INPUT CAPACITOR, C
I
f
=
c
1
2
Z C
p
i
i
–3 dB
f
c
(2)
C =
i
1
2 Z f
p
i c
(3)
Single-Ended Output Capacitor, C
O
Output Filter and Frequency Response
SLOS578 – MAY 2008
........................................................................................................................................................................................................
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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
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 20 Hz.
is
reconfigured as
.
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
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.
In single-ended (SE) applications, the dc blocking capacitor forms a high-pass filter with the speaker impedance.
The frequency response rolls off with decreasing frequency at a rate of 20 dB/decade. The cutoff frequency is
determined by
f
c
=
π
C
O
Z
L
shows some common component values and the associated cutoff frequencies:
Table 3. Common Filter Responses
C
SE
- DC Blocking Capacitor (
µ
F)
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
6
470
680
1500
8
330
470
1000
For the best frequency response, a flat-passband output filter (second-order Butterworth) may be used. The
output filter components consist of the series inductor and capacitor to ground at the LOUT and ROUT pins.
There are several possible configurations, depending on the speaker impedance and whether the output
configuration is single-ended (SE) or bridge-tied load (BTL).
lists the recommended values for the filter
components. It is important to use a high-quality capacitor in this application. A rating of at least X7R is required.
14
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