THAT4305 Datasheet, PDF (12/20 Pages) List of Unclassifed Manufacturers – Pre-trimmed Analog Engine® IC

Page 12
THAT4305 Pre-trimmed Low-power Analog Engine
Applications
In this datasheet, we will show detailed circuits
for the 4305 in a relatively simple above-threshold
compressor, and in two simple companding sys-
tems. As mentioned above, many other configura-
tions of the 4305 are possible. THAT intends to
publish additional circuits in forthcoming applica-
tions notes. Please check with THAT's applications
engineering department to see if your application
has been covered yet, and for personalized assis-
tance with specific designs.
Feedforward Compressor/Limiter
The circuit in Figure 16 shows a typical hard-
knee, feedforward compressor/limiter. In addition
to compression ratio, the sidechain includes con-
trols for threshold and make-up gain as well.
The Signal Path
The input of the VCA (pin 15) is a virtual ground,
and R14 converts the input signal into a current
flowing into the VCA. The maximum total signal
current, (I_IN + I_OUT) is 1.8 mA with ±15 V supplies,
so R14 is sized to keep the maximum current at
unity gain to below this level. With peak input volt-
age swing limited by the ±15 V supply rails, the
20 kΩ resistor at R14 limits maximum i_IN to about
750 µA. At 0 dB gain, this will cause the same cur-
rent at the VCA output, which is converted back to
15 V by U2 and R13 (also 20 kΩ). C2 provides AC
coupling, required to block any DC currents that
might otherwise flow into the VCA input. This pre-
vents changes in gain from modulating this current,
which could produce audible "thumps".
The compensation circuit of R28 and C16 is re-
quired for stability. The VCA must “see” a source im-
pedance no greater than 5 kΩ above 1 MHz. R28 in
parallel with R14 accomplishes this. C16 is chosen
to prevent the added noise gain of the lower source
impedance from increasing noise within the audio
band.
Note that such compensation is unnecessary
when the voltage-to-current converting resistor
(R14) is 5 kΩ or less. For example, if the input sig-
nal were limited to lower voltages, the input volt-
age-to-current converting resistor (R14) could be
reduced in value, possibly eliminating the need for
R28 and C16.
U2, along with C4 and R13 forms a trans-
impedance amplifier that converts the VCA's output
current into a voltage. C4 prevents the VCA's output
capacitance from destabilizing the op-amp in this
configuration.
+15V
R13
C16 _R28
11
U1A
100p ^6k2
9 _THAT4305
EC+
In
C2 _R14
Vcc
13
10u ^20k
VCA In VCA Out
15
Gnd
Vee
EC-
6
8
12
-15V
U1B
20k
C4
22p NPO
U2
C6
22p
Out
+15V
-20 dB
Gain
+20 dB
R12
10k
R8
620k
Increase
R18
5k1
C5
C1
R1
10u
33k
THAT4305
-15V
2 _{RMS In}
5
R17
R2
R9
R3
RMS Out
5k1
10k
1k
10k
CT
4
D1
D2
1N4148
INF:1 CR
Compression
100n
U4
+15V
C13
CTIME
10u _+15V
1N4148
C15
Ratio _R11
1:1 CR ^10k
22u
-40 dBu R10
10k _R7
Threshold
430k
20 dBu _Increase
22p
U3
-15V
Figure 16. 4305-based feedforward compressor
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com