f=

1

2

Z C

p

i

i

Ci

IN

Zi

Zf

Input

Signal

TPA3121D2

www.ti.com

SLOS537B – MAY 2008 – REVISED JANUARY 2014

Several things can be done to relieve power-supply pumping. The lowest impact is to operate the two inputs out

of phase 180° and reverse the speaker connections. Because most audio is highly correlated, this causes the

supply pumping to be out of phase and not as severe. If this is not enough, the amount of bulk capacitance on

the supply must be increased. Also, improvement is realized by hooking other supplies to this node, thereby,

sinking some of the excess current. Power-supply pumping should be tested by operating the amplifier at low

frequencies and high output levels.

Gain Setting via GAIN0 and GAIN1 Inputs

The gain of the TPA3121D2 is set by two input terminals, GAIN0 and GAIN1.

The gains listed in Table 2 are realized by changing the taps on the input resistors and feedback resistors inside

the amplifier. This causes the input impedance (Zi) to be dependent on the gain setting. The actual gain settings

are controlled by ratios of resistors, so the gain variation from part-to-part is small. However, the input impedance

from part-to-part at the same gain may shift by ±20% due to shifts in the actual resistance of the input resistors.

For design purposes, the input network (discussed in the next section) should be designed assuming an input

impedance of 8 k

Ω, which is the absolute minimum input impedance of the TPA3121D2. At the higher gain

settings, the input impedance could increase as high as 72 k

Ω.

Table 2. Gain Setting

INPUT IMPEDANCE

AMPLIFIER GAIN (dB),

GAIN1

GAIN0

(k

Ω),

TYPICAL

TYPICAL

0

0

20

60

0

1

26

30

1

0

32

15

1

1

36

9

INPUT RESISTANCE

Changing the gain setting can vary the input resistance of the amplifier from its smallest value, 10 k

Ω ±20%, to

the largest value, 60 k

Ω ±20%. As a result, if a single capacitor is used in the input high-pass filter, the –3-dB

cutoff frequency may change when changing gain steps.

The –3-dB frequency can be calculated using Equation 1. Use the Zi values given in Table 2.

(1)

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