G1427 Datasheet, PDF (14/16 Pages) Global Mixed-mode Technology Inc – 2W Stereo Audio Amplifier

Global Mixed-mode Technology Inc.
G1427
Where 170kΩ is the output impedance of the
mid-rail generator, C_B is the mid-rail bypass ca-
pacitor, C_I is the input coupling capacitor, R_I is the
input impedance, R_F is the gain setting impedance
which is on the feedback path. C_B is the most im-
portant capacitor. Besides it is used to reduce the
popping, C_B can also determine the rate at which
the amplifier starts up during startup or recovery
from shutdown mode.
Output coupling capacitor
G1427 can drive clean, low distortion SE output power
with gain –1V/V into headphone loads (generally 16Ω
or 32Ω) as in Figure 3. Please refer to Electrical
Characteristics to see the performances. A coupling
capacitor is needed to block the dc-offset voltage, al-
lowing pure ac signals into headphone loads. Choos-
ing the coupling capacitor will also determine the -3dB
point of the high-pass filter network, as Figure 4.
De-popping circuitry of G1427 is shown as below
Figure 2. The PNP transistor limits the voltage drop
across the 120kΩ by slewing the internal node
slowly when power is applied. At start-up, the volt-
age at BYPASS capacitor is 0. The PNP is ON to
pull the mid-point of the bias circuit down. So the
capacitor sees a lower effective voltage, and thus
the charging is slower. This appears as a linear
ramp (while the PNP transistor is conducting), fol-
lowed by the expected exponential ramp of an R-C
circuit.
For better performance, C_B is recommended to be
at least 1.5 times of input coupling capacitor C_I. For
example, if using 1µF input coupling capacitor,
2.2µF ceramic or tantalum low-ESR capacitors are
recommended to achieve the better THD perform-
ance.
f_C=1/(2πR_LC_C)
For example, a 220µF capacitor with 32Ω headphone
load would attenuate low frequency performance be-
low 22.6Hz. So the coupling capacitor should be well
chosen to achieve the excellent bass performance
when in SE mode operation.
VDD
Vo(PP)
C_C
R_L
Vo(PP)
Bypass
120 kΩ
VDD
100 kΩ
100 kΩ
Figure 3
-3 dB
Figure 2
fc
Figure 4
Ver: 1.3
Sep 23, 2005
TEL: [phone]
http://www.gmt.com.tw
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