*

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*

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*

S

1

2

721

Audio ICs

BA3121 / BA3121F / BA3121N

FCircuit operation

Car-audio systems are earthed to the car body, and for

this reason, electrical noise generated by the car elec-

trics can enter the power amplifier input via the chassis,

and become audible.

The BA3121 makes use of the common-mode rejection

characteristics of an operational amplifier to eliminate

this noise. Without the BA3121 noise enters the power

amplifier input directly, when used, the CMMR of opera-

tional amplifiers 1-A and 2-A eliminates the noise.

Principles of noise elimination:

become zero. However, due to mismatching between the

tolerances in the operational amplifier, a noise voltage

does result.

With the BA3121, the elimination level of the noise is ex-

Therefore, CMRR

y 41dB can be guaranteed.

FOperation notes

(1)

Maintain a ratio of 2: 1 for the values of the capaci-

ripple rejection ratio stable. If this ratio is maintained, the

ripple rejection ratio will not vary significantly even if the

capacitance values are halved.

(2)

in the example is doubled, the bass-region CMMR will be

+6dB, and if it is halved, it will be

*6dB (see Fig. 16).