Datasheet

C =
i
1
2 Z fp
i c
f =
c
1
2 Z Cp
i i
-3dB
f
c
f=
1
2 Z Cp
i i
C
i
IN
Z
i
Z
f
Input
Signal
TPA3111D1
www.ti.com SLOS618E –AUGUST 2009–REVISED AUGUST 2012
INPUT RESISTANCE
Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 9 kΩ ±20%, to the
largest value, 60 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 2. Use the Z
I
values given in Table 1.
(2)
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 3.
(3)
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 60 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 3 is
reconfigured as Equation 4.
(4)
In this example, C
I
is 0.13 µF; so, one would likely choose a value of 0.15 μF as this value is commonly used. If
the gain is known and is constant, use Z
I
from Table 1 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. If a
ceramic capacitor is used, use a high quality capacitor with good temperature and voltage coefficient. An X7R
type works well and if possible use a higher voltage rating than required. This will give a better C vs voltage
characteristic. 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 3 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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