Datasheet

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INPUT RESISTANCE
C
I
IN
Z
I
Z
F
Input
Signal
f
*3 dB
+
1
2p C
I
Z
I
(4)
INPUT CAPACITOR, C
I
f
c(highpass)
+
1
2p Z
I
C
I
−3 dB
f
c
(5)
C
I
+
1
2p Z
I
f
c
(6)
C
I
≤ C
BYP
/ 10
(7)
TPA2000D2
SLOS291F – MARCH 2000 – REVISED MARCH 2007
Each gain setting is achieved by varying the input resistance of the amplifier, which can range from its smallest
value to over 6 times that value.
The -3 dB frequency can be calculated using Equation 4 :
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 5 .
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 k Ω and the specification calls for a flat bass response down to 80 Hz. Equation 5 is
reconfigured as Equation 6 .
In this example, C
I
is 0.1 µF, so one would likely choose a value in the range of 0.1 µF to 1 µF. 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. 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
DD
/2, which is likely higher than the source dc level. Note that it is
important to confirm the capacitor polarity in the application.
C
I
should be 10 times smaller than the bypass capacitor to reduce clicking and popping noise from power on/off
and entering and leaving shutdown. After sizing CI for a given cutoff frequency, size the bypass capacitor up to
10 times that of the input capacitor.
12
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