G1431
Global Mixed-mode Technology Inc.
Application Information
Gain setting via GAIN0 and GAIN1 inputs
The internal gain setting is determined by two input
terminals, GAIN0 and GAIN1. The gains listed in Table
1 are realized by changing the taps on the input resis-
tors inside the amplifier. This will cause the internal
input impedance, ZI, to be dependent on the gain set-
ting. Although the real input impedance will shift by
30% due to process variation from part-to-part, the
actual gain settings are controlled by the ratios of the
resistors and the actual gain distribution from part-to-
part is quite good.
Table 2
Zi (kΩ)
30
AV (dB)
21.6
15.6
10
45
70
90
6
Input Capacitor
In the typical application, an input capacitor Ci is re-
quired to allow the amplifier to bias the input signal to
the proper dc level for optimum operation. In this case,
Ci and the input impedance of the amplifier, Zi, form a
high-pass filter with the –3dB determined by the equa-
tion: f-3dB= 1/ 2πRI Ci
Table 1
GAIN0
GAIN1
AV (dB)
6
0
0
1
1
0
1
0
1
The value of Ci is important to consider as it directly
affects the bass performance of the application circuit.
For example, if the input resistor is 15kΩ, the input
capacitor is 1µF, the flat bass response will be down to
10.6Hz.
10
15.6
21.6
Input Resistance
The typical input impedance at each gain setting is
given in the Table 2. Each gain setting is achieved by
varying the input resistance of the amplifier, which can
be over 6 times from its minimum value to the maxi-
mum value. As a result, if a single capacitor is used in
the input high pass filter, the –3dB or cut-off frequency
will be also change over 3.5 times. To reduce the
variation of the cut-off frequency, an additional resistor
can be connected from the input pin of the amplifier to
the ground, as shown in the figure below. With the
extra resistor, the cut-off frequency can be
Because the small leakage current of the input ca-
pacitors will cause the dc offset voltage at the input to
the amplifier that reduces the operation headroom,
especially at the high gain applications. The low-
leakage tantalum or ceramic capacitors are suggested
to be used as the input coupling capacitors. When
using the polarized capacitors, it is important to let the
positive side connecting to the higher dc level of the
application.
Power Supply Decoupling
re-calculated using equation :
f-3dB= 1/ 2πC(R||RI).
The G1431 is a high-performance CMOS audio ampli-
fier that requires adequate power supply decoupling to
make sure the output total harmonic distortion (THD)
as low as possible. The optimum decoupling is using
two capacitors with different types that target different
types of noise on the power supply leads. For high
frequency transients, spikes, a good low ESR ceramic
capacitor works best, typically 0.1µF/1µF used and
placed as close as possible to the G1431 VDD lead. A
larger aluminum electrolytic capacitor of 10µF or
greater placed near the device power is recommended
for filtering low-frequency noise.
Using small external R can reduce the variation of the
cut-off frequency. But the side effect is small external
R will also let (R||RI) become small, the cut-off fre-
quency will be larger and degraded the bass-band
performance. The other side effect is with extra power
dissipation through the external resistor R to the
ground. So using the external resistor R to flatting the
variation of the cut-off frequency, the user must also
consider the bass-band performance and the extra
power dissipation to choose the accepted external
resistor R value.
Optimizing DEPOP Operation
Circuitry has been implemented in G1431 to minimize
the amount of popping heard at power-up and when
coming out of shutdown mode. Popping occurs when-
ever a voltage step is applied to the speaker and
making the differential voltage generated at the two
ends of the speaker. To avoid the popping heard, the
bypass capacitor should be chosen promptly,
1/(CBx170kΩ) ≦ 1/(CI*(RI+RF)). Where 170kΩ is the
output impedance of the mid-rail generator, CB is the
mid-rail bypass capacitor, CI is the input coupling ca-
pacitor, RI is the input impedance, RF is the gain set-
C
Zi
Zf
Input Signal
IN
R
TEL: 886-3-5788833
http://www.gmt.com.tw
Ver: 1.3
Sep 23, 2005
11
GMT [ GLOBAL MIXED-MODE TECHNOLOGY INC ]
