Technical Note
6/8
BA3121F, BA3123F
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2009.08 - Rev.A
© 2009 ROHM Co., Ltd. All rights reserved.
●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
as: CMMR = 20log (e
0
/e
I
)(e
I
= e
1
= e
2
)
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.
