TPA6120A2 Datasheet, PDF (10/31 Pages) Texas Instruments – HIGH FIDELITY HEADPHONE AMPLIFIER

TPA6120A2
SLOS431B – MARCH 2004 – REVISED FEBRUARY 2015
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Typical Application (continued)
10.2.1.1 Design Requirements
• ±12V Operation from bipolar power supply
• Differential voltage source
• Be transparent to the user
10.2.1.2 Detailed Design Procedure
The output of the PCM1792 is current, not voltage, so the OPA4134 is used to convert the current input to a
voltage output. The OPA4134 (SBOS058), is a low-noise, high-speed, high-performance operational amplifier. C_F
and R_F are used to set the cutoff frequency of the filter. The RC combination in Figure 13 has a cutoff frequency
of 59 kHz. All four amplifiers of the OPA4134 are used so the TPA6120A2 can be driven differentially.
The output of the OPA4134 goes into the TPA6120A2. The TPA6120A2 is configured for use with differential
inputs, stereo use, and a gain of 2V/V. Note that the 0.1µF capacitors are placed at every supply pin of the
TPA6120A2, as well as the 39.2Ω series output resistor.
Each output goes to one channel of a pair of stereo headphones, where the listener enjoys crisp, clean, virtually
noise free music with a dynamic range greater than the human ear is capable of detecting.
10.2.1.2.1 Resistor Values
R_F = 1 kW
R_I = 1 kW
V_CC−
−
R_O = 39.2 W
V_I
+
R_S = 50 W
R_L
V_CC+
Figure 14. Single-Ended Input With A Noninverting Gain Of 2V/V
In the most basic configuration (see Figure 14), four resistors must be considered, not including the load
impedance. The feedback and input resistors, R_F and R_I, respectively, determine the closed-loop gain of the
amplifier. R_O is a series output resistor designed to protect the amplifier from any capacitance on the output path,
including board and load capacitance. R_S is a series input resistor.
The series output resistor should be between 10Ω and 100Ω. The output series resistance eases the work of the
output power stage by increasing the load when low impedance headphones are connected, as well as isolating
any capacitance on the following traces and headphone cable.
Because the TPA6120A2 is a current-feedback amplifier, take care when choosing the feedback resistor. TI
recommends a lower level of 800Ω for the feedback resistance. No capacitors should be used in the feedback
path, as they will form a short circuit at high frequencies.
The value of the feedback resistor should be chosen by using Figure 17 as a guideline. 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 10kΩ. The typical value for the feedback resistor for
the TPA6120A2 is 1kΩ. 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 1kΩ to 2kΩ, 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.
10
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