Datasheet

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SINGLE-ENDED OPERATION
Audio
Input
V
DD
6
5
8
V
O
1
V
O
2
V
DD
2
3
4
IN+
IN−
BYPASS
V
DD
/2
C
I
R
I
R
F
C
S
C
B
250-mW
External
Speaker
C
SE
= 0.1 µF
R
SE
= 50 Ω
C
C
−
+
−
+
Gain  

R
F
R
I

(11)
1

C
B
 25 kΩ


1

C
I
R
I


1
R
L
C
C
(12)
OUTPUT COUPLING CAPACITOR, C
C
f
out high

1
2 R
L
C
C
(13)
TPA4861
SLOS163C – SEPTEMBER 1996 – REVISED JUNE 2004
Figure 38 is a schematic diagram of the recommended SE configuration. In SE mode configurations, the load
should be driven from the primary amplifier output (V
O
1, terminal 5).
Figure 38. Singled-Ended Mode
Gain is set by the R
F
and R
I
resistors and is shown in Equation 11 . Because the inverting amplifier is not used to
mirror the voltage swing on the load, the factor of 2 is not included.
The phase margin of the inverting amplifier into an open circuit is not adequate to ensure stability, so a
termination load should be connected to V
O
2. This consists of a 50-Ω resistor in series with a 0.1-µF capacitor to
ground. It is important to avoid oscillation of the inverting output to minimize noise and power dissipation.
The output coupling capacitor required in single-supply SE mode also places additional constraints on the
selection of other components in the amplifier circuit. The rules described earlier still hold with the addition of the
following relationship:
In the typical single-supply SE configuration, an output coupling capacitor (C
C
) is required to block the dc bias at
the output of the amplifier thus preventing dc currents in the load. As with the input coupling capacitor, the output
coupling capacitor and impedance of the load form a high-pass filter governed by Equation 13 .
The main disadvantage, from a performance standpoint, is that the load impedances are typically small, which
drives the low-frequency corner higher. Large values of C
C
are required to pass low frequencies into the load.
Consider the example where a C
C
of 68 µF is chosen and loads vary from 8 Ω, 32 Ω, and 47 kΩ. Table 2
summarizes the frequency response characteristics of each configuration.
18