Technical Note

6/8

BA3121F, BA3123F

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2009.08 - Rev.A

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●Circuit operation

Fig.14 Flow of noise in car audio systems

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

this reason, electrical noise generated by the car electrics

can enter the power amplifier input via the chassis,

and become audible.

The BA3121F/BA3123F makes use of the common-mode

rejection characteristics of an operational amplifier to eliminate

this noise. Without the BA3121F/BA3123F noise enters

the power amplifier input directly, when used, the CMMR of

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

Principles of noise elimination:

To obtain the output voltage (eO)

Fig.15 The principle of noise rejection

With the BA3121F/BA3123F, the elimination level of

the noise is expressed

Therefore, CMRR ≧ 41dB can be guaranteed.

From ① and ②

Ideally, if R1R4 = R2R3, and e1 = e2, the noise voltage will

become zero. However, due to mismatching between the

resistors, difference in the noise voltages (e1 and e2), and

tolerances in the operational amplifier, a noise voltage

does result.