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TPA102_14 Datasheet, PDF (17/26 Pages) Texas Instruments – 150-mW STEREO AUDIO POWER AMPLIFIER
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TPA102
www.ti.com
SLOS213D – AUGUST 1998 – REVISED SEPTEMBER 2004
APPLICATION INFORMATION
GAIN SETTING RESISTORS, Rf and Ri
The gain for the TPA102 is set by resistors Rf and Ri
according to Equation 1.
ǒ Ǔ Gain + *
Rf
Ri
(1)
Given that the TPA102 is a MOS amplifier, the input
impedance is very high. Consequently input leakage
currents are not generally a concern. However, noise
in the circuit increases as the value of Rf increases.
In addition, a certain range of Rf values is required for
proper start-up operation of the amplifier. Considering
these factors, it is 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.
Effective Impedance
+
RfRi
Rf ) Ri
(2)
For example, if the input resistance is 20 kΩ and the
feedback resistor is 20 kΩ, the gain of the amplifier is
-1, and the effective impedance at the inverting
terminal is 10 kΩ, a value within the recommended
range.
For high performance applications, metal-film re-
sistors are recommended because they tend to have
lower noise levels than carbon resistors. For values
of Rf above 50 kΩ, the amplifier tends to become
unstable due to a pole formed from Rf and the
inherent input capacitance of the MOS input struc-
ture. For this reason, a small compensation capacitor
of approximately 5 pF should be placed in parallel
with Rf. This, in effect, creates a low-pass filter
network with the cutoff frequency defined by
Equation 3.
fc(lowpass)
+
1
2 p Rf CF
(3)
For example, if Rf is 100 kΩ and CF is 5 pF then
fc(lowpass) is 318 kHz, which is well outside the audio
range.
fc(highpass)
+
1
2 p Ri Ci
(4)
The value of Ci directly affects the bass (low fre-
quency) performance of the circuit. Consider the
example where Ri is 20 kΩ and the specification calls
for a flat bass response down to 20 Hz. Equation 4 is
reconfigured as Equation 5.
Ci
+
1
2p Rifc(highpass)
(5)
In this example, Ci 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
path from the input source through the input network
formed by Ri, Ci, and the feedback resistor (Rf) to the
load. This leakage current creates a dc offset voltage
at the input to the amplifier that reduces useful
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.
The dc level there is held at VDD/2—likely higher than
the source dc level. It is important to confirm the
capacitor polarity in the application.
POWER SUPPLY DECOUPLING, C(S)
The TPA102 is a high-performance CMOS audio
amplifier that requires adequate power-supply de-
coupling to minimize the output total harmonic distor-
tion (THD). Power-supply decoupling also prevents
oscillations when long lead lengths are used between
the amplifier and the speaker. The optimum decoup-
ling is achieved by using two capacitors of different
types that target different types of noise on the power
supply leads. For higher frequency transients, spikes,
or digital hash on the line, a good low equival-
ent-series-resistance (ESR) ceramic capacitor, typi-
cally 0.1 µF, placed as close as possible to the
device VDD 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.
INPUT CAPACITOR, Ci
In the typical application, an input capacitor, Ci, is
required to allow the amplifier to bias the input signal
to the proper dc level for optimum operation. In this
case, Ci and Ri form a high-pass filter with the corner
frequency determined in Equation 4.
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