OPA1641-Q1 Datasheet, PDF (21/31 Pages) Texas Instruments – SoundPlus JFET-Input, Automotive-Grade, Audio Operational Amplifiers

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OPA1641-Q1, OPA1642-Q1
SBOS791 – JUNE 2017
8.2 Typical Application
8.2.1 Single-Supply Electret Microphone Preamplifier for Speech
Electret microphones are commonly used in automotive hands-free phone systems because of their small size,
low cost, and relatively good signal-to-noise ratio (SNR). The low noise and distortion of the OPA1641-Q1 makes
the device a good choice for preamplifier circuits for electret microphones. The circuit shown in Figure 37 is a
single-supply preamplifier circuit for electret microphones with a bandwidth from 100 Hz to 3 kHz for capturing
speech.
+9 V
RBIAS ¹⁴
k
CIN
Electret
Microphone
0.1 F
+9 V
RDIV
100 k
RDIV
100 k
CFB
1 nF
RFB
61.9 k
+9 V
0.1 F
± ^VS+
+
OPA1641-Q1
VS±
CDIV
160 nF
Output
Copyright © 2016, Texas Instruments Incorporated
Figure 37. Preamplifier Circuit for Electret Microphones Using a Single Power Supply Voltage
8.2.1.1 Design Requirements
• 9-V single supply
• 1-V_RMS output for 100-dB_SPL input
• Approximately 100-Hz to 3-kHz, –3-dB Bandwidth
• Microphone sensitivity: 8 µA / Pa
• Microphone operating voltage: 2 V to 10 V
• Microphone bias current: 500 µA
8.2.1.2 Detailed Design Procedure
In this circuit, the op amp is configured as a transimpedance amplifier which converts the signal current of the
microphone into an output voltage. The bandwidth of this circuit is limited to the vocal range as is common in
telephony systems. The gain of the circuit is determined by the feedback resistor (R_FB), which must be calculated
according to the microphone sensitivity. For this design, a microphone output current of 8 µA per Pascal (Pa) of
air pressure was selected. Using this value, the output current for a sound pressure level of 100 dB_SPL, or 2 Pa
air pressure, is calculated in Equation 1.
i_mic
8 PA _{u 2 Pa = 16 PA}
1 Pa
(1)
R_FB is then calculated from this current to produce 1-V_RMS output for a 100-dB_SPL input signal in Equation 2.
R_FB
V_O
i_mic
1 V_RMS
16 PA
62500 o 61.9 k:
(2)
The feedback capacitor (C_FB) is calculated to limit the bandwidth of the amplifier to 3 kHz in Equation 3.
C_FB
1
2 ˜ S ˜ R_FB ˜ f_H
1
857 u10 ¹² o 1 nF
2 ˜ S ˜ (61.9 k:) ˜ (3 kHz)
(3)
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