10.2.1.2.1 Resistor Values

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.

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 (for example, 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 10Ω; 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 100Ω. Care should be taken in selecting the thermal capacity of the output series resistor, as it will create a potential divider with the load and dissipate power.

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 50Ω resistor was chosen because it provides ample protection without interfering in any noticeable way with the signal. Not shown is another 50Ω 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 50Ω source. See Figure 15.

Figure 16 shows the TPA6120A2 connected with differential inputs. Differential inputs are useful because they take the greatest advantage of the high CMRR of the device. The two feedback resistor values must be kept the same, as do the input resistor values.

Special note regarding mono operation:

• If both amplifiers are powered on, but only one channel is to be used, the unused amplifier MUST have a feedback resistor from the output to the negative input. Additionally, the positive input should be grounded as close to the pin as possible. Terminate the output as close to the output pin as possible with a 25Ω load to ground.
• These measures should be followed to prevent the unused amplifier from oscillating. If it oscillates, and the power pins of both amplifiers are tied together, the performance of the amplifier could be seriously degraded.

10.2.1.2.2 Checking For Oscillations And Instability

Checking the stability of the amplifier setup is recommended. High frequency oscillations in the megahertz region can cause undesirable effects in the audio band.

Sometimes, the oscillations can be quite clear. An unexpectedly large draw from the power supply may be an indication of oscillations. These oscillations can be seen with an oscilloscope. However, if the oscillations are not obvious, or there is a chance that the system is stable but close to the edge, placing a scope probe with 10pF of capacitance can make the oscillations worse, or actually cause them to start.

A network analyzer can be used to determine the inherent stability of a system. An output versus frequency curve generated by a network analyzer can be a good indicator of stability. At high frequencies, the curve shows whether a system is oscillating, close to oscillation, or stable. In Figure 17 the system is stable because the high frequency rolloff is smooth and has no peaking. Increasing R_F decreases the frequency at which this rolloff occurs (see the Resistor Values section). Another scenario shows some peaking at high frequency. If the peaking is 2dB, the amplifier is stable as there is still 45 degrees of phase margin. As the peaking increases, the phase margin shrinks, causing the amplifier and the system to approach instability. The same system that normally has a 2dB peak has an increased peak when a capacitor is added to the output, indicating that the system is either on the verge of oscillation or is oscillating; corrective action is required.

10.2.1.2.3 Thermal Considerations

There is no one to one relationship between output power and heat dissipation, so the following equations must be used:

Where

Where

Therefore,

For stereo operation, the efficiency does not change because both P_L and P_SUP are doubled, affecting the amount of power dissipated by the package in the form of heat.

A simple formula for calculating the power dissipated, P_DISS, is shown in Equation 7:

In stereo operation, P_SUP is twice the quantity that is present in mono operation.

The maximum ambient temperature, T_A, depends on the heat-sinking ability of the system. R_θJA for a 20-pin DWP, whose thermal pad is properly soldered down, is shown in Thermal Information. Also see Figure 18.