UTC571
LINEAR INTEGRATED CIRCUIT
YOUW ANG ELECTRONICS CO.LTD
1
COMPANDER
DESCRIPTION
The UTC571 is a versatile low cost dual gain control
circuit in which either channel may be used as a dynamic
range compressor or expandor. Each channel has a full-
wave rectifier to detect the average value of the signal, a
linerarized temperature-compensated variable gain cell,
and an operational amplifier.
The UTC571 is well suited for use in cellular radio and
radio communications systems, modems, telephone, and
satellite broadcast/receive audio systems.
FEATURES
Complete compressor and expandor in one IChip
Temperature compensated
Greater than 110dB dynamic range
Operates down to 6VDC
System levels adjustable with external components
Distortion may be trimmed out
Dynamic noise reduction systems
Voltage-controlled amplifier
DIP-16
APPLICATIONS
Cellular radio
High level limiter
Low level expandor--noise gate
Dynamic filters
CD Player
PIN CONFIGURATION
16
15
14
13
12
11
10
9
1
2
3
4
5
6
7
8
UT
C5
7
1
RECT CAP 2
RECT IN 2
AG CELL IN 2
VCC
INV.IN 2
RES.R
3
2
OUTPUT 2
THD TRIM 2
RECT CAP 1
RECT IN 1
AG CELL IN 1
GND
INV.IN 1
RES.R
3
1
OUTPUT 1
THD TRIM 1
UTC571
LINEAR INTEGRATED CIRCUIT
YOUW ANG ELECTRONICS CO.LTD
2
BLOCK DIAGRAM
VARIABLE
GAIN
RECTIFIER
VREF
1.8V
R1 10K
R2 20K
R3 20K
R4 20K
RECT CAP
RECT IN
G IN
THD TRIM
R3 INVERTER IN
OUTPUT
ABSOLUTE MAXIMUM RATINGS
(Ta=25
C )
Characteristic
Symbol
Value
Unit
Maximum Operating Voltage
V
CC
18
V
Operating Temperature
T
A
0~70
C
Power dissipation
P
D
400
mW
AC ELECTRICAL CHARACTERISTICS
(Ta=25
C, Vcc=+6V,unless otherwise stated
)
Characteristic
Symbol
Test Condition
Min
Typ.
Max
Unit
Supply Voltage
V
CC
6
18
v
Supply Current
I
CC
No signal
3.2
4.8
mA
Output Current capability
I
OUT
20
mA
Output Slew Rate
SR
.5
V/
s
Gsin Cell Distortion
Untrimmed
Trimmed
0.5
0.1
2.0
%
Resister Tolerance
5
15
%
Internal Reference Voltage
1.7
1.85
2.0
V
Output DC Shift
Untrimmed
30
150
mV
Expandor Output Noise
No signal, 15Hz-20kHz
20
60
V
Unity Gain Level
1kHz
-1.5
0
+1.5
dBm
Gain Change
0.1
dB
Reference Drift
+2,-25 +20,-50
mV
Resistor Drift
+8,-0
%
Tracking Error(measured relative
to value at unity gain) Equals [VO-
VO(unity gain)]dB-V2dBm
Rectifier input,
V2=+6dBm,V1=0dB
V2=-30dBm, V1=0dB
+0.2
+0.2
-1,+1.5
dB
Channel Separation
60
dB
Note: 1. Input to V 1 and V 2 grounded.
2. Measured at 0dBm, 1kHz.
3. Expandor AC input change from no signal to 0dBm.
4. Relative to value at T A = 2
C.
5. Electrical characteristics for the UTC571 only are specified over -40 to +8
C temperature range.
6. 0dBm = 775mV RMS .
UTC571
LINEAR INTEGRATED CIRCUIT
YOUW ANG ELECTRONICS CO.LTD
3
FUNCTION DESCRIPTION
CIRCUIT DESCRIPTION
The UTC571 compandor building blocks, as shown in the block diagram, are a full-wave rectifier, a variable
gain cell, an operational amplifier and a bias system. The arrangement of these blocks in the IC result in a
circuit which can perform well with few external components, yet can be adapted to many diverse applications.
The full-wave rectifier rectifies the input current which flows from the rectifier input, to an internal summing
node which is biased at V
REF
. The rectified current is averaged on an external filter capacitor tied to the C
RECT
terminal, and the average value of the input current controls the gain of the variable gain cell. The gain will thus
be proportional to the average value of the input signal for capacitively-coupled voltage inputs as shown in the
following equation. Note that for capacitively-coupled inputs there is no offset voltage capable of producing a
gain error. The only error will come from the bias current of the rectifier (supplied internally) which is less than
0.1
A.
G
R1
avg
|
V
V
|
REF
IN
-
or
G
R1
avg
|
V
|
IN
The speed with which gain changes to follow changes in input signal levels is determined by the rectifier filter
capacitor. A small capacitor will yield rapid response but will not fully filter low frequency signals. Any ripple on
the gain control signal will modulate the signal passing through the variable gain cell. In an expander or
compressor application, this would lead to third harmonic distortion, so there is a trade-off to be made between
fast attack and decay times and distortion. For step changes in amplitude, the change in gain with time is shown
by this equation.
G(t)=(G
initial
-G
final
)e-t/
+G
final
;
=10k
C
RECT
The variable gain cell is a current-in, current-out device with the ratio I
OUT
/I
IN
controlled by the rectifier. I
IN
is
the current which flows from the DG input to an internal summing node biased at V
REF
. The following equation
applies for capacitively-coupled inputs. The output current, I
OUT
, is fed to the summing node of the op amp.
R2
V
R2
V
V
I
IN
REF
IN
IN
=
-
=
A compensation scheme built into the DG cell compensates for temperature and cancels out odd harmonic
distortion. The only distortion which remains is even harmonics, and they exist only because of internal offset
voltages. The THD trim terminal provides a means for nulling the internal offsets for low distortion operation.
The operational amplifier (which is internally compensated) has the non-inverting input tied to V
REF
, and the
inverting input connected to the DG cell output as well as brought out externally. A resistor, R 3 , is brought out
from the summing node and allows compressor or expander gain to be determined only by internal components.
The output stage is capable of 20mA output current. This allows a +13dBm (3.5V
RMS
) output into a 300W
load which, with a series resistor and proper transformer, can result in +13dBm with a 600
output impedance.
A bandgap reference provides the reference voltage for all summing nodes, a regulated supply voltage for the
UTC571
LINEAR INTEGRATED CIRCUIT
YOUW ANG ELECTRONICS CO.LTD
4
rectifier and DG cell, and a bias current for the DG cell. The low tempco of this type of reference provides very
stable biasing over a wide temperature range.
The typical performance characteristics illustration shows the basic input-output transfer curve for basic
compressor or expander circuits.
-40
-30
-20
-10
0
+10
-80
-70
-60
-50
-40
-20
-30
-10
0
+10
+20
COMPRESSOR OUTPUT LEVEL OR
EXPANDOR INPUT LEVEL(dBm)
C
O
M
P
R
E
SS
OR
IN
P
U
T
L
EV
EL
O
R
EX
PA
N
D
O
R
OU
T
P
U
T
L
EVE
L
(
d
B
m
)
Basic Input-Output Transfer Curve
TYPICAL TEST CIRCUIT
20k
10k
2.2
F
2.2
F
0.1
F
10
F
3.14
2.15
4
1.16
8.2k
5.12
200pF
8.9
7.10
6.11
Vcc=15V
20k
30k
G
V
REF
V1
V2
V0
2.2
F
Typical Test Circuit
UTC571
LINEAR INTEGRATED CIRCUIT
YOUW ANG ELECTRONICS CO.LTD
5
INTRODUCTION
Much interest has been expressed in high performance electronic gain control circuits. For non-critical
applications, an integrated circuit operational transconductance amplifier can be used, but when high-performance
is required, one has to resort to complex discrete circuitry with many expensive, well-matched components.
This paper describes an inexpensive integrated circuit, the UTC571 Compandor, which offers a pair of high
performance gain control circuits featuring low distortion (<0.1%), high signal-to-noise ratio (90dB), and wide
dynamic range (110dB).
CIRCUIT BACKGROUND
The UTC571 Compandor was originally designed to satisfy the requirements of the telephone system. When
several telephone channels are multiplexed onto a common line, the resulting signal-to-noise ratio is poor and
companding is used to allow a wider dynamic range to be passed through the channel. Figure 1 graphically shows
what a compandor can do for the signal-to-noise ratio of a restricted dynamic range channel. The input level range
of +20 to -80dB is shown undergoing a 2-to-1 compression where a 2dB input level change is compressed into a
1dB output level change by the compressor. The original 100dB of dynamic range is thus compressed to a 50dB
range for transmission through a restricted dynamic range channel. A complementary expansion on the receiving
end restores the original signal levels and reduces the channel noise by as much as 45dB.
-80
-40
0dB
+20
INPUT
LEVEL
OUTPUT
LEVEL
-80
-40
0d
B
+20
CO
MP
RE
S
S
I
O
N
E
XPAN
SIO
N
NOISE
Figure 1. Restricted Dynamic Range Channel
The significant circuits in a compressor or expander are the rectifier and the gain control element. The phone
system requires a simple full-wave averaging rectifier with good accuracy, since the rectifier accuracy determines
the (input) output level tracking accuracy. The gain cell determines the distortion and noise characteristics, and the
phone system specifications here are very loose. These specs could have been met with a simple operational
transconductance multiplier, or OTA, but the gain of an OTA is proportional to temperature and this is very
undesirable. Therefore, a linearized Tran conductance multiplier was designed which is insensitive to temperature
and offers low noise and low distortion performance. These features make the circuit useful in audio and data
systems as well as in telecommunications systems.
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