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OPA1652_16 Datasheet, PDF (21/42 Pages) Texas Instruments – Audio Operational Amplifiers
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OPA1652, OPA1654
SBOS477A – DECEMBER 2011 – REVISED AUGUST 2016
8.2 Typical Application
The low noise and distortion of the OPA165x family of audio operational amplifiers make them an excellent
choice for a number of analog audio circuits. One such circuit is shown in Figure 41 which illustrates a power
amplifier circuit suitable for high-fidelity headphone applications.
+12 V
Input
RIN 100 k
0.1 F
CBYP
+
±
OPA1652
0.1 F
RBW
CBYP
100 F
CBULK
0.1 F
CBYP
BUF634
0.1 F
CBYP
100 F
CBULK
Output
-12V
R1
R2
200
200
Copyright © 2016, Texas Instruments Incorporated
Figure 41. Composite Power Amplifier for Headphones
8.2.1 Design Requirements
• Gain: 6 dB
• Output Voltage: > 2 VRMS, 32-Ω Load
• Output Impedance: < 1 Ω
• THD+N: < –110dB (1 kHz, 2 VRMS, 32-Ω Load)
8.2.2 Detailed Design Procedure
The power amplifier circuit (single channel shown) features a BUF634 high-speed buffer amplifier inside the
feedback loop of an OPA1652 to increase the amount of available output current. The bandwidth and power
consumption of the BUF634 can be set with an external resistor, RBW. For this circuit, a 0-Ω resistor was used for
RBW, which configures the BUF634 for its widest bandwidth and highest performance. The gain of the circuit is
determined by the feedback resistors R1 and R2 as shown in Equation 1:
AV
1 R2
R1
(1)
To achieve the design goal of 6-dB voltage gain (2 V/V), R1 and R2 must have equal values. These resistors also
contribute noise thermal noise to the circuit. The voltage noise spectral density of the feedback resistors, referred
to the amplifier input, is given in Equation 2:
eNR 4kT R2 || R1
(2)
Ideally, the thermal noise contributions of the resistors would not significantly degrade the noise performance of
the circuit. Selecting resistor values such that the resistor noise is less than one-third the input voltage noise of
the op amp (Equation 3) ensures that any increase in the circuit noise as a result of the feedback resistor
contributions is minimal.
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