Component Selection
4-4
4.3
Component Selection
The TPA6120A2 is a high-fidelity audio amplifier. The composition of
components is important when dealing with high fidelity. The best practice is
to operate the device with a split power supply. This eliminates the need for
capacitors in the signal path, which can introduce distortion. Capacitors can
also fail after a certain period of time. The failure mode of many capacitors is
to essentially turn into a short. If capacitors must be used in the signal path
because the device is being used in a single power supply configuration,
polystyrene and PPS film capacitors are good choices.
Thin film resistors have excellent noise and distortion performance.
4.4
Input Bias Voltage
The TPA6120A2 EVM is designed to keep a constant dc voltage on the
positive input pins of the amplifier. This is done with resistors R9 and R10
which effectively tie the positive input pins to ground. This is necessary in most
applications to protect the system and its components from unexpectedly high
dc currents, and to prevent damage to headphones from both large continuous
dc currents and a large POP.
When a source is not connected to the positive node, and the positive node
is not tied to ground, it floats, typically up to the rail. A 12-V offset with a 32-
Ω
load can draw up to 375 mA of current, depending on the size of the output
series resistor. When a source is then connected, or turned on and thus taken
out of a high impedance state, the offset voltage quickly drops to near 0 V. This
large, fast change can cause a loud, potentially headphone damaging POP.
The solution is to keep the positive input pin of the amplifier at a constant
voltage, regardless of the state of the audio source.
With headphones, permanent damage to the voice coil can begin to occur
when the dc voltage across them exceeds 100 mV. The maximum input bias
current of the TPA6120A2 is 12
µ
A. Therefore, the maximum output offset with
the audio source off is:
ǒ
(12
m
A
R9)
*
ǒ
12
m
A
R
F
ø
R
I
Ǔ
Ǔ
Gain
Once the audio source is turned on, the output offset voltage drops to just a
few millivolts.
The value for R9 must be chosen carefully. The value has a direct impact on
the output offset voltage and the loading of the audio source. In the
configuration of the TPA6120A2EVM, where the input and feedback resistors
(R1 and R3) are each 1-k
Ω
, a 500-
Ω
resistor would be the best choice for offset
performance. However, a load of this size can degrade the performance of the
audio source. The audio source should have no problems driving a large load
such as a 100-k
Ω
resistor. However, a load of this size can cause as much as
2.4 V of output offset voltage (when the source is left floating).
