C =

i

1

2

Z f

p

i

c

f

=

c

1

2

Z C

p

i

i

–3dB

fc

TPA3121D2

SLOS537B – MAY 2008 – REVISED JANUARY 2014

www.ti.com

INPUT CAPACITOR, Ci

In the typical application, an input capacitor (Ci) is required to allow the amplifier to bias the input signal to the

proper dc level for optimum operation. In this case, Ci and the input impedance of the amplifier (Zi) form a high-

pass filter with the corner frequency determined in Equation 2.

(2)

The value of Ci is important, as it directly affects the bass (low-frequency) performance of the circuit. Consider

the example where Zi is 20 kΩ and the specification calls for a flat bass response down to 20 Hz. Equation 2 is

reconfigured as Equation 3.

(3)

In this example, Ci is 0.4 μF; so, one would likely choose a value of 0.47 μF as this value is commonly used. If

the gain is known and is constant, use Zi from Table 2 to calculate Ci. A further consideration for this capacitor is

the leakage path from the input source through the input network (Ci) 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.

Single-Ended Output Capacitor, CO

In single-ended (SE) applications, the dc blocking capacitor forms a high-pass filter with the speaker impedance.

The frequency response rolls of with decreasing frequency at a rate of 20 dB/decade. The cutoff frequency is

determined by

fc = 1/2πCOZL

(4)

Table 3 shows some common component values and the associated cutoff frequencies:

Table 3. Common Filter Responses

CSE - DC Blocking Capacitor (μF)

Speaker Impedance (

Ω)

fc = 60 Hz (–3 dB)

fc = 40 Hz (–3 dB)

fc = 20 Hz (–3 dB)

4

680

1000

2200

6

470

680

1500

8

330

470

1000

Output Filter and Frequency Response

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). Table 4 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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