Datasheet
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APPLICATION INFORMATION
GAIN SETTING RESISTORS, R
f
and R
i
f
c(highpass)
1
2 R
i
C
i
(4)
Gain
R
f
R
i
(1)
C
i
1
2 R
i
f
c(highpass)
(5)
Effective Impedance
R
f
R
i
R
f
R
i
(2)
POWER SUPPLY DECOUPLING, C
(S)
f
c(lowpass)
1
2 R
f
C
F
(3)
INPUT CAPACITOR, C
i
TPA6112A2
SLOS342A – DECEMBER 2000 – REVISED SEPTEMBER 2004
The gain for the TPA6112A2 is set by resistors R
f
The value of C
i
directly affects the bass (low fre-
and R
i
according to Equation 1 .
quency) performance of the circuit. Consider the
example where R
i
is 20 k Ω and the specification calls
for a flat bass response down to 20 Hz. Equation 4 is
reconfigured as Equation 5 .
Given that the TPA6112A2 is a MOS amplifier, the
input impedance is very high. Consequently input
leakage currents are not generally a concern. How-
In this example, C
i
is 0.40 µF, so one would likely
ever, noise in the circuit increases as the value of R
f
choose a value in the range of 0.47 µF to 1 µF. A
increases. In addition, a certain range of R
f
values is
further consideration for this capacitor is the leakage
required for proper start-up operation of the amplifier.
path from the input source through the input network
Considering these factors, it is recommended that the
formed by R
i
, C
i
, and the feedback resistor (R
f
) to the
effective impedance seen by the inverting node of the
load. This leakage current creates a dc offset voltage
amplifier be set between 5 k Ω and 20 k Ω . The
at the input to the amplifier that reduces useful
effective impedance is calculated using Equation 2 .
headroom, especially in high-gain applications (gain
>10). For this reason a low-leakage tantalum or
ceramic capacitor is the best choice. When polarized
capacitors are used, connect the positive side of the
capacitor to the amplifier input in most applications.
For example, if the input resistance is 20 k Ω and the
The dc level there is held at V
DD
/2—likely higher than
feedback resistor is 20 k Ω , the gain of the amplifier is
the source dc level. It is important to confirm the
-1, and the effective impedance at the inverting
capacitor polarity in the application.
terminal is 10 k Ω , a value within the recommended
range.
For high performance applications, metal-film re-
The TPA6112A2 is a high-performance CMOS audio
sistors are recommended because they tend to have
amplifier that requires adequate power-supply de-
lower noise levels than carbon resistors. For values
coupling to minimize the output total harmonic distor-
of R
f
above 50 k Ω , the amplifier tends to become
tion (THD). Power-supply decoupling also prevents
unstable due to a pole formed from R
f
and the
oscillations when long lead lengths are used between
inherent input capacitance of the MOS input struc-
the amplifier and the speaker. The optimum decoup-
ture. For this reason, a small compensation capacitor
ling is achieved by using two capacitors of different
of approximately 5 pF should be placed in parallel
types that target different types of noise on the power
with R
f
. This, in effect, creates a low-pass filter
supply leads. For higher frequency transients, spikes,
network with the cutoff frequency defined by
or digital hash on the line, a good low equival-
Equation 3 .
ent-series-resistance (ESR) ceramic capacitor, typi-
cally 0.1 µF, placed as close as possible to the
device V
DD
lead, works best. For filtering
lower-frequency noise signals, a larger aluminum
For example, if R
f
is 100 k Ω and C
F
is 5 pF then
electrolytic capacitor of 10 µF or greater placed near
f
c(lowpass)
is 318 kHz, which is well outside the audio
the power amplifier is recommended.
range.
In the typical application, an input capacitor, C
i
, is
required to allow the amplifier to bias the input signal
to the proper dc level for optimum operation. In this
case, C
i
and R
i
form a high-pass filter with the corner
frequency determined in Equation 4 .
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