
GAIN CONTROL
HEADPHONE AMPLIFIERS
f
=
c
1
2 R C
p
L
O
(1)
O
C
L
1
C
=
2
R
p
(2)
SLOS597 – DECEMBER 2008 ........................................................................................................................................................................................... www.ti.com
The TPA6132A2 has four gain settings which are controlled with pins G0 and G1. The following table gives an
overview of the gain function.
G0 VOLTAGE
G1 VOLTAGE
AMPLIFIER GAIN
≤ 0.5 V
–6 dB
≥ 1.3 V
≤ 0.5 V
0 dB
≤ 0.5 V
≥ 1.3 V
3 dB
≥ 1.3 V
6 dB
Table 1. Windows Vista Premium Mobile Mode Specifications
Windows Premium Mobile Vista
Device Type
Requirement
TPA6132A2 Typical Performance
Specifications
THD+N
≤ –65 dB FS [20 Hz, 20 kHz]
–75 dB FS[20 Hz, 20 kHz]
Analog Speaker Line Jack
Dynamic Range with Signal
(RL = 10 k, FS = 0.707
≤ –80 dB FS A-Weight
–100 dB FS A-Weight
Present
Vrms)
Line Output Crosstalk
≤ –60 dB [20 Hz, 20 kHz]
–90 dB [20 Hz, 20 kHz]
THD+N
≤ –45 dB FS [20 Hz, 20 kHz]
–65 dB FS [20 Hz, 20 kHz]
Analog Headphone Out Jack
Dynamic Range with Signal
(RL = 32, FS = 0.300
≤ –80 dB FS A-Weight
–94 dB FS A-Weight
Present
Vrms)
Headphone Output Crosstalk
≤ –60 dB [20 Hz, 20 kHz]
–90 dB [20 Hz, 20 kHz]
Single-supply headphone amplifiers typically require dc-blocking capacitors to remove dc bias from their output
voltage. The top drawing in
Figure 25 illustrates this connection. If dc bias is not removed, large dc current will
flow through the headphones which wastes power, clip the output signal, and potentially damage the
headphones.
These dc-blocking capacitors are often large in value and size. Headphone speakers have a typical resistance
between 16
and 32 . This combination creates a high-pass filter with a cutoff frequency as shown in
Equation 1, where RL is the load impedance, CO is the dc-block capacitor, and fC is the cutoff frequency. For a given high-pass cutoff frequency and load impedance, the required dc-blocking capacitor is found as:
Reducing fC improves low frequency fidelity and requires a larger dc-blocking capacitor. To achieve a 20 Hz
cutoff with 16
headphones, CO must be at least 500 F. Large capacitor values require large packages,
consuming PCB area, increasing height, and increasing cost of assembly. During start-up or shutdown the
dc-blocking capacitor has to be charged or discharged. This causes an audible pop on start-up and power-down.
Large dc-blocking capacitors also reduce audio output signal fidelity.
Two different headphone amplifier architectures are available to eliminate the need for dc-blocking capacitors.
The Capless amplifier architecture is similar provides a reference voltage to the headphone connector shield pin
as shown in the middle drawing of
Figure 25. The audio output signals are centered around this reference
voltage, which is typically half of the supply voltage to allow symmetrical output voltage swing.
When using a Capless amplifier do not connect the headphone jack shield to any ground reference or large
currents will result. This makes Capless amplifiers ineffective for plugging non-headphone accessories into the
headphone connector. Capless amplifiers are useful only with floating GND headphones.
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Copyright 2008, Texas Instruments Incorporated