TS472_06 Datasheet, PDF (15/24 Pages) STMicroelectronics – Very low noise microphone preamplifier with 2.0V bias output and active low standby mode

TS472
5.3
Application information
Lower cut-off frequency
The lower cut-off frequency F_CL of the microphone preamplifier depends on the input
capacitors C_in and output capacitors C_out. These input and output capacitors are mandatory
in an application because of DC voltage blocking.
The input capacitors C_in in series with the input impedance of the TS472 (100kΩ) are
equivalent to a first order high pass filter. Assuming that F_CL is the lowest frequency to be
amplified (with a 3dB attenuation), the minimum value of C_in is:
C_in
=
--------------------------¹----------------------------
2π ⋅ F_CL ⋅ 100× 10³
The capacitors C_out in series with the output resistors R_out (or an input impedance of the
next stage) are also equivalent to a first order high pass filter. Assuming that F_CL is the
lowest frequency to be amplified (with a 3dB attenuation), the minimum value of C_out is:
C_out=
---------------------¹---------------------
2π ⋅ F_CL ⋅ R_out
Figure 33. Lower cut-off frequency vs. input
capacitors
1000
ZinMAX
Typical Zin
Figure 34. Lower cut-off frequency vs. output
capacitors
1000
Rout=10kΩ
100
100
ZinMIN
Rout=100kΩ
10
1
10
100
Cin (nF)
10
1
10
100
1000
Cout (nF)
Note:
Figure 33 and Figure 34 give directly the lower cut-off frequency (with 3dB attenuation)
versus the value of the input or output capacitors
In case F_CL is kept the same for calculation, take into account that the 1st order high-pass
filter on the input and the 1st order high-pass filter on the output create a 2nd order high-
pass filter in the audio signal path with an attenuation of 6dB on F_CL and a rolloff of
40dB⁄ decade.
5.4
Low-noise microphone bias source
The TS472 provides a very low noise voltage and power supply rejection BIAS source
designed for biasing an electret condenser microphone cartridge. The BIAS output is
typically set at 2.0 V_DC (no load conditions), and can typically source 2mA with respect to
drop-out, determined by the internal resistance 100Ω (for detailed load regulation curves
see Figure 8).
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