SA571 Datasheet, PDF (6/11 Pages) NXP Semiconductors – Compandor

SA571
Figure 6 shows how the circuit is hooked up to realize an
expandor. The input signal, V_IN, is applied to the inputs of
both the rectifier and the DG cell. When the input signal
drops by 6.0 dB, the gain control current will drop by a factor
of 2, and so the gain will drop 6.0 dB. The output level at
V_OUT will thus drop 12 dB, giving us the desired 2−to−1
expansion.
R₃
*C_IN1
R₂
DG
V_IN
R₄
*C_{IN2 R1}
−
+
V_OUT
V_REF
*C_RECT
NOTE:
ǒ Ǔ GAIN +
2 R₃ V_IN (avg) ²
R₁ R₂ I_B
I_B = 140mA
*EXTERNAL COMPONENTS
Figure 6. Basic Expander
Figure 7 shows the hook−up for a compressor. This is
essentially an expandor placed in the feedback loop of the op
amp. The DG cell is setup to provide AC feedback only, so
a separate DC feedback loop is provided by the two R_DC and
C_DC. The values of R_DC will determine the DC bias at the
output of the op amp. The output will bias to:
ǒ Ǔ V_OUT DC +
1
)
R_DC1
)
R₄
R_DC2
V_REF
ǒ Ǔ V_OUT DC +
1 ) ^RDCTOT
30kW
1.8V
The output of the expander will bias up to:
ǒ Ǔ V_OUT DC +
1
)
R₃
R₄
V_REF
ǒ Ǔ V_OUT DC +
1
)
20kW
30kW
1.8V + 3.0V
The output will bias to 3.0 V when the internal resistors are
used. External resistors may be placed in series with R₃,
(which will affect the gain), or in parallel with R₄ to raise the
DC bias to any desired value.
R₂
DG
R₁
R_DC *
C_IN^* _R3
V_IN
R₄
−
V_REF ⁺
C_RECT *
R_DC *
C_DC *
C_F^*
V_OUT
ǒ Ǔ NOTE: GAIN +
1
R₁ R₂ I_B ²
2R₃ V_INavg
I_B = 140mA
*EXTERNAL COMPONENTS
Figure 7. Basic Compressor
Circuit Details − Rectifier
Figure 8 shows the concept behind the full−wave
averaging rectifier. The input current to the summing node
of the op amp, V_IN/R₁, is supplied by the output of the op
amp. If we can mirror the op amp output current into a
unipolar current, we will have an ideal rectifier. The output
current is averaged by R₅, CR, which set the averaging time
constant, and then mirrored with a gain of 2 to become I_G,
the gain control current.
I = V_IN / R₁ V+
R₁
−
V_IN
+
C_R
R₅
10kW
I_G
Figure 8. Rectifier Concept
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