Datasheet

Gain + *
ǒ
R
f
R
i
Ǔ
f
c(highpass)
+
1
2p R
i
C
i
C
i
+
1
2p R
i
f
c(highpass)
Effective Impedance +
R
f
R
i
R
f
) R
i
f
c(lowpass)
+
1
2p R
f
C
F
TPA6110A2
SLOS314B –DECEMBER 2000– REVISED MARCH 2011 www.ti.com
APPLICATION INFORMATION
GAIN SETTING RESISTORS, R
f
and R
i
INPUT CAPACITOR, C
i
The gain for the TPA6110A2 is set by resistors R
f
In the typical application, an input capacitor, C
i
, is
and R
i
according to Equation 1. 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.
(1)
Given that the TPA6110A2 is a MOS amplifier, the
(4)
input impedance is very high. Consequently input
leakage currents are not generally a concern.
The value of C
i
directly affects the bass (low
However, noise in the circuit increases as the value
frequency) performance of the circuit. Consider the
of R
f
increases. In addition, a certain range of R
f
example where R
i
is 20 kΩ and the specification calls
values is required for proper start-up operation of the
for a flat bass response down to 20 Hz. Equation 4 is
amplifier. Considering these factors, it is
reconfigured as Equation 5.
recommended that the effective impedance seen by
the inverting node of the amplifier be set between 5
kΩ and 20 kΩ. The effective impedance is calculated
using Equation 2.
(5)
In this example, C
i
is 0.40 µF, so one would likely
choose a value in the range of 0.47 µF to 1 µF. A
further consideration for this capacitor is the leakage
(2)
path from the input source through the input network
For example, if the input resistance is 20 kΩ and the
formed by R
i
, C
i
, and the feedback resistor (R
f
) to the
feedback resistor is 20 kΩ, the gain of the amplifier is
load. This leakage current creates a dc offset voltage
-1, and the effective impedance at the inverting
at the input to the amplifier that reduces useful
terminal is 10 kΩ, a value within the recommended
headroom, especially in high-gain applications (gain
range.
>10). For this reason a low-leakage tantalum or
ceramic capacitor is the best choice. When polarized
For high performance applications, metal-film
capacitors are used, connect the positive side of the
resistors are recommended because they tend to
capacitor to the amplifier input in most applications.
have lower noise levels than carbon resistors. For
The dc level there is held at V
DD
/2—likely higher than
values of R
f
above 50 kΩ, the amplifier tends to
the source dc level. It is important to confirm the
become unstable due to a pole formed from R
f
and
capacitor polarity in the application.
the inherent input capacitance of the MOS input
structure. For this reason, a small compensation
POWER SUPPLY DECOUPLING, C
(S)
capacitor of approximately 5 pF should be placed in
parallel with R
f
. This, in effect, creates a low-pass
The TPA6110A2 is a high-performance CMOS audio
filter network with the cutoff frequency defined by
amplifier that requires adequate power-supply
Equation 3.
decoupling to minimize the output total harmonic
distortion (THD). Power-supply decoupling also
prevents oscillations when long lead lengths are used
between the amplifier and the speaker. The optimum
(3) decoupling is achieved by using two capacitors of
different types that target different types of noise on
For example, if R
f
is 100 kΩ and C
F
is 5 pF then
the power supply leads. For higher frequency
f
c(lowpass)
is 318 kHz, which is well outside the audio
transients, spikes, or digital hash on the line, a good
range.
low equivalent-series-resistance (ESR) ceramic
capacitor, typically 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 electrolytic capacitor of 10 µF or greater
placed near the power amplifier is recommended.
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