THAT4305 Datasheet, PDF (11/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Document 600067 Rev 00
Page 11
slightly with temperature unless the applied voltage
is appropriately compensated.
If we let T_dB=20, A_dB=0 dB, and k=0.75, this be-
havior yields the transfer function shown in Fig-
ure 13. As predicted by the above equations, this
results in a 4:1 compression ratio above the thresh-
old of -20 dB (relative to the RMS detector's 0 dB
reference level. The output level increases by 10 dB
over a 40 dB change in input level.
20
10
0
-10
-20
-30
-40
Out
-50
In
-60
-70
-80
-90
-100
-100 -80 -60 -40 -20
0
20
dB In
Figure 13. Transfer function of a feedforward compressor
In Figure 14, we have again extended the basic
feedback compressor with a threshold adjustment
(T_dB), a threshold (the ideal diode), a means to vary
the sidechain gain (k), and a make-up gain adjust-
ment (AdB) just as in Figure 8. However, in this
case, the detector level is based on the compressor’s
output.
Once again we start with
Out_dB = In_dB + G_dB
For output signals above the level determined by
the threshold setting,
Out_dB > T_dB .
So for signals above the threshold,
G_dB = −k(Out_dB − T_dB) + A_dB .
Substituting yields
Out_dB = In_dB − k(Out_dB − T_dB) + A_dB ,
which can be reduced to
Out_dB
=
In_dB + T_dB + A_dB
(1+ k)
.
For output signals below the level set by the
threshold setting, the signal at the output of the
ideal diode threshold is 0 (dB), so
G_dB = A_dB , and
Out_dB = In_dB + A_dB
If we let T_dB=10, A_dB=20, and k=10, this behav-
ior yields the transfer function shown in Figure 15.
The compression ratio of 11:1 allows a rise of only
about 4.5 dB over a 50 dB range.
In_dB
G_dB
S
A _dB
Out _dB
RMS
Ideal
Diode
Out _dB
-k
S⁺
-
T_dB
Figure 14. Feedback compressor with threshold, gain, and
ratio adjustments
0
-50
Out
In
-100
-100
-50
0
dB Out
Figure 15. Transfer function of a feedback compressor
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