OPA1622 Datasheet, PDF (24/36 Pages) Texas Instruments – OPA1622 SoundPlus™ High-Fidelity, Bipolar-Input, Audio Operational Amplifier

OPA1622
SBOS727A – NOVEMBER 2015 – REVISED NOVEMBER 2015
www.ti.com
In order to accommodate higher impedance headphones, increase the gain of the circuit to produce greater
output voltages. However, increasing the gain also increases the noise of the circuit and limits the dynamic range
of the circuit into lower impedance headphones. For this reason, some designers choose to select the
headphone amplifier gain by using a switch.
Capacitors C₁ and C₂ limit the bandwidth of the circuit to prevent the unnecessary amplification of interfering
signals. The maximum value of these capacitors is determined by the limitations on frequency response
magnitude deviation detailed in the Design Requirements section. C₁ and C₂ combine with resistors R₂ and R₄ to
form a pole, as shown in Equation 9:
f_P
1
2S(R₂,R₄ )(C₁,C₂ )
(9)
Calculate the minimum pole frequency allowable to meet the magnitude deviation requirements using
Equation 10:
f_P t
f
§ 1 ·²
¨© _G ¸¹
t
1
20 kHz
§ 1 ·²
¨© _0.999 ¸¹
t 416.6 kHz
1
where
• G represents the gain in decimal for a –0.01-dB deviation at 20 kHz.
(10)
Use Equation 11 to calculate the upper limit for the value of C₁ and C₂ in order to meet the goal for minimal
magnitude deviation at 20 kHz.
C₁,C₂
d
1
2S(R₂,R₄ )F_P
d
2S(768
1
d 497 pF o 470 pF
N+]
(11)
8.2.3 Application Curves
The circuit described in Figure 52 is constructed using 0.1% tolerance thin-film resistors (0603 package) and
surface-mount film capacitors. The performance of the circuit is measured using a high-performance audio
analyzer and is displayed in Figure 54 through Figure 59. The maximum output power for three common
headphone impedances is shown in Table 2
Table 2. Maximum Output Power and THD+N Before Clipping for Common Headphone Impedances
LOAD IMPEDANCE
(Ω)
16
32
600
MAXIMUM OUTPUT POWER BEFORE CLIPPING
(mW)
95
150
11.3
THD+N AT MAXIMUM OUTPUT POWER
(dB)
–114.2
–118.7
–119.4
The maximum output power delivered to low impedance headphone loads (16 Ω and 32 Ω) is limited by the
output current capabilities of the amplifier. For the 600-Ω case, the maximum power delivered is limited by the
output voltage capability of the amplifier and depends greatly on the power-supply voltages used. Figure 55
shows the maximum output voltage achievable for each load before the onset of clipping (±5-V supplies),
indicated by a sharp increase in distortion.
As more current is delivered by the output transistors of an amplifier, additional distortion is produced. At low
frequencies, this distortion is corrected by the feedback loop of the amplifier. However, as the loop gain of the
amplifier begins to decline at high frequencies, the overall distortion begins to climb. The unique output stage
design of the OPA1622 greatly reduces the additional distortion at high frequency when delivering large currents,
as shown in Figure 56. High-ordered harmonics (above the 2nd and 3rd) are also kept to a minimal level at high
output powers, as shown in Figure 57 through Figure 59.
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