Philips Semiconductors

Product specification

SA575

Low voltage compandor

1997 Nov 07

5

voltage between Pins 4 and 9 will cause small offset error current in

the rectifier cell. This will affect the accuracy of the gain cell. This

can be improved by using an extra capacitor from the input to Pin 4

and eliminating the DC connection between Pins 4 and 9.

The expandor gain expression and the attack and release time

constant is given by Equation 1 and Equation 2, respectively.

3.8k x 100

µA

Expandor gain =

Equation 2.

Equation 1.

COMPRESSOR

The typical compressor configuration is shown in Figure 4. In this

mode, the rectifier cell and variable gain cell are in the feedback

path. R6 and R7 provide the DC feedback to the summing amplifier.

The input is AC coupled through C12 and output is AC coupled

through C8. In a system with inputs and outputs AC coupled, C8

and C12 could be eliminated and only R6, R7, C7, and C13 would

be required. If the external components R6, R7 and C7 are

eliminated, then the output of the summing amplifier will motor-boat

in absence of signals or at extremely low signals. This is because

there is no DC feedback path from the output to input. In the

presence of an AC signal this phenomenon is not observed and the

circuit will appear to function properly.

The compressor gain expression and the attack and release time

constant is given by Equation 3 and Equation 4, respectively.

3.8k x 100

µA

1/2

Compressor gain =

Equation 3.

Equation 4.

AUTOMATIC LEVEL CONTROL

The typical Automatic Level Control circuit configuration is shown in

Figure 5. It can be seen that it is quite similar to the compressor

schematic except that the input to the rectifier cell is from the input

path and not from the feedback path. The input is AC coupled

through C12 and C13 and the output is AC coupled through C8.

Once again, as in the previous cases, if the system input and output

signals are already AC coupled, then C12, C13 and C8 could be

eliminated. Concerning the compressor, removing R6, R7 and C7

to stabilize the summing amplifier at higher input levels. This circuit

provides an input dynamic range greater than 60dB with the output

within +0.5dB typical. The necessary design expressions are given

by Equation 5 and Equation 6, respectively.

3.8k x 100

µA

Equation 5.

Equation 6.

ALC gain =

2.2

µF

10

µF

10

µF

VREF

Σ

EXP IN

EXP OUT

10k

C5

9

4

3.8k

5

C4

C3

10k

7

6

8

SR00705

Figure 3. Typical Expandor Configuration