SILAN SA2159

SA2159
VOLTAGE CONTROLLED AMPLIFIER FOR ELECTRONIC VOLUME CONTROL
DESCRIPTION
The SA2159 is a voltage-controlled amplifier (VCA) circuit. The chip is a
high-performance current–in/current-out devices including two opposingpolarity, voltage-sensitive control ports. Due to super low-noise
application, the circuit adopts high hFE. Complementary NPN/PNP pairs
configuration. It requires little external support circuitry and selects a
space-efficient plastic 8-pin single–in–line (SIP) package. The SA2159
VCA combines a number of advantages such as low noise, low distortion,
SIP-8
low offset and high gain-bandwidth to offer discrete performance at IC
prices.
FEATURES
* Wide dynamic range:>115dB
APPLICATIONS
* Wide gain range:>130dB
* Logarithmic gain control
* Faders
* Low distortion:(0.008%@0dB gain, 0.035%@15dB gain)
* Expanders
* Wide gain-bandwidth: 6MHz
* Compressors
* Single in–line package
* Equalizers
* Dual gain-control ports(pos/neg)
* Oscillators
* Filters
ORDERING INFORMATIONS
* Automation systems
Device
Package
SA2159
SIP-8
BLOCK DIAGRAM
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
Http: www.silan.com.cn
Rev: 1.0
2005.04.22
Page 1 of 11
SA2159
ABSOLUTE MAXIMUM RATINGS (Tamb=25°C)
Characteristic
Symbol
Ratings
Unit
Positive Supply Voltage
VCC
+15
V
Negative Supply Voltage
VEE
-15
V
Supply Current
ICC
30
mA
Power Dissipation
PD
330
mW
Operating Temperature Range
TOP
-20~+75
°C
Storage Temperature Range
TST
-40~+125
°C
RECOMMENDED OPERATING CONDITIONS
Characteristics
Symbol
Test conditions
Min
Typ
Max
Unit
Supply Voltage
VCC
+4
+12
+15
V
Supply Voltage
VEE
-4
-12
-15
V
Bias Current
ISET
VCC- VEE=24V
-
2
2.4
mA
IIN+IOUT
ISET=2.4mA
-
125
550
µA
Signal Current
ELECTRICAL CHARACTERISTICS (Unless otherwise specified, VCC+=+15V, VCC-=-15V, Tamb =25°C)
Characteristics
Symbol
Test conditions
Min
Typ
Max
Unit
Supply Current
ICC
No signal
-
2
3
mA
Equiv. Input Bias Current
IB
No signal
-
5
20
nA
VOFF(IN)
No signal
-
+10
-
mV
-
1
-
20
Input Offset Voltage
ROUT=20KΩ
Output Offset Voltage
VOFF(OUT) 0 dB gain
THD adj for min
Gain Cell Idling Current
IIDLE
mV
-
µA
Tamb=20°C
Gain-Control Constant
Gain-Control Temp Co
-60dB<gain<+40 dB
EC+
Pins 2&4(fig.13)
5.8
5.9
6.1
mV/dB
EC-
Pin 3
-5.8
-5.9
-6.1
mV/dB
-
-0.33
-
%/°C
-
0.5
2
%
110
115
-
dB
0dBain
-
-97
-89
dBV
+15dBain
-
-88
-82
dBV
AV=0dB, THD<0.07%
-4
0
+4
mV
0dB gain
-
0.007
0.07
%
+20dB gain
-
0.02
0.10
%
-20dB gain
-
0.02
0.15
%
∆EC/∆Tamb Tamb~27°C
Gain-Control Linearity
-60~40dB gain
Offset Isolation(Fig.14)
EC+=-360mV, EC-=+360mV
20Hz-20kHz
Output Noise
Symmetry Control Voltage
en(OUT)
VSYM
ROUT=20KΩ
IOUT=30µA,1KHz
Total Harmonic Distortion
THD
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
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Rev: 1.0
2005.04.22
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SA2159
PIN CONFIGURATION
PIN DESCRIPTION
Pin No.
Symbol
Description
1
IN
2
EC+
Positive exponential control
3
EC-
Negative exponential control
4
EC+(SYM)
5
V-
6
GND
7
V+
8
OUT
Signal input
Symmetry control
Negative power
Ground
Positive power
Output
FUNCTION DESCRIPTION
OPERATING THEORY
The SA2159 VCA is designed for high performance in audio frequency applications which requires exponential
gain control, low distortion, wide dynamic range and low DC bias modulation. The gain control part controls gain
by converting an input current signal to a bipolar logged voltage, adding a DC control voltage and reconverting
the summed voltage back to a current through a bipolar antilog circuit.
The simplified internal circuit diagram of the IC is shown in figure 1. The AC input signal current flows in the
input pin 1, which is maintained at a virtual ground potential by the internal op amp.
Figure 1. Simplified internal circuit diagram
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SA2159
Positive input currents
For positive input currents (IIN defined as flowing into pin 1), the op amp turns Q1 and D1 off, while
simultaneously turns Q3 and D3 on. Thus, the input signal current is forced to flow through Q3 and D3.
Logging & Antilogging
We know the basic properties Vbe of the transistor and the collector current Ic as:
Vbe = VT ln (Ic/Is )
It is assumed that D3/Q3 and Q4/D4 are matched, we therefore can conclude:
V3 = EC- - 2VT In (Ic3 / Is)
Where V3 is the voltage at the junction of D3 and D4, VT is the thermal voltage, kT/q, Ic3 is the current in the
collector of Q3, and IS is the reverse-saturation current of Q3.
In typical applications (see typical applications diagram.), pin 4 is normally at or very near ground. Pin 8 is
connected to a virtual ground. In this way, the voltage at the cathodes of D3 and D4 will cause an exponentiallyrelated current to flow in D4 and Q4, and out through pin 8. Mathematically:
V3 = EC+ - 2VT In (Ic4 / Is )
Exponential gain control
It can be derived from said two equations:
V3 = EC- - 2VT In (Ic3 / Is) = EC+ - 2VT In (Ic4 / Is)
EC+ - EC- = 2VT In (Ic4 / Is) - 2VT In (Ic3 / Is) = 2VT In (Ic4 / Ic3)
Rearranging terms,
Ic4 = ic3 e (EC+ - EC-) / 2VT
Av = Ic4 / Ic3 = e (EC+ - EC-) / 2VT
Where AV is the current gain, which is exponential with the difference in the voltage EC+ and ECFor pin 4 at or very near ground, at room temperature, and converting to a base of 10 for the exponential, this
reduces to:
Av = 10 - EC- / 0.118
Another way of expressing this relationship is:
Gain = 20 log Av = - EC- / 0.0059, where the unit of Gain is dB.
dB Gain
+30
0
-30
-60
-90
-540
-EC
mV
-360
-180
0
+180
Figure 2. Gain versus control voltage at 25°C
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SA2159
Negative input currents
For negative input currents, the operation of Q1/D1 and Q3/D3 is symmetric with Q2/D2 and Q4/D4. pin 2 is
connected to ground, or very nearly ground potential (see the section below on symmetry adjustment for more
detail), Since the top pair versus the bottom pair transistors are inverted (NPN/PNP) and the bases are crossconnected between the input (left) half and the output (right) half of each pair, the polarity (positive/negative, in
dB) of the gain is the same.
Symmetry adjustment
The layout design construction of the devices assures relatively good matching between the paired transistors,
but even small VBE mismatches can cause a dc output current to flow in pin 8, which will ultimately manifest itself
as a DC offset voltage. The DC offset voltage will be modulated by gain commands, and may become audible as
“thumps”if gain commands changes very large and fast.
Transistor matching also affects distortion. If the top half of the gain cell is perfectly matched, while the bottom
half is not well match, then the gain commanded by the voltage at pin 3 will affect the two halves of the core
differently. Since positive and negative halves of input signals are handled by separate parts of the core, this
gives rise to even order distortion production.
Figure 3. Typical THD versus symmetry voltage
Accordingly, the base of Q1 and Q4 are brought out to pin 2 and pin 4, respectively. This allows applying a
small static voltage differential to the two bases. The applied voltage differential must be set to the sum of the
Vbe mismatches around the core. Typical applications diagram includes a typical symmetry voltage application
circuitry. The symmetry trim pot controls primarily even-order harmonic distortion, and is adjusted for minimum
distortion in the output waveform.
Opposite polarity control
As may be seen from the equation, the bases of Q1 and Q4 can also be used as an additional control port,
which can be used as opposite control port of pin 3. Both pins must be applied to the control voltage
simultaneously, while the voltage applied between the two pins must be small difference. (A typical connection is
shown in Figure 4.) Either pin 3, or pins 2 and 4, or both ports together may be used for gain control.
Mathematically:Av = 10 - EC- / 0.118, or Gain = 20 log Av = - EC- / 0.0059, where the unit of Gain is dB.
HANGZHOU SILAN MICROELECTRONICS CO.,LTD
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Rev: 1.0
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SA2159
Figure 4. Using Both Control Ports Pin 2 and Pin4
Control port source impedance
The control ports (pin 2 through 4) are connected directly to the bases of the logging and /or antilogging
transistors. Since the collector currents of transistors are signal-related, the base currents will also be signalrelated. If the source impedance of the control voltage is large, the signal-related base currents will cause signalrelated voltage to appear at the control ports, which will affect precise logging and antilogging, in turn causing
distortion. At high signal levels, even 100Ω can spoil the good performance of these parts. Due to signal current
based on a transistor β of approximately 300(NPN) or 100(PNP), the distortion caused by a specific and non-zero
source impedance by determining the base voltage modulation can be estimated.
In SA2159 VCA design, the source impedance of pin 2 and pin 3 are zero, while the source impedance of pin 4
is 50Ω because pin 4 is intended for connection to the symmetry control.
DC input signals
Any dc currents in the feedback loop of the internal op amp will show up as dc terms in the output signal, and
will be modulated by gain commands. The bias current compensation circuit inside the chip offers the input bias
current to the input differential amplifier. Ac input coupling is strongly recommended to prevent an input dc
current supplied from external application circuit of the VCA.
Figure 5. DC offset vs. gain, after symmetry adjustment
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SA2159
Current programming
An external Iset can set the size of the current source at the bottom of the core. Iset is normally determined by
a resistor between V- and pin 5 (Vpin 5 = -2.7V, typically). Iset should be 200µA larger than the total of the peak
input and output signal currents because Iset divides into two portions: approximately 200µA is used for internal
biasing, and the rest is available for the current source at the bottom of the core.
Note that since the sum of the input and output currents plus ISET must be supplied through the output
impedance of the internal op amp, this impedance is approximately 2kΩ, the voltage available to drive the core
will lower.
Headroom
The logarithmic characteristics of the core transistors also limit maximum signal currents. The devices used in
the SA2159 are specially designed to conform to an ideal log-linear curve over a wide range of currents, but their
limit value is approximately 1mA. Distortion will increase with increasing current levels because of falling log
conformance. Figure 6 through 8 show typical distortion versus 1 kHz signal level for the SA2159 at -15dB, 0 dB,
and +15dB gain. Thus, the maximum signal level for a particular design will be determined by the acceptable
distortion.
Figure 6. 1KHz THD +Noise Vs. input,-15dB gain
Figure 7. 1kHz THD+Noise Vs. level, 0dB gain
Figure 8. 1kHz THD+Noise Vs. Input, +15dB gain
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SA2159
APPLICATIONS
Input
As mentioned above, because input and output signals are currents, not voltages, the current input/output
mode provides great flexibility in application.
Internal negative feedback loop provides a virtual ground to the input pin 1 (See Figure 1). Within the linear
range of the device, the input resistor (shown as 20kΩ in typical application circuit) should be scaled to convert
the available ac input voltage to a current. In order to obtain best distortion performance, peak input currents
should be kept under 1 mA. The input impedances must be less than 30kΩ to assure the circuit stability.
The feedback impedances around the internal op amp (essentially Q1/D1 and Q3/D3) are fixed. Low values for
the input resistor will require more closed-loop gain from the op amp.
Since the open-loop gain naturally falls off at high frequencies, this resistor should be kept to 10kΩ or above in
order to prevent high frequency distortion when the gain is too much. Distortion vs. frequency for a 1V signal at
0dB gain with a 20 kΩ input resistor is shown in Figure 9.
As mentioned above, any dc input currents will cause dc signals in the output. The dc signals will be modulated
by gain command, in turn causing audible thump. Therefore, in order to control quality audio applications,
capacitive coupling must be adopted in actual application, which can give acceptable low frequency performance.
Figure 9. Typical THD Vs. frequency, 0dB gain
Output
The output pin (pin 8) should be connected to a virtual ground node, so that current flowing in it may be
converted to a voltage. Choose the external op amp can improve audio performance.
A small feedback capacitor around the output op amp is necessary to cancel the output capacitance of the
VCA, so that most op amps can operate steadily. The capacitance at pin 8 is typically 30 pf.
Power supplies
The positive supply is connected directly to pin 7 without special bypassing circuit, but it is good practice to
include a small (~1µf) electrolytic close to the VCA IC on the PCB. There is no particularly dependence between
performance and supply voltage. The lowest supply voltage is determined by the sum of the input and output
currents plus ISET, which must be supplied through the resistor (~2kΩ) at the top of the core transistors while still
allowing enough voltage swing to bias the internal op amp and the core transistors themselves. Reducing signal
currents may help accommodate low supply voltages.
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SA2159
The process characteristics and internal power consumption determine the highest supply voltage. +15V is the
nominal limit.
A resistive current source determining the current available for the core is connected to the negative supply
terminal. As mentioned before, this source must supply 200µA current over the sum of the required signal
currents including input signal current, output signal current and the bias to run the rest of the IC. 2.4mA is
recommended for most pro audio applications where +15V supplies are common and headroom is important.
Bypassing at pin 5 is not necessary because pin 5 is a current supply, not a voltage supply.
Pin 6 is used as a ground reference for the VCA which connect the non-inverting input of the internal op amp,
as a portion of the internal bias network. It may not be used as an additional input pin.
Voltage control
Pin 3 is the primary voltage control pin. This point controls gain is inversely proportional to applied voltage:
positive voltage causes loss, negative voltage causes gain. The current gain of the VCA is unity when pin 3 is at
0V and varies with voltage at approximately -5.9m/dB, at room temperature.
As implied by the equation for Av at the foot of page 3, the gain is sensitive to temperature. The constant of
proportionality is 0.33% of the decibel gain commanded, per degree Celsius, referenced to 27°C (300K). The
formula is:
Gain = (EC+ - EC-) / (0.0059 * 1.0033 * T).
Where ∆T is the difference between the actual temperature and room temperature (27°C)
For most audio applications, this change with temperature is of little consequence. However, if necessary, it
may be compensated by a resistor which varies its value by 0.33%/°C.
When pin 3 is used for voltage control, Pin 2 is connected to ground and pin 4 is used to apply a small
symmetry voltage (~±4 mV) to correct for VBE mismatches within the VCA IC. Therefore, in order to obtain
optimum performance, pin 4 connects with an external impedance of approximately 50Ω. A trim pot is used to
adjust the voltage between pin 4 and pin 2 as shown in typical application circuit. Voltage adjustment range is ±4
mV.
Pin 2 and pin4 can be used together as an opposite sense voltage control port (See Figure 4). Pin 3 may be
grounded and pin 2 driven against the symmetry-adjustment voltage. The change of voltage at pin 4 does have a
small effect on the symmetry voltage, but this is of little practical consequence in most applications.
The chip can combine all control ports together with differential drive (See Figure 10). While the driving circuitry
is more complex, this configuration offers better performance at high attenuation levels (<-90dB) where the single
control port circuits begin to saturate Q1 (for EC- drive) or Q3 (for EC+ drive). When either of these transistors
saturates, the internal op amp will accommodate the change in current demand by responding with a small
change in its input offset voltage. This leads to an accumulation of charge on the input capacitor, which in turn
can cause thump when the high attenuation is suddenly removed(e.g., when a muted channel is opened).
Differential control drive avoids the large dc levels otherwise required to command high attenuation (+600mV or 100dB gain at pin 3 alone, vs. ±300mV when using both pin 3 and pins 2 and 4).
Control port drive impedance
In order to reduce distortion, it is necessary to use low source impedance at the control port. Thus, this often
suggests an op amp is used to drive the control port directly (see below under noise considerations). However,
due to falling loop gain at high frequencies, the closed-loop output impedance of an op amp typically rises. The
output impedance is therefore inductive at high frequencies. Excessive inductance can cause the VCA to
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2005.04.22
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SA2159
oscillate internally. In such cases, a 100Ω resistor in series with a 1.5nf capacitor between the control port and
ground will usually suffice to prevent the instability.
+15V
47p
7
3
SA2159
VCA
20k
V+
ECOUT
E C+(sym)
8
EC+
4
GND
2
V6
5
Input
IN
10
20k
51
5.1K
LF351
+15V
Rsym
150K
1k
Output
50K
SYM
ADJ
1k
-15V
-15V
EC+
Figure 10. Using both control ports (differential drive)
Noise considerations
For good audio designers, it is important to consider the effects of noisy devices on the signal path. As is well
known, this includes not only active devices, but also impedance. High value resistors have inherent thermal
noise. And the wrong choice of impedance levels will easily bring on the noise performance of an otherwise quiet
circuit being spoiled.
However, the effect of noisy circuitry and high impedance levels in the control path of voltage-control circuitry is
easily neglected. When no signal is present at the signal input, The SA2159 VCA noise at the control input is
rejected like double-balanced multipliers. Most everyone measures noise in the absence of signal, so noise of
control circuitry is often neglected. However, noise at the control port of these parts will cause noise modulation
of the signal. If driving the control ports is not quiet signals, this problem will become very grave.
Quiet electronics throughout be control-voltage circuitry can be used to avoid excessive noise. One useful
technique is to process control voltages at a multiple of the eventual control constant (e.g., 59mV/dB –ten times
higher than the VCA requires). And then attenuate the control signal just before the final drive amplifier. It is
necessary to pay attention to Impedance levels and noisy op amps.
Figure 11. Typical frequency response vs. gain
Figure 12. Typical noise (20kHz NBW) vs. gain
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SA2159
TYPICAL APPLICATION CIRCUIT
PACKAGE OUTLINE
SIP-8
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Unit: mm
Rev: 1.0
2005.04.22
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