Datasheet
SLOS336C − DECEMBER 2000 − REVISED OCTOBER 2002
17
POST OFFICE BOX 655303 • DALLAS, TEXAS 75265
APPLICATION INFORMATION
input capacitor, C
I
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 8.
(8)
−3 dB
f
c
f
c
+
1
2p R
I
C
I
The value of C
I
is important to consider, as it directly affects the bass (low frequency) performance of the circuit.
Consider the example where R
I
is 10 kΩ and the specification calls for a flat bass response down to 40 Hz.
Equation 8 is reconfigured as equation 9.
(9)
C
I
+
1
2p R
I
f
c
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 path from the input source through the input network (R
I
, C
I
) and
the feedback resistor (R
F
) 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. For this reason a low-leakage tantalum or
ceramic capacitor is the best choice. When polarized capacitors are used, the positive side of the capacitor
should face the amplifier input in most applications, as the dc level there is held at V
DD
/2, which is likely higher
than the source dc level. It is important to confirm the capacitor polarity in the application.
power supply decoupling, C
S
The TPA751 is a high-performance CMOS audio amplifier that requires adequate power supply decoupling to
ensure the output total harmonic distortion (THD) is as low as possible. Power supply decoupling also prevents
oscillations for long lead lengths between the amplifier and the speaker. The optimum decoupling 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 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 audio
power amplifier is recommended.