Page 1
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT Corporation
THAT Analog Engine
®
IC Dynamics Processor
THAT 4301, 4301A
FEATURES
·
High-Performance Voltage
Controlled Amplifier
·
High-Performance RMS-Level
Detector
·
Three General-Purpose Opamps
·
Wide Dynamic Range: >115 dB
·
Low THD: <0.03%
·
Low Cost: $4.39 (‘000s)
·
DIP & Surface-Mount Packages
APPLICATIONS
·
Compressors
·
Limiters
·
Gates
·
Expanders
·
De-Essers
·
Duckers
·
Noise Reduction Systems
·
Wide-Range Level Meters
Description
THAT 4301 Dynamics Processor, dubbed
“THAT Analog Engine,” combines in a single IC
all the active circuitry needed to construct a
wide range of dynamics processors. The 4301
includes a high-performance, exponentially-controlled VCA, a log-responding
RMS-level sensor and three general- purpose
opamps.
The VCA provides two opposing-polarity, voltage-sensitive control ports. Dynamic range exceeds 115 dB, and THD is typically 0.003% at 0
dB gain. In the 4301A, the VCA is selected for
low THD at extremely high levels. The RMS de
tector provides accurate rms-to-dc conversion
over an 80 dB dynamic range for signals with
crest factors up to 10. One opamp is dedicated
as a current-to-voltage converter for the VCA,
while the other two may be used for the signal
path or control voltage processing.
The combination of exponential VCA gain control and logarithmic detector response — “decibel-linear” response — simplifies the
mathematics of designing the control paths of
dynamics processors. This makes it easy to design audio compressors, limiters, gates, expanders, de-essers, duckers, noise reduction
systems and the like. The high level of integra
-
tion ensures excellent temperature tracking be
tween the VCA and the detector, while
minimizing the external parts count.
OUT
CTIT
IN
-
+
-
1
18
11
17
14
13
12
15
16
19
20
2
5
4
9
10 8
6
7
-
OA1
+
VCC
THAT4301
EC-
EC+
IN
OUT
SYM
VCA
OA3
+
OA2
GND
VEE
RMS
Figure 1. Block Diagram (pin numbers are for DIP only)
Page 2
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 2 Rev. 04/10/02
SPECIFICATIONS
1,2
Absolute Maximum Ratings (TA= 25°C)
Positive Supply Voltage (VCC) +18 V
Negative Supply Voltage (V
EE
) -18 V
Supply Current (I
CC
)20mA
Power Dissipation (P
D
)(TA= 75°C) 700 mW
Operating Temperature Range (T
OP
) 0 to +70°C
Storage Temperature Range (T
ST
) -40 to +125°C
Overall Electrical Characteristics
Parameter Symbol Conditions Min Typ Max Units
Positive Supply Voltage V
CC
+7 — +15 V
Negative Supply Voltage V
EE
-7 — -15 V
Positive Supply Current I
CC
—1218 mA
Negative Supply Current I
EE
— -12 -18 mA
Thermal Resistance q
J-C
SO-Package — 140 — °C/W
VCA Electrical Characteristics
3
4301 4301A
Parameter Symbol Conditions Min Typ Max Min Typ Max Units
Input Bias Current I
B(VCA)
No Signal — 30 400 — 30 400 pA
Input Offset Voltage V
OFF(VCA In)
No Signal — ±4 ±15 — ±4 ±15 mV
Input Signal Current I
IN(VCA)
or I
OUT(VCA)
— 175 750 — 175 750 mArms
Gain at 0V Control G
0
EC+=EC–= 0.000V -0.4 0.0 +0.4 -0.4 0.0 +0.4 dB
Gain-Control Constant T
A
= 25°C (T
CHIP
@ 55°C)
-60 dB < gain < +40dB
E
C+
/Gain (dB) EC+& SYM 6.4 6.5 6.6 6.4 6.5 6.6 mV/dB
EC-/Gain (dB) E
C-
-6.4 -6.5 -6.6 -6.4 -6.5 -6.6 mV/dB
Gain-Control TempCo DEC/ DT
CHIP
Ref T
CHIP
= 27°C — +0.33 — — +0.33 — %/°C
Gain-Control Linearity -60 to +40 dB gain — 0.5 2 — 0.5 2 %
Off Isolation EC+=SYM=-375mV, EC-=+375mV 110 115 — 110 115 — dB
Output Offset Voltage Change DV
OFF(OUT)
R
out
= 20kW
0 dB gain — 1 3 — 1 3 mV
+15 dB gain — 2 10 — 2 10 mV
+30 dB gain — 5 25 — 5 25 mV
Gain Cell Idling Current I
IDLE
—20— —20— mA
Output Noise e
n(OUT)
20 Hz-20 kHz
R
out
= 20kW
0 dB gain — -96 -94 — -96 -94 dBV
+15 dB gain — -85 -83 — -85 -83 dBV
Total Harmonic Distortion THD V
IN
= 0 dBV, 1 kHz
0 dB gain — 0.003 0.007 — 0.003 0.007 %
1. All specifications subject to change without notice.
2. Unless otherwise noted, T
A
=25°C, VCC= +15V, VEE= -15V; VCA
SYM
adjusted for min THD@1V,1kHz, 0 dB gain.
3. Test circuit is the VCA section only from Figure 2.
Page 3
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 4301 Dynamics Processor IC Page 3
SPECIFICATIONS
1,2
(Cont’d.)
VCA Electrical Characteristics3(Cont’d.)
4301 4301A
Parameter Symbol Conditions Min Typ Max Min Typ Max Units
Total Harmonic Distortion (cont’d.) THD V
IN
= +10 dBV, 1 kHz
0 dB gain — 0.03 0.07 — 0.03 0.07 %
–15 dB gain — 0.035 0.09 — 0.035 0.09 %
V
OUT
= +10 dBV, 1 kHz
+15 dB gain — 0.035 0.09 — 0.035 0.09 %
V
IN
= +19.5 dBV, 1 kHz
0 dB gain — — — — 0.05 0.09 %
Symmetry Control Voltage V
SYM
minimum THD -2.5 0 +2.5 -2.5 0 +2.5 mV
RMS Detector Electrical Characteristics
4
Parameter Symbol Conditions Min Typ Max Units
Input Bias Current I
B (RMS)
No Signal — 30 400 pA
Input Offset Voltage V
OFF(RMS In)
No Signal — ±4 ±15 mV
Input Signal Current I
IN(RMS)
— 175 750 mA
Input Current for 0 V Output I
in0
IT= 7.5mA 6 8.5 12 mA
Output Scale Factor E
O
/ 20log(Iin/I
in0
) 31.6nA< IIN< 1mA
TA= 25°C (T
CHIP
»55°C) 6.4 6.5 6.6 mV/dB
Scale Factor Match (RMS to VCA) -20 dB < VCA Gain < +20 dB
1mA<I
in (DET)
<100mA .985 1 1.015
Output Linearity f
IN
= 1kHz
1mA<I
in
< 100mA — 0.1 — dB
100nA < I
in
< 316mA — 0.5 — dB
31.6nA < Iin< 1mA — 1.5 — dB
Rectifier Balance f
IN
= 100 Hz, t = .001 s
1mA< Iin< 100mA –20 — 20 %
Crest Factor 1ms pulse repetition rate
0.2 dB error — 3.5 —
0.5 dB error — 5 —
1.0 dB error — 10 —
Maximum Frequency for 2 dB Additional Error I
in
³ 10mA — 100 — kHz
I
in
³ 3mA — 45 — kHz
Iin³ 300nA — 7 — kHz
Timing Current Set Range I
T
1.5 7.5 15 mA
Voltage at ITPin IT= 7.5mA -10 +20 +50 mV
Timing Current Accuracy ICT/I
T
IT= 7.5mA 0.90 1.1 1.30
Filtering Time Constant t T
CHIP
=55°C
()0 026.
C
I
T
T
s
Output Temp. Coefficient DEo/ DT
CHIP
Re: T
CHIP
=27°C — 0.33 — %/°C
Output Current I
OUT
–300mV < V
OUT
< +300mV ±90 ±100 — mA
4. Except as noted, test circuit is the RMS-Detector section only from Figure 2.
Page 4
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 4 Rev. 04/10/02
Specifications
1,2
(Cont’d)
Opamp Electrical Characteristics
5
OA1 OA2 OA3
Parameter Symbol Conditions Min Typ Max Min Typ Max Min Typ Max Units
Input Offset Voltage V
OS
— ±0.5 ±6—±0.5 ±6—±0.5 ±6mV
Input Bias Current I
B
— 150 500 — 150 500 — 150 500 nA
Input Offset Current I
OS
— 15 50 — 15 50 N/A nA
Input Voltage Range I
VR
— ±13.5 — — ±13.5 — N/A V
Common Mode Rej. Ratio CMRR RS<10k — 100 — — 100 — N/A
Power Supply Rej. Ratio PSRR VS=±7V to ±15V — 100 — — 100 — — 100 —
Gain Bandwidth Product GBW (@50kHz) — 5 — — 5 — — 5 — MHz
Open Loop Gain A
VO
RL=10k — 115 — — 110 — — 125 —
RL=2k N/A N/A — 120 —
Output Voltage Swing V
O@RL
=5kW — ±13 — — ±13 — — ±14 — V
VO@RL=2kW N/A N/A — — ±13 — V
Short Circuit Output Current — 4 — — 4 — — 12 — mA
Slew Rate SR — 2 — — 2 — — 2 — V/ms
Total Harmonic Distortion THD 1kHz, A
V
=1, RL=10kW — 0.0007 0.003 — 0.0007 0.003 — 0.0007 0.003 %
1kHz, AV=–1, RL=2kW N/A N/A — 0.0007 0.003 %
Input Noise Voltage Density e
n
fO=1kHz — 6.5 10 — 7.5 12 — 7.5 12
nV Hz
Input Noise Current Density i
n
fO=1kHz — 0.3 — — 0.3 — — 0.3 —
pA Hz
50K
R5
47uF
C1
10uF
C4
22uF
C6
300K
R4
51
R3
1%
R1
1%
R2
47pF
C2
1%
10K0
R6
47uF
C3
1%
2M00
R7
100n
C7
100n
C8
Ct
OA2
OA1
VEE
VCC
GND
-
+
VCA
-
-
+
VCA SYM
IN
SIGNAL
Ec-
OUT
SIGNAL
OUT
RMS
+15V
+15V
-15V
-15V
-15V
20K0
20K0
THAT4301
IN
RMS
It
SYM
OUT
IN
EC-
EC+
OA3
OUT
+
Figure 2. VCA and RMS detector test circuit
5. Test circuit for opamps is a unity-gain follower configuration, with load resistor R
L
as specified.
Page 5
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 4301 Dynamics Processor IC Page 5
Pin Name DIP P in SO Pin
RMS In 1 3
I
T(ITime
)24
No Connection 3 5
RMS Out 4 6
C
T(CTime
)57
OA2 -In 6 9
OA2 Out 7 10
OA2 +In 8 11
GND 9 12
VEE 10 13
VCC 11 18
OA3 Out 12 19
VCA Out 13 20
SYM 14 22
E
C+
15 23
Pin Name DIP P in SO Pin
E
C-
16 24
VCA In 17 25
OA1 Out 18 26
OA1 -In 19 27
OA1 +In 20 28
No Connection 1
No Connection 2
No Connection 8
No Connection 14
No Connection 15
No Connection 16
No Connection 17
No Connection 21
No Connection 29
No Connection 30
Table 1. Pin Connections
1
B
A
C
D
P
J
I
K
N
L
M
O
H
F
G
E
ITEM
A
B
C
D
E
F
G
H
I
J
K
L
M
N
O
P
MILLIMETERS INCHES
24.8 Max.
0.98 Max
24.2 +/-0.2 0.95 +/-0.008
6.4 +/-0.2 0.25 +/-0.008
7.62 +/-0.25
0.30 +/-0.01
2.54 +/-0.15 0.10 +/-0.006
0.46 +0.15 -0.1 0.02 +0.006 -0.004
1.0 +/-0.15
0.04 +/-0.006
1.5 Typ.
0.06 Typ.
0.98 Typ.
0.04 Typ.
1.5 0.06
1.75
0.07
3.25 +/-0.15 0.13 +/-0.006
4.7 Max.
0.19 Max.
0.51 Min.
0.02 Min.
2.8 Min.
0.11 Min.
0.25 +0.15 -0.05 0.01 +0.006 -0.002
0-15
Figure 3. Plastic dual in-line package outline
B
C
J
G
I
H
ED
A
1
+ 0.1 - 0.05
Typ .
+/- 0.2
+ 0.1 - 0.05
+/- 0.4
+/- 0.3
J0.2
I
0.8
H0.15
G2.3
F 0.85 MAX.
E1.0
D
0.4
C10.3
B7.5
15.4A
MILLIMETERSITEM
F
+/- 0.1
+/- 0.15
INCHES
0.60 +/- 0.012
0.29 +/- 0.008
0.41 +/- 0.016
0.002 +0.004 -0.002
0.039 Typ.
0.033 Max.
0.09 +/- 0.006
0.006 +/- 0.004
0.031
0.008 +0.004 -0.002
0-10
Figure 4. Plastic surface-mount package outline
Ordering Information
Plastic DIP 4301P 4301PA
Plastic Surface Mount 4301S Inquire
Page 6
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 6 Rev. 04/10/02
Figure 5. VCA Gain vs. Control Voltage (Ec-) at 25°C Figure 6. VCA 1kHz THD+Noise vs. Input, -15 dB Gain
Representative Data
Figure 7. VCA 1kHz THD+Noise vs. Input, +15 dB Gain Figure 8. VCA 1kHz THD+Noise vs. Input, 0 dB Gain
Figure 9. VCA THD vs. Frequency, 0 dB Gain, 1Vrms Input Figure 10. RMS Output vs. Input Level, 1 kHz & 10 kHz
Figure 11. Departure from Ideal Detector Law vs. Level Figure 12. Detector Output vs. Frequency at Various Levels
Page 7
Theory of Operation
THAT 4301 Dynamics Processor combines THAT
Corporation’s proven Voltage-Controlled Amplifier
(VCA) and RMS-Level Detector designs with three
general-purpose opamps to produce an Analog En
-
gine useful in a variety of dynamics processor appli
cations. For details of the theory of operation of the
VCA and RMS-Detector building blocks, the inter
ested reader is referred to THAT Corporation’s data
sheets on the 2150 Series VCAs and the 2252
RMS-Level Detector. Theory of the interconnection of
exponentially-controlled VCAs and log-responding
level detectors is covered in THAT Corporation’s ap
plication note AN101, The Mathematics of
Log-Based Dynamic Processors.
The VCA — in Brief
THAT 4301 VCA is based on THAT Corporation’s
highly successful complementary log-antilog gain cell
topology, as used in THAT 2150-Series IC VCAs, and
the modular 202 Series VCAs. THAT 4301 is inte
grated using a fully complementary, BiFET process.
The combination of FETs with high-quality, comple
mentary bipolar transistors (NPNs and PNPs) allows
additional flexibility in the design of the VCA over
previous efforts.
Input signals are currents to the VCA IN pin. This
pin is a virtual ground, so in normal operation an input voltage is converted to input current via an appropriately sized resistor (R
1
in Figure 2, Page 4).
Because dc offsets present at the input pin and any
dc offset in preceeding stages will be modulated by
gain changes (thereby becoming audible as thumps),
the input pin is normally ac-coupled (C
1
in Figure 2).
The VCA output signal is also a current, inverted
with respect to the input current. In normal opera
tion, the output current is converted to a voltage via
inverter OA
3
, where the ratio of the conversion is de
termined by the feedback resistor (R
2
, Figure 2) con
nected between OA
3
‘s output and its inverting input.
The signal path through the VCA and OA
3
is
noninverting.
The gain of the VCA is controlled by the voltage
applied to E
C–,EC+
, and SYM. Gain (in decibels) is
proportional to E
C+–EC-
, provided EC+and SYM are
at essentially the same voltage (see below). The con
stant of proportionality is –6.5 mV/dB for the voltage
at E
C–
, and 6.5 mV/dB for the voltage at EC+and SYM.
As mentioned, for proper operation, the same
voltage must be applied to E
C+
and SYM, except for a
small (±2.5 mV) dc bias applied between these pins.
This bias voltage adjusts for internal mismatches in
the VCA gain cell which would otherwise cause small
differences between the gain of positive and negative
half-cycles of the signal. The voltage is usually ap
-
plied via an external trim potentiometer (R
5
in Figure
2), which is adjusted for minimum signal distortion
at unity (0 dB) gain.
The VCA may be controlled via E
C-
, as shown in
Figure 2, or via the combination of E
C+
and SYM.
This connection is illustrated in Figure 13. Note that
this figure shows only that portion of the circuitry
needed to drive the positive VCA control port; cir
-
cuitry associated with OA
1
,OA2and the RMS detector
has been omitted.
While the 4301’s VCA circuitry is very similar to
that of the THAT 2150 Series VCAs, there are several
important differences, as follows:
1) Supply current for the VCA is fixed internally.
Approximately 2mA is available for the sum of input
and output signal currents. (This is also the case in a
2150 Series VCA when biased as recommended.)
2) The signal current output of the VCA is inter
nally connected to the inverting input of an on-chip
opamp. In order to provide external feedback around
this opamp, this node is brought out to a pin.
3) The control-voltage constant is approximately
6.5 mV/dB, due primarily to the higher internal oper
ating temperature of the 4301 compared to that of
the 2150 Series.
4) The input stage of the 4301 VCA uses inte
grated P-channel FETs rather than a bias-current
corrected bipolar differential amplifier. Input bias
currents have therefore been reduced.
The RMS Detector — in Brief
The 4301’s detector computes rms level by recti
fying input current signals, converting the rectified
current to a logarithmic voltage, and applying that
voltage to a log-domain filter. The output signal is a
dc voltage proportional to the decibel-level of the rms
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
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THAT 4301 Dynamics Processor IC Page 7
50K
R5
47uF
C1
300K
R4
51
R3
1%20K0
R1
1%20K0
R2
47pFC2
CtIt
THAT4301
OUT
SYM
OUT
OA3
OA2
OA1
VEE
VCC
GND
IN
RMS
-
+
EC+EC-
IN
VCA
-
-
++
VCA SYM
Signal In
Positive Control In
Signal
Out
Figure 13. Driving the VCA via the Positive Control Port
Page 8
value of the input signal current. Some ac component
(at twice the input frequency) remains superimposed
on the dc output. The ac signal is attenuated by a
log-domain filter, which constitutes a single-pole
rolloff with cutoff determined by an external capaci
-
tor and a programmable dc current.
As in the VCA, input signals are currents to the
RMS IN pin. This input is a virtual ground, so a re
-
sistor (R
6
in Figure 2) is normally used to convert in
put voltages to the desired current. The level detector
is capable of accurately resolving signals well below
10 mV (with a 10 kW input resistor). However, if the
detector is to accurately track such low-level signals,
ac coupling is normally required.
The log-domain filter cutoff frequency is usually
placed well below the frequency range of interest. For
an audio-band detector, a typical value would be
5 Hz, or a 32 ms time constant (t). The filter’s time
constant is determined by an external capacitor at
-
tached to the C
T
pin, and an internal current source
(I
CT
) connected to CT. The current source is pro
-
grammed via the I
T
pin: current in ITis mirrored to
I
CT
with a gain of approximately 1.1. The resulting
time constant t is approximately equal to 0.026 C
T/IT
.
Note that, as a result of the mathematics of RMS detection, the attack and release time constants are
fixed in their relationship to each other.
The dc output of the detector is scaled with the
same constant of proportionality as the VCA gain
control: 6.5 mV/dB. The detector’s 0 dB reference
(I
in0
, the input current which causes 0 V output), is
determined by I
T
as follows: I
in0
=
9.6 mA I
t
The detector output stage is capable of sinking or sourcing
100 mA.
Differences between the 4301’s RMS-Level Detec
-
tor circuitry and that of the THAT 2252 RMS Detec
-
tor are as follows:
1) The rectifier in the 4301 RMS Detector is inter
nally balanced by design, and cannot be balanced
via an external control. The 4301 will typically bal
ance positive and negative halves of the input signal
within ±1.5%, but in extreme cases the mismatch
may reach ±15%. However, a 15% mismatch will
not significantly increase ripple-induced distortion
in dynamics processors over that caused by signal
ripple alone.
2) The time constant of the 4301’s RMS detector is
determined by the combination of an external ca
-
pacitor (connected to the C
T
pin) and an internal,
programmable current source. The current source
is equal to 1.1 I
T
. Normally, a resistor is not con
-
nected directly to the C
T
pin on the 4301.
3) The 0 dB reference point, or level match,isnot
adjustable via an external current source. However,
as in the 2252, the level match is affected by the
timing current, which, in this case, is drawn from
the I
T
pin and mirrored internally to CT.
4) The input stage of the 4301 RMS detector uses
integrated P-channel FETs rather than a
bias-current corrected bipolar differential amplifier.
Input bias currents are therefore negligible, improv
-
ing performance at low signal levels.
The Opamps — in Brief
The three opamps in the 4301 are intended for
general purpose applications. All are 5 MHz opamps
with slew rates of approximately 2V/ms. All use bipo
lar PNP input stages. However, the design of each is
optimized for its expected use. Therefore, to get the
most out of the 4301, it is useful to know the major
differences among these opamps.
OA
3
, being internally connected to the output of
the VCA, is intended for current-to-voltage conversion. Its input noise performance, at
75.
nV
Hz
, com-
plements that of the VCA, adding negligible noise at
unity gain. Its output section is capable of driving a
2kW load to within 2V of the power supply rails,
making it possible to use this opamp directly as the
output stage in single-ended designs.
OA
1
is the quietest opamp of the three. Its input
noise voltage, at
65.
nV
Hz
, makes it the opamp of
choice for input stages. Note that its output drive ca
pability is limited (in order to reduce the chip’s
power dissipation) to approximately ±3 mA. It is
comfortable driving loads of 5 kW or more to within
1V of the power supply rails.
OA
2
is intended primarily as a control-voltage
processor. Its input noise parallels that of OA
3
, and
its output drive capability parallels that of OA
1
.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 8 Rev. 04/10/02
Page 9
Applications
The circuit of Figure 14, Page 9, shows a typical
application for THAT 4301. This simple compres
sor/limiter design features adjustable hard-knee
threshold, compression ratio, and static gain
1
. The
applications discussion in this data sheet will center
on this circuit for the purpose of illustrating impor
-
tant design issues. However, it is posslble to config
ure many other types of dynamics processors with
THAT 4301. Hopefully, the following discussion will
imply some of these possibilities.
Signal Path
As mentioned in the section on theory, the VCA
input pin is a virtual ground with negative feedback
provided internally. An input resistor (R
1
, 20kW)is
required to convert the ac input voltage to a current
within the linear range of the 4301. (Peak VCA input
currents should be kept under 1 mA for best distor
-
tion performance.) The coupling capacitor (C
1
,47mf)
is strongly recom
-
mended to block dc cur
rent from preceeding
stages (and from offset
voltage at the input of
the VCA). Any dc current
into the VCA will be
modulated by varying
gain in the VCA, showing
up in the output as
“thumps”. Note that C
1
,
in conjunction with R
1
,
will set the low frequency limit of the cir
-
cuit.
The VCA output is
connected to OA
3
, config
-
ured as an inverting cur
rent-to-voltage converter.
OA
3
‘s feedback compo
nents (R
2
,20kW, and
C
2
, 47 pf) determine the
constant of cur
rent-to-voltage conver
sion. The simplest way
to deal with this is to
recognize that when the
VCA is set for unity
(0 dB) gain, the input to
output voltage gain is
simply R
2/R1
, just as in
the case of a single in
-
verting stage. If, for some reason, more than 0 dB
gain is required when the VCA is set to unity, then
the resistors may be skewed to provide it. Note that
the feedback capacitor (C
2
)isrequired for stability.
The VCA output has approximately 45 pf of capaci
tance to ground, which must be neutralized via the
47 pf feedback capacitor across R
2
.
The VCA gain is controlled via the E
C–
terminal,
whereby gain will be proportional to the negative of
the voltage at E
C–
. The EC+terminal is grounded, and
the SYM terminal is returned nearly to ground via a
small resistor (R
3
,51W). The VCA SYM trim (R5,
50 kW) allows a small voltage to be applied to the
SYM terminal via R
4
(300 kW). This voltage adjusts
for small mismatches within the VCA gain cell,
thereby reducing even-order distortion products. To
adjust the trim, apply to the input a middle-level,
middle-frequency signal (1 kHz at1Visagood
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 4301 Dynamics Processor IC Page 9
EC+
EC-
IN
OUT
SYM
VCA
+
OA3
-
OUT
1%20K0
R2
51
R3
R16
1%4k99
+
OA2
-
GND
C5
100N
1%
590K
R17
1%
10K0
R15
+15
R18
CCW
CW
GAIN
10K
-15
1%
1K43
R14
COMPRESSION
CW
CCW
R13
10K
C3
47uF
R6
-15
10K0 1%
22uF
C6
IN
IT
OUT
CT
RMS
R7
2M00
1%
-15
10uF
C4
VEE
VCC
THAT4301
+15
100n
C7
C8
100n
1%4k99
R8
+
OA1
-
-15
2M00 1%
R10
1%383K
R11
CR1
CR2
10K0 1%
R9
R12
CW
10K
CCW
THRESHOLD
+15
20K0 1%
R1
47uF
C1
300K
R4
-15
+15
R5
50K
VCA SYM
C2
47pF
IN
Figure 14. Typical Compressor/Limiter Application Circuit
1. More information on this compressor design, along with suggestions for converting it to soft-knee operation,
is given in AN100, Basic Compressor Limiter Design. The designs in AN100 are based on THAT Corporation’s
2150-Series VCAs and 2252 RMS Detector, but are readily adaptable to the 4301 with only minor modifications. In
fact, the circuit presented here is functionally identical to the hard-knee circuit published in AN100.
Page 10
choice with this circuit) and observe THD at the sig
-
nal output. Set the trim for minimum THD.
RMS-Level Detector
The RMS detector’s input is similar to that of the
VCA. An input resistor (R
6
,10kW) converts the ac in
-
put voltage to a current within the linear range of the
4301. (Peak detector input currents should be kept
under 1 mA for best linearity.) The coupling capacitor
(C
3
,47mf) is recommended to block dc current from
preceeding stages (and from offset voltage at the in
put of the detector). Any dc current into the detector
will limit the low-level resolution of the detector, and
will upset the rectifier balance at low levels. Note
that, as with the VCA input circuitry, C
3
in conjunc
tion with R
6
will set the lower frequency limit of the
detector.
The time response of the RMS detector is deter
mined by the capacitor attached to C
T(C4
,10mf) and
the size of the current in pin I
T
(determined by R7,
2MW and the negative power supply, –15V). Since the
voltage at I
T
is approximately 0 V, the circuit of Figure
14 produces 7.5 mAinI
T
. The current in ITis mir
-
rored with a gain of 1.1 to the C
T
pin, where it is
available to discharge the timing capacitor (C
4
). The
combination produces a log filter with time constant
equal to approximately 0.026 C
T/IT
(~35 ms in the
circuit shown).
The waveform at C
T
will follow the logged (decibel) value of the input signal envelope, plus a dc offset of about 1.3 V (2 V
BE
). This allows a polarized
capacitor to be used for the timing capacitor, usually
an electrolytic. The capacitor used should be a
low-leakage type in order not to add significantly to
the timing current.
The output stage of the RMS detector serves to
buffer the voltage at C
T
and remove the 1.3 V dc off
-
set, resulting in an output centered around 0 V for in
put signals of about 85 mV. The output voltage
increases 6.5 mV for every 1 dB increase in input sig
nal level. This relationship holds over more than a
60 dB range in input currents.
Control Path
A compressor/limiter is intended to reduce its
gain as signals rise above a threshold. The output of
the RMS detector represents the input signal level
over a wide range of levels, but compression only oc
-
curs when the level is above the threshold. OA
1
is
configured as a variable threshold detector to block
envelope information for low-level signals, passing
only information for signals above threshold.
OA
1
is an inverting stage with gain of 2 above
threshold and 0 below threshold. Neglecting the ac
-
tion of the THRESHOLD control (R
12
) and its associ
-
ated resistors (R
11
and R10), positive signals from the
RMS detector output drive the output of OA
1
nega
-
tive. This forward biases CR
2
, closing the feedback
loop such that the junction of R
9
and CR2(the output
of the threshold detector) sits at -(R
9/R8
) RMS
OUT
. For
the circuit of Figure 14, this is –2 RMS
OUT
. Negative
signals from the RMS detector drive the output of
OA
1
positive, reverse biasing CR2and forward biasing
CR
1
. In this case, the junction of R9and CR2rests at
0 V, and no signal level informaion is passed to the
threshold detector’s output.
In order to vary the threshold, R
12
, the THRESH
-
OLD control, is provided. Via R
11
(383 kW), R12adds
up to ±39.2 mA of current to OA
1
‘s summing junction,
requiring the same amount of opposite-polarity cur
rent from the RMS detector output to counterbalance
it. At 4.99 kW, the voltage across R
8
required to pro
duce a counterbalancing current is ± 195 mV, which
represents a ±30 dB change in RMS detector input
level.
Since the RMS detector’s 0 dB reference level is
85 mV, the center of the THRESHOLD pot’s range
would be 85 mV, were it not for R
10
(2 MW), which
provides an offset. R
10
adds an extra –7.5 matoOA1‘s
summing junction, which would be counterbalanced
by 37.4 mV at the detector output. This corresponds
to 5.8 dB, offsetting the THRESHOLD center by this
much to 165 mV, or approximately -16 dBV.
The output of the threshold detector represents
the signal level above the determined threshold, at a
constant of about 13 mV/dB (from
[R
9/R8
] 6.5 mV/dB). This signal is passed on to the
COMPRESSION control (R
13
), which variably attenu-
ates the signal passed on to OA
2
. Note that the gain of
OA
2
, from the wiper of the COMPRESSION control to
OA
2
‘s output, is R16/R15(0.5), precisely the inverse of
the gain of OA
1
. Therefore, the COMPRESSION con
-
trol lets the user vary the above-threshold gain be
-
tween the RMS detector output and the output of OA
1
from zero to a maximum of unity.
The gain control constant of the VCA, 6.5 mV/dB,
is exactly equal to the output scaling constant of the
RMS detector. Therefore, at maximum COMPRES
SION, above threshold, every dB increase in input
signal level causes a 6.5 mV increase in the output of
OA
2
, which in turn causesa1dBdecrease in the VCA
gain. With this setting, the output will not increase
despite large increases in input level above threshold.
This is infinite compression. For intermediate set
-
tings of COMPRESSION,a1dBincrease in input sig
nal level will cause less thana1dBdecrease in gain,
thereby varying the compression ratio.
The resistor R
14
is included to alter the taper of
the COMPRESSION pot to better suit common use. If
a linear taper pot is used for R
13
, the compression ra
tio will be 1:2 at the middle of the rotation. However,
1:2 compression in an above-threshold compressor
is not very strong processing, so 1:4 is often pre
ferred at the midpoint. R
14
warps the taper of R13so
that 1:4 compression occurs at approximately the
midpoint of R
13
‘s rotation.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 10 Rev. 04/10/02
Page 11
The GAIN control (R18) is used to provide static
gain or attenuation in the signal path. This control
adds up to ±130 mV offset to the output of OA
2
(from
-V
+
R
R
16
17
to -V
-
R
R
16
17
), which is approximately
±20 dB change in gain of the VCA. C
5
is used to atten
-
uate the noise of OA
2
,OA1and the resistors R
8
through R16used in the control path. All these active
and passive components produce noise which is
passed on to the control port of the VCA, causing
modulation of the signal. By itself, the 4301 VCA pro
-
duces very little noise modulation, and its perfor
mance can be significantly degraded by the use of
noisy components in the control voltage path.
Overall Result
The resulting compressor circuit provides
hard-knee compression above threshold with three
essential user-adjustable controls. The threshold of
compression may be varied over a ±30 dB range from
about –46 dBV to +14 dBV. The compression ratio
may be varied from 1:1 (no compression) to ¥:1.
And, static gain may be added up to ±20 dB. Audio
performance is excellent, with THD running below
0.05% at middle frequencies even with 10 dB of com
pression, and an input dynamic range of over
115 dB.
Perhaps most important, this example design
only scratches the surface of the large body of appli
cations circuits which may be constructed with THAT
4301. The combination of an accurate,
wide-dynamic-range, log-responding level detector
with a high-quality, exponentially-responding VCA
produces a versatile and powerful analog engine. The
opamps provided in the 4301 enable the designer to
configure these building blocks with few external
components to construct gates, expanders, de-essers,
noise reduction systems and the like.
For further information, samples and pricing,
please contact us at the address below.
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
THAT 4301 Dynamics Processor IC Page 11
Page 12
Notes
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
Tel: +1 (508) 478-9200; Fax: +1 (508) 478-0990; Web: www.thatcorp.com
Page 12 Rev. 04/10/02
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