Vi =
R
4
(R3R4)
S e
2
eO =
R
1R2
R1
S Vi
R2
R1
e1
eO =
R
1R2
R1
S e2
R2
R1
e1
eO =
R
1R4R2R3
R1 (R3R4)
S e
2
R2
R1
S (e1e2)
R4
(R3R4)
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:
To obtain the output voltage (e
O)
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 (e
1 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 (e
O /ei) (ei = e1 = e2)
Therefore, CMRR 41dB can be guaranteed.
FOperation notes
(1) Maintain a ratio of 2: 1 for the values of the capaci-
tors connected to pin 2 (V
m1) 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 (V
m1)
in the example is doubled, the bass-region CMMR will be
+6dB, and if it is halved, it will be 6dB (see Fig. 16).
