TPA0233
2-W MONO AUDIO POWER AMPLIFIER
WITH HEADPHONE DRIVE
SLOS278D – JANUARY 2000 – REVISED NOVEMBER 2002
APPLICATION INFORMATION
gain setting via input resistance
The gain of the input stage is set by the user-selected input resistor and a 50-kΩ internal feedback resistor.
However, the power stage is internally configured with a gain of –1.25 V/V in stereo mode, and –2.5 V/V in mono
mode. Thus, the feedback resistor (R
F
) is effectively 62.5 kΩ in stereo mode and 125 kΩ in mono mode.
Therefore, the overall gain can be calculated using equations (1) and (2) stereo.
A
+
–125 kW
V
R
I
A
+
–62.5 kW
V
R
I
(Mono)
(1)
(Stereo)
(2)
The –3 dB frequency can be calculated using equation 3.
ƒ
–3 dB
+
1
2p R C
I i
(3)
If the filter must be more accurate, the value of the capacitor should be increased while the value of the resistor
to ground should be decreased. In addition, the order of the filter could be increased.
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 the input resistance of the amplifier, R
I
, form a
high-pass filter with the corner frequency determined in equation 4.
–3 dB
f
c(highpass)
+
1
2
p
R
I
C
i
(4)
fc
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 2 is reconfigured as equation 5.
1
C
+
i
2p R f c
I
(5)
In this example, C
I
is 0.4
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 (C
i
) and the
feedback network 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. Note that it is important to confirm the capacitor polarity in the application.
12
POST OFFICE BOX 655303
•
DALLAS, TEXAS 75265
TI [ TEXAS INSTRUMENTS ]
