RESISTANCE
Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 16 kΩ ±20%, to
the largest value, 32 kΩ ±20%. As a result, if a single capacitor is used in the input high-pass filter, the –3 dB or
cutoff frequency may change when changing gain steps.
The –3-dB frequency can be calculated using Equation 1. Use the Z
I
values given in Table 2.
(1)
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 µF; 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 2 V, 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.
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