that4301 THAT Corporation, that4301 Datasheet - Page 7

• Manufacturer Part Number: that4301
• Description: That Analog Engine Ic Dynamics Processor
• Manufacturer: THAT Corporation

THAT 4301 Analog Engine IC Dynamics Processor
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
Log-Based Dynamic Processors.
The VCA — in Brief
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.
pin is a virtual ground, so in normal operation an in-
put voltage is converted to input current via an ap-
propriately sized resistor (R
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
with respect to the input current. In normal opera-
tion, the output current is converted to a voltage via
inverter OA
termined by the feedback resistor (R
nected between OA
The signal path through the VCA and OA
noninverting.
applied to E
proportional to E
at essentially the same voltage (see below). The con-
stant of proportionality is –6.5 mV/dB for the voltage
at E
voltage must be applied to E
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
THAT 4301 Dynamics Processor combines THAT
THAT 4301 VCA is based on THAT Corporation’s
Input signals are currents to the VCA IN pin. This
The VCA output signal is also a current, inverted
The gain of the VCA is controlled by the voltage
As mentioned, for proper operation, the same
C–
, and 6.5 mV/dB for the voltage at E
note
3
, where the ratio of the conversion is de-
C–
, E
C+
C+
AN101,
3
, and SYM. Gain (in decibels) is
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
‘s output and its inverting input.
– E
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
C-
, provided E
C+
The
1
and SYM, except for a
in Figure 2, Page 4).
Mathematics
2
C+
, Figure 2) con-
1
in Figure 2).
and SYM are
Theory of Operation
C+
5
and SYM.
in Figure
3
of
is
2), which is adjusted for minimum signal distortion
at unity (0 dB) gain.
Figure 2, or via the combination of E
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
has been omitted.
that of the THAT 2150 Series VCAs, there are several
important differences, as follows:
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.)
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.
6.5 mV/dB, due primarily to the higher internal oper-
ating temperature of the 4301 compared to that of
the 2150 Series.
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
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
Figure 13. Driving the VCA via the Positive Control Port
Positive Control In
Signal In
The VCA may be controlled via E
While the 4301’s VCA circuitry is very similar to
1) Supply current for the VCA is fixed internally.
2) The signal current output of the VCA is inter-
3) The control-voltage constant is approximately
4) The input stage of the 4301 VCA uses inte-
The 4301’s detector computes rms level by recti-
VCC
VEE
+
-
OA1
47uF
IN
C1
It
THAT4301
RMS
20K0
R1
1%
OUT
Ct
IN
EC-
1
, OA
R3
51
VCA
GND
50K
2
R5
SYM
and the RMS detector
EC+
300K
OUT
R4
VCA SYM
C-
+
+
-
-
OA3
OA2
C2
20K0
, as shown in
C+
R2
47pF
1%
and SYM.
Page 7
Signal
Out