TS4909 datasheet(23/32 Pages) STMICROELECTRONICS

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TS4909

Application information

23/32

Lower cut-off frequency

The lower cut-off frequency FCL of the TS4909 depends on input capacitors Cin1,2. In the

single-ended configuration, FCL depends on output capacitors Cout1,2 as well.

The input capacitor Cin in series with the input resistor Rin of the amplifier is 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:

In the single-ended configuration, the capacitor Cout in series with the load resistor RL is

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:

Note:

If FCL is kept the same for calculation purposes, it must be taken in 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 6dB on FCL and a roll-

off of 40db

⁄ decade.

4.3

Gain using the typical application schematics

In the flat region (no Cin effect), the output voltage of a channel is:

The gain AV is:

Note:

The configuration (either single-ended or phantom ground) has no effect on the value of the

gain.

π F

⋅⋅

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

out

π F

⋅⋅

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

Figure 78.

Lower cut-off frequency vs. input

capacitor

Figure 79.

Lower cut-off frequency vs. output

capacitor

100

1000

100

10k

Rin=50k

Rin=100k

Rin=20k

Cin (nF)

Rin=10k

0.1

100

1000

100

10k

=32

=300

=600

=16

Cout

(

μF)

OUT

feed

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

–

⎝⎠

⎛⎞

⋅

feed

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

–

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