G1428
| Model | G1428 |
| Description | 2W Stereo Audio Amplifier |
| PDF file | Total 16 pages (File size: 339K) |
| Chip Manufacturer | GMT |
Global Mixed-mode Technology Inc.
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.
De-popping circuitry of G1428 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 1uF input coupling capacitor,
2.2
µ
F ceramic or tantalum low-ESR capacitors are
recommended to achieve the better THD perform-
ance.
VDD
100 kΩ
120 kΩ
Bypass
100 kΩ
-3 dB
G1428
Output coupling capacitor
G1428 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.
f
C
=1/(2πR
L
C
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)
Figure 3
f
c
Figure 2
Figure 4
Ver: 1.2
Mar 31, 2005
TEL: 886-3-5788833
http://www.gmt.com.tw
14
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