Technical Note
12/21
BD88400GUL,BD88410GUL,BD88415GUL,BD88420GUL
www.rohm.com
2011.03 – Rev. A
© 2011 ROHM Co., Ltd. All rights reserved.
●Functional descriptions
The conventional headphone amplifier composition is occupied to Fig.57. In this composition, the signal is output by using
the middle point bias circuit based on the middle point bias. Therefore, the output coupling capacitor that removes the DC
voltage difference and does the AC coupling is necessary. This coupling capacitor and the impedance of the headphone
composes the high-pass filter. Therefore, the signal degradation in the low frequency region learns by experience. The
output coupling capacitor should be a large capacity, because the cutoff frequency of this high-pass filter becomes the
following formula (1).
CL
c
CR2
1
f
π
(1)
* Cc is the coupling capacitor, and RL is the impedance of the headphone.
Moreover, POP noise by the middle point bias start-up is generated and the degradation of PSRR learns by experience.
+
-
VDD
GND
+
Vout
Input
time [s]
Vout [V]
VDD/2
Middle Point
Bias Circuit
VDD
0
Vhp
0
Vhp [V]
time [s]
Cc
Fig.57 Conventional headphone amplifier composition
The composition of the series of BD884xxGUL is occupied to Fig.58. In this composition, the signal is output by using a
negative voltage based on the ground level. Therefore, the amplifier output can be connected directly with the headphone.
And, the output coupling capacitor becomes unnecessary. Additionally, the signal degradation in the low frequency region
with the coupling capacitor is not generated, and the deep bass is achieved.
Moreover, POP noise is controlled because of no middle point bias start-up. And, the degradation of PSRR doesn't occur by
being based on the ground.
+
-
HPVDD
Vout
Input
time [s]
Vout [V]
VDD
Charge
Pump
VSS
0
CF : Flying
Capacitor
CH : Hold
Capacitor
Vhp
0
Vhp [V]
time [s]
HPVDD
Fig.58 Composition of the series of BD884xxGUL
