BA3121 データシートの表示（PDF） - ROHM Semiconductor

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BA3121

2-channel head switch for radio cassette recorders

ROHM Semiconductor 

Audio ICs

FCircuit operation

BA3121 / BA3121F / BA3121N

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:

To obtain the output voltage (eO)

Vi =

R4

(R3)R4)

S e2

1

eO

=

*

R2

R1

e1 )

R1)R2

R1

S Vi 2

From 1 and 2

eO

=

*

R2

R1

e1 ) R1)R2

R1

eO

=

*

R2

R1

S (e1*e2) )

S

R4

(R3)R4)

S e2

R1R4*R2R3

R1 (R3)R4)

S e2

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.

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

pressed as: CMMR = 20log (eO / ei) (ei = e1 = e2)

Therefore, CMRR y 41dB can be guaranteed.

FOperation notes

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

tors connected to pin 2 (Vm1) and pin 6 (Vm2) to keep the

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) If the value of the capacitor connected to pin 2 (Vm1)

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

+6dB, and if it is halved, it will be *6dB (see Fig. 16).

721

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