Texas Instruments | TIA microphone amplifier circuit | Application notes | Texas Instruments TIA microphone amplifier circuit Application notes

Texas Instruments TIA microphone amplifier circuit Application notes
Analog Engineer's Circuit: Amplifiers
SBOA291 – December 2018
TIA microphone amplifier circuit
Design Goals
Input pressure
(Max)
Output Voltage
(Max)
100dB SPL(2Pa)
1.228Vrms
Supply
Frequency Response Deviation
Vcc
Vee
@ 20Hz
@20kHz
5V
0V
–0.5dB
–0.1dB
Design Description
This circuit uses an op amp in a transimpedance amplifier configuration to convert the output current from
an electret capsule microphone into an output voltage. The common mode voltage of this circuit is
constant and set to mid–supply eliminating any input–stage cross over distortion.
C3 15p
Vcc
Vcc
R4 75k
R1 5.9k
V3 5
U2 TLV6741
C4 3.3u
Vcc
Vcc
VB
C2 3.3u
Electret_Mic
R2 100k
+
Vin
R3 100k
Out
Gnd
+
Vout
+
R5 10k
C1 4.7u
Design Notes
1. Use the op amp in the linear output operating range, which is usually specified under the AOL test
conditions.
2. Use low–K capacitors (tantalum, C0G, etc.) and thin film resistors help to decrease distortion.
3. Use a battery to power this circuit to eliminate distortion caused by switching power supplies.
4. Use low value resistors and low noise op amp to achieve high performance low noise designs.
5. The voltage connected to R1 to bias the microphone does not have to match the supply voltage of the
op amp. Using a larger microphone bias voltage allows for a larger value or R1 which decreases the noise
gain of the op amp circuit while still maintaining normal operation of the microphone.
6. Capacitor C1 should be large enough that its impedance is much less than resistor R1 at audio
frequency. Pay attention to the signal polarity when using tantalum capacitors.
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TIA microphone amplifier circuit
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Design Steps
The following microphone is chosen as an example to design this circuit.
Microphone parameter
Value
Sensitivity @ 94dB SPL (1 Pa)
–35 ± 4 dBV
Current Consumption (Max)
0.5mA
Impedance
2.2kΩ
Standard Operating Voltage
2Vdc
1. Convert the sensitivity to volts per Pascal.
2. Convert volts per Pascal to current per Pascal.
3. Max output current occurs at max sound pressure level of 2Pa.
4. Calculate the value of resistor R4 to set the gain
5. Calculate the value for the bias resistor R1. In the following equation, Vmic is the standard operating
voltage of the microphone
6. Calculate the high frequency pole according to the allowed deviation at 20 kHz. In the following
equation, G_pole1 is the gain at frequency "f".
7. Calculate C3 based on the pole frequency calculated in step 6.
8. Calculate the corner frequency at low frequency according to the allowed deviation at 20 Hz. In the
following equation, G_pole2 is the gain contributed by each pole at frequency "f" respectively. There
are two poles, so divided by two.
9. Calculate the input capacitor C1 based on the cut off frequency calculated in step 8.
10. Assuming the output load R5 is 10kΩ, calculate the output capacitor C4 based on the cut off frequency
calculated in step 8.
11. Set the amplifier input common mode voltage to mid–supply voltage. Select R2 and R3 as 100kΩ. The
equivalent resistance equals to the parallel combination of the two resistors:
12. Calculate the capacitor C2 to filter the power supply and resistor noise. Set the cutoff frequency to
1Hz.
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Design Simulations
AC Simulation Results
T
Gain (dB)
-4.35
-4.46dB @ 20kHz
-0.11dB deviation
-4.94dB @ 20Hz
-0.59dB deviation
-5.45
-6.56
100
10
1k
Frequency (Hz)
10k
100k
Transient Simulation Results
The input voltage represents the SPL of an input signal to the microphone. A 2 Vrms input signal represents
2 Pascal.
T
2.83
Vin
-2.83
1.70
Vout
-1.73
5.00m
6.00m
7.00m
8.00m
9.00m
10.00m
Time (s)
Noise Simulation Results
The following simulation results show 22.39µVrms of noise at 22kHz. The noise is measured at a bandwidth
of 22kHz to represent the measured noise using an audio analyzer with the bandwidth set to 22kHz.
Total noise (V)
T
42.94u
22.05uVrms @ 22kHz
21.47u
0.00
1
10
100
1k
Frequency (Hz)
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10k
100k
TIA microphone amplifier circuit
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References:
1. Analog Engineer's Circuit Cookbooks
2. SPICE Simulation File SBOC526
3. TI Precision Designs TIPD181
4. TI Precision Labs
Design Featured Op Amp
TLV6741
Vss
1.8V to 5.5V
VinCM
Vee to Vcc–1.2V
Vout
Rail–to–rail
Vos
150µV
Iq
890µA/Ch
Ib
10pA
UGBW
10MHz
SR
4.75V/µs
#Channels
1
www.ti.com/product/tlv6741
Design Alternate Op Amp
OPA172
OPA192
Vss
4.5V to 36V
4.5V to 36V
VinCM
Vee–0.1V to Vcc–2V
Vee–0.1V to Vcc+0.1V
Vout
Rail–to–rail
Rail–to–rail
Vos
±200µV
±5µV
Iq
1.6mA/Ch
1mA/Ch
Ib
8pA
5pA
UGBW
10MHz
10MHz
SR
10V/µs
20V/µs
#Channels
1, 2, 4
1, 2, 4
www.ti.com/product/op www.ti.com/product/op
a172
a192
4
TIA microphone amplifier circuit
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