2.2 mF

15

H

m

15 mH

OUTP

OUTN

L1

L2

C2

C3

2.2 mF

Figure 8-5. Typical LC Output Filter, Cutoff Frequency of 27 kHz, Speaker Impedance = 6 Ω

8.2.2.4 Input Resistance

The typical input resistance of the amplifier is fixed to 20 kΩ ±15% for 26dB Gain and 40kΩ ±15% for 20dB

Gain .

Ci

IN

Zi

Zf

Input

Signal

8.2.2.5 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.

f

=

c

1

2

Z C

p

i

i

-3dB

fc

(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 (26dB Gain) and the specification calls for a flat bass response down to 20 Hz.

Equation 2 is reconfigured as Equation 3.

C =

i

1

2

Z f

p

i

c

(3)

In this example, Ci is 0.4 µF; so, one would likely choose a value of 0.39 μF as this value is commonly used. 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. 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 3 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.

TPA3139D2

SLOS810A – OCTOBER 2019 – REVISED AUGUST 2020

www.ti.com

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