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).

C

in

1

2

π F

CL

100

3

× 10

⋅⋅

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

=

C

out

1

2

π F

CL

R

out

⋅⋅

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

=

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

Cin (nF)

ZinMIN

1

10

100

1000

10

100

1000

Rout=10k

Ω

Cout (nF)

Rout=100k

Ω