OPA1641-Q1 Datasheet, PDF (22/31 Pages) Texas Instruments – SoundPlus JFET-Input, Automotive-Grade, Audio Operational Amplifiers

OPA1641-Q1, OPA1642-Q1
SBOS791 – JUNE 2017
www.ti.com
Typical Application (continued)
R_BIAS is necessary to divert the microphone signal current through capacitor C_IN rather than flowing from the
power supply (V_CC). Larger values of R_BIAS allow for a smaller capacitor to be used for C_IN and reduces the
overall noise of the circuit. However, the maximum value for R_BIAS is limited by the microphone bias current and
minimum operating voltage.
The value of R_BIAS is calculated in Equation 4.
R_BIAS
V_CC V_MIC
I_BIAS
9 V 2 V _{14 k:}
500 PA
(4)
Input capacitor C_IN forms a high-pass filter in combination with resistor R_BIAS. The filter corner frequency
calculation is shown in Equation 5 to place the high-pass corner frequency at 100 Hz.
C_IN
1
2 ˜ S ˜ R_BIAS ˜ f_L
1
113.7 u10 ⁹ o 100 nF
2 ˜ S ˜ (14 k:) ˜ (100 Hz)
(5)
The voltage divider network at the op amp noninverting input is used to bias the op amp output to the midsupply
point (V_CC / 2) to maximize the output voltage range of the circuit. This result is easily achieved by selecting the
same value for both resistors in the divider. The absolute value of those resistors is limited by the acceptable
power-supply current drawn by the voltage divider. Choosing 50 µA as an acceptable limit of supply current gives
a value of 100 kΩ for the resistors in the divider, as Equation 6 shows.
R_DIV
t
V_CC
2 ˜I_DIV
t
9V
2 ˜ 50 PA
t 90
k: o 100
k:
(6)
Finally, to minimize the additional noise contribution from the voltage divider, a capacitor is placed at the op amp
noninverting input. This capacitor forms a low-pass filter with the parallel combination of the voltage divider
resistors. Selecting a filter corner frequency of 20 Hz minimizes the noise contribution of the voltage divider
inside the amplifier passband; see Equation 7.
C_DIV
1
2˜
S
˜
§
¨©
R_DIV
2
·
¸¹
˜
f_L
1
2
˜
S
˜
§
¨©
100 k:
2
·
¸¹
˜
(20
Hz)
1.592 u10 ⁷ o 160 nF
(7)
22
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