Application information TS472
14/25 Doc ID 11015 Rev 6
4.3 Lower cut-off frequency
The lower cut-off frequency FCL of the microphone preamplifier depends on the input
capacitors Cin and output capacitors Cout. These input and output capacitors are mandatory
in an application because of DC voltage blocking.
The input capacitors Cin in series with the input impedance of the TS472 (100 kΩ) are
equivalent to a first order high-pass filter. Assuming that FCL is the lowest frequency to be
amplified (with a 3 dB attenuation), the minimum value of Cin is:
The capacitors Cout in series with the output resistors Rout (or an input impedance of the
next stage) are also equivalent to a first order high-pass filter. Assuming that FCL is the
lowest frequency to be amplified (with a 3 dB attenuation), the minimum value of Cout is:
Figure 33 and Figure 34 give directly the lower cut-off frequency (with 3 dB attenuation)
versus the value of the input or output capacitors.
Note: If FCL is kept the same for calculation purposes, 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 6 dB on FCL and a roll-off of
40 dB/decade.
4.4 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 VDC (no load conditions), and can typically source 2 mA with respect to
drop-out, determined by the internal 100 Ω resistance (for detailed load regulation curves
see Figure 8).
Cin
1
2πFCL 100 3
×10⋅⋅
------------------------------------------------------=
Cout
1
2πFCL Rout
⋅⋅
-------------------------------------------=
Figure 33. Lower cut-off frequency vs. input
capacitors
Figure 34. Lower cut-off frequency vs. output
capacitors
1 10 100
10
100
1000
Typical Zin
ZinMAX
Lower Cut-off frequency (Hz)
Cin (nF)
ZinMIN
1 10 100 1000
10
100
1000
Rout=10k
Ω
Lower Cut-off frequency (Hz)
Cout (nF)
Rout=100k
Ω