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1

C

B

R

SOURCE

v

1

C

I RI

1

R

LCC

(6)

MIDRAIL BYPASS CAPACITOR, CB

1

C

B

R

SOURCE

v

1

C

I RI

(7)

6.25 v 50

(8)

OUTPUT COUPLING CAPACITOR, CC

f

(out high) +

1

2p R

L CC

(9)

TPA112

SLOS212E – AUGUST 1998 – REVISED JUNE 2004

APPLICATION INFORMATION (continued)

Where CBYPASS is the value of the bypass capacitor, and RSOURCE is the equivalent source impedance of the

voltage divider (the parallel combination of the two resistors). For example, if the voltage divider is constructed

using two 20-k

Ω resistors, then R

SOURCE is 10 kΩ.

The midrail bypass capacitor CB serves several important functions. During start-up, CB determines the rate at

which the amplifier starts up. This helps to push the start-up pop noise into the subaudible range (so slow it can

not be heard). The second function is to reduce noise produced by the power supply caused by coupling into the

output drive signal. This noise is from the midrail generation circuit internal to the amplifier. The capacitor is fed

from the resistor divider with equivalent resistance of RSOURCE. To keep the start-up pop as low as possible, the

relationship shown in Equation 7 should be maintained.

As an example, consider a circuit where CB is 1 µF, RSOURCE = 160 kΩ, CI is 1 µF, and RI is 20 kΩ. Inserting

these values into the Equation 8 results in:

which satisfies the rule. Recommended values for bypass capacitor CB are 0.1 µF to 1 µF, ceramic or tantalum

low-ESR, for the best THD and noise performance.

In the typical single-supply, single-ended (SE) configuration, an output coupling capacitor (CC) is required to

block the dc bias at the output of the amplifier, thus preventing dc currents in the load. As with the input coupling

capacitor, the output coupling capacitor and impedance of the load form a high-pass filter governed by

Equation 9.

The main disadvantage, from a performance standpoint, is that the typically small load impedances drive the

low-frequency corner higher. Large values of CC are required to pass low frequencies into the load. Consider the

example where a CC of 68 µF is chosen and loads vary from 32 Ω to 47 kΩ. Table 1 summarizes the frequency

response characteristics of each configuration.

Table 1. Common Load Impedances vs Low Frequency

Output Characteristics in SE Mode

RL

CC

LOWEST FREQUENCY

32

Ω

68 µF

73 Hz

10,000

Ω

68 µF

0.23 Hz

47,000

Ω

68 µF

0.05 Hz

As Table 1 indicates, headphone response is adequate and drive into line level inputs (a home stereo for

example) is good.

The output coupling capacitor required in single-supply, SE mode also places additional constraints on the

selection of other components in the amplifier circuit. With the rules described earlier still valid, add the following

relationship:

•

Output Pulldown Resistor, RC + RO

– Placing a 100-

Ω resistor, R

C, from the output side of the coupling capacitor to ground ensures the coupling

capacitor, CC, is charged before a plug is inserted into the jack. Without this resistor, the coupling capacitor

would charge rapidly upon insertion of a plug, leading to an audible pop in the headphones.

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