Datasheet

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−
+
R
F
= 1 k
V
CC+
R
I
= 1 k
R
O
= 10 
R
L
V
I
V
CC−
−
+
R
F
= 1 k
V
CC+
R
I
= 1 k
R
O
= 10 
R
L
V
I−
V
CC−
V
I+
R
I
= 1 k
R
F
= 1 k
TPA6120A2
SLOS431 – MARCH 2004
APPLICATION INFORMATION (continued)
The value of the feedback resistor should be chosen by using Figure 27 through Figure 32 as guidelines. The
gain can then be set by adjusting the input resistor. The smaller the feedback resistor, the less noise is
introduced into the system. However, smaller values move the dominant pole to higher and higher frequencies,
making the device more susceptible to oscillations. Higher feedback resistor values add more noise to the
system, but pull the dominant pole down to lower frequencies, making the device more stable. Higher impedance
loads tend to make the device more unstable. One way to combat this problem is to increase the value of the
feedback resistor. It is not recommended that the feedback resistor exceed a value of 10 kΩ. The typical value
for the feedback resistor for the TPA6120A2 is 1 kΩ. In some cases, where a high-impedance load is used along
with a relatively large gain and a capacitive load, it may be necessary to increase the value of the feedback
resistor from 1 kΩ to 2 kΩ, thus adding more stability to the system. Another method to deal with oscillations is to
increase the size of R
O
.
CAUTION:
Do not place a capacitor in the feedback path. Doing so can cause oscillations.
Capacitance at the outputs can cause oscillations. Capacitance from some sources, such as layout, can be
minimized. Other sources, such as those from the load (e.g., the inherent capacitance in a pair of headphones),
cannot be easily minimized. In this case, adjustments to R
O
and/or R
F
may be necessary.
The series output resistor should be kept at a minimum of 10 Ω. It is small enough so that the effect on the load
is minimal, but large enough to provide the protection necessary such that the output of the amplifier sees little
capacitance. The value can be increased to provide further isolation, up to 100 Ω.
The series resistor, R
S
, should be used for two reasons:
1. It prevents the positive input pin from being exposed to capacitance from the line and source.
2. It prevents the source from seeing the input capacitance of the TPA6120A2.
The 50-Ω resistor was chosen because it provides ample protection without interfering in any noticeable way with
the signal. Not shown is another 50-Ω resistor that can be placed on the source side of R
S
to ground. In that
capacity, it serves as an impedance match to any 50-Ω source.
Figure 25. Single-Ended Input With a Noninverting Gain of -1 V/V
Figure 26. Differential Input With a Noninverting Gain of 2 V/V
Figure 26 shows the TPA6120A2 connected with differential inputs. Differential inputs are useful because they
take the greatest advantage of the device's high CMRR. The two feedback resistor values must be kept the
same, as do the input resistor values.
14