TS472_06 datasheet(15/24 Pages) STMICROELECTRONICS

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TS472

Application information

15/24

5.3

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 (100kΩ) are

equivalent to a first order high pass filter. Assuming that FCL is the lowest frequency to be

amplified (with a 3dB 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 3dB attenuation), the minimum value of Cout is:

Figure 33 and Figure 34 give directly the lower cut-off frequency (with 3dB attenuation)

versus the value of the input or output capacitors

Note:

In case FCL 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 FCL 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 VDC (no load conditions), and can typically source 2mA with respect to

drop-out, determined by the internal resistance 100

Ω (for detailed load regulation curves

see Figure 8).

π F

100

× 10

⋅⋅

------------------------------------------------------

out

π F

out

⋅⋅

-------------------------------------------

Figure 33.

Lower cut-off frequency vs. input

capacitors

Figure 34.

Lower cut-off frequency vs. output

capacitors

100

1000

Typical Zin

ZinMAX

Cin (nF)

ZinMIN

100

1000

100

1000

Rout=10k

Cout (nF)

Rout=100k

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