Application information

TS4909

24/35

Doc ID 11972 Rev 9

Lower cut-off frequency

The lower cut-off frequency F

CL

of the TS4909 depends on input capacitors C

in1,2

. In the

single-ended configuration, F

CL

depends on output capacitors C

out1,2

as well.

The input capacitor C

in

in series with the input resistor R

in

of the amplifier is equivalent to a

first-order high-pass filter. Assuming that F

CL

is the lowest frequency to be amplified (with a

3 dB attenuation), the minimum value of C

in

is:

In the single-ended configuration, the capacitor C

out

in series with the load resistor R

L

is

equivalent to a first-order high-pass filter. Assuming that F

CL

is the lowest frequency to be

amplified (with a 3 dB attenuation), the minimum value of C

out

is:

Note:

If FCL is kept the same for calculation purposes, it must be taken 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.

4.3

Gain using the typical application schematics

In the flat region (no C

in

effect), the output voltage of a channel is:

The gain A

V

is:

Note:

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

gain.

C

in

1

2

π F

CL

R

in

⋅⋅

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

=

C

ou t

1

2

π F

CL

R

L

⋅⋅

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

=

Figure 78. Lower cut-off frequency vs. input

capacitor

Figure 79. Lower cut-off frequency vs. output

capacitor

1

1 0

1 00

10 00

10

10 0

1k

10 k

R in= 50k

Ω

R in=10 0k

Ω

R in=20 k

Ω

Cin (nF)

R in=10 k

Ω

0.1

1

10

100

1000

10

100

1k

10k

R

L

=32

Ω

R

L

= 300

Ω

R

L

=6 00

Ω

R

L

=16

Ω

Cout (μF)

V

OUT

V

IN

R

fe ed

R

in

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

–





⋅

V

IN

A

V

⋅

==

A

V

R

f eed

R

in

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

–

=