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TA2022

STEREO 90W (4Ω) CLASS-T™ DIGITAL AUDIO AMPLIFIER DRIVER USING DIGITAL POWER PROCESSING (DPP™) TECHNOLOGY

Tripath Technology Inc. 

Tripath Technology, Inc. - Technical Information

INPUT1

INPUT2

RF

CI

RI

RF

CI

RI

OAOUT1

INV1

TA2022

2

V5

-

+

BIASCAP

INV2

OAOUT2

AGND

V5

+

-

AGND

Figure 1: Input Stage

The gain of the input stage, above the low frequency high pass filter point, is that of a simple inverting

amplifier: It should be noted that the input opamps are biased at approximately 2.5VDC. Thus, the

polarity of CI must be followed as shown in Figure 1 for a standard ground referenced input signal

A VINPUTSTAG E = − R F

RI

Input Capacitor Selection

CI can be calculated once a value for RI has been determined. CI and RI determine the input low

frequency pole. Typically this pole is set below 10Hz. CI is calculated according to:

CI= 1

2π fP R I

where:

RI = Input resistor value in ohms.

fP = Input low frequency pole (typically 10Hz or below).

Modulator Feedback Design

The modulator converts the signal from the input stage to the high-voltage output signal. The

optimum gain of the modulator is determined from the maximum allowable feedback level for the

modulator and maximum supply voltages for the power stage. Depending on the maximum supply

voltage, the feedback ratio will need to be adjusted to maximize performance. The values of RFBA,

RFBB and RFBC (see explanation below) define the gain of the modulator. Once these values are

chosen, based on the maximum supply voltage, the gain of the modulator will be fixed even with as

the supply voltage fluctuates due to current draw.

For the best signal-to-noise ratio and lowest distortion, the maximum modulator feedback voltage

should be approximately 4Vpp. This will keep the gain of the modulator as low as possible and still

allow headroom so that the feedback signal does not clip the modulator feedback stage.

Figure 2 shows how the feedback from the output of the amplifier is returned to the input of the

modulator. The input to the modulator (FBKOUT1/FBKGND1 for channel 1) can be viewed as inputs

to an inverting differential amplifier. RFBA and RFBB bias the feedback signal to approximately 2.5V and

RFBC scales the large OUT1/OUT2 signal to down to 4Vpp.

16

TA2022 – KLI/1.2/07-04

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