Datasheet

INPUT RESISTANCE
C
i
IN
Z
i
Z
f
Input
Signal
f=
1
2 Z Cp
i i
(1)
INPUT CAPACITOR, C
i
f =
c
1
2 Z Cp
i i
–3dB
f
c
(2)
C =
i
1
2 Z fp
i c
(3)
TPA3125D2
SLOS611 – DECEMBER 2008 ........................................................................................................................................................................................... www.ti.com
Table 2. Gain Setting
INPUT IMPEDANCE
AMPLIFIER GAIN (dB),
GAIN1 GAIN0 (k Ω ),
TYPICAL
TYPICAL
0 0 20 60
0 1 26 30
1 0 32 15
1 1 36 9
Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 9 k Ω ± 20%, to the
largest value, 60 k Ω ± 20%. As a result, if a single capacitor is used in the input high-pass filter, the – 3-dB cutoff
frequency will change when changing gain steps.
The – 3-dB frequency can be calculated using Equation 1 . Use the Z
I
values given in Table 2 .
In the typical application, 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 .
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 k Ω and the specification calls for a flat bass response down to 20 Hz. Equation 2 is
reconfigured as Equation 3 .
In this example, C
i
is 0.4 µ F; so, one would likely choose a value of 0.47 µ F 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
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