wide-range exponential control of gain and attenuation
with low signal distortion. The parts are trimmed at
wafer stage for low THD and control-voltage
feedthrough without further adjustment.
closely matched NPN/PNP pairs, to deliver discrete
performance at IC prices. The parts are available in
three grades, selected for factory trimmed distortion,
alowing the user to optimize cost vs. performance. The
2180 Series is available in an 8-pin single-in-line (SIP)
2. Unless otherwise noted, T
exponential gain control, low distortion, wide dynamic
range and low control-voltage feedthrough. These parts
control gain by converting an input current signal to a
bipolar logged voltage, adding a dc control voltage, and
re-converting the summed voltage back to a current
through a bipolar antilog circuit.
flows in pin1, the input pin. An internal operational
transconductance amplifier (OTA) works to maintain
pin 1 at a virtual ground potential by driving the emitters
of Q1 and (through the Voltage Bias Generator) Q3.
Q3/D3 and Q1/D1 act to log the input current, producing
a voltage, V3, which represents the bipolar logarithm of
the input current. (The voltage at the junction of D1 and
D2 is the same as V3, but shifted by four forward V
of V3, creating an output current which is a precise rep-
lica of the input current. If pin 2 (Ec+) and pin 3 (Ec-)
are held at ground, the output current will equal the in-
put current. For pin 2 positive or pin 3 negative, the out-
put current will be scaled larger than the input current.
For pin 2 negative or pin 3 positive, the output current is
scaled smaller than the input.
(Ec-), or both, may be used to control gain. Gain is expo-
nentially proportional to the voltage at pin 2, and expo-
pin 3. Therefore, pin 2 (Ec+) is the positive control port,
while pin 3 (Ec-) is the negative control port. Because of
the exponential characteristic, the control voltage sets
gain linearly in decibels. Figure 6 shows the decibel cur-
rent gain of a 2180 versus the voltage at Ec+, while Fig-
ure 7 shows gain versus the Ec-.
in a semiconductor junction (in particular, between a
any log-antilog VCA depends on its temperature.
same manner.) Note that the gain at Ec = 0 V is 0 dB, re-
gardless of temperature. Changing temperature changes
the scale factor of the gain by 0.33%/°C, which pivots the
curve about the 0 dB point.
(25°C) and the actual temperature, and Gain is the
gain in decibels. At room temperature, this reduces to
split into two paths: 570 mA is used for biasing the IC,
and the remainder becomes Icell as shown in Figure 5.
Icell is further split in two parts: about 20 mA biases the
core transistors (Q1 through Q4), the rest is available for
input and output signal current
This control-voltage feedthrough is more pronounced
with gain; the A version of the part produces the least
feedthrough, the C version the most. See Figure 9 for
typical curves for dc offset vs. gain
distortion over a wide range of gain, cut and signal lev-
els. Figures 10 through 12 show typical distortion per-
formance for representative samples of each grade of the
part. Figure 13 shows the harmonic content of the dis-
tortion in a typical B-grade part.
ceptual difficulty for designers first exposed to this con-
vention, the current input/output mode provides great
flexibility in application.
The input resistor (shown as 20 kW in Figure 2, Page 3)
should be scaled to convert the available ac input voltage
to a current within the linear range of the device. Gen-
erally, peak input currents should be kept under 1 mA
for best distortion performance.
+15 dB and -15 dB gain. The circuit of Figure 2, Page 3
was used to generate these curves.
which may be used. Note that, with 20 kW cur-
rent-to-voltage converting resistors, distortion remains
low even at 10 V rms input at 0 dB or -15 dB gain, and
at 1.7 V rms input at +15 dB gain (~10 V rms output).
This is especially true in the A and B grades of the
tors used at the input and output in Figure 2, Page 3. For
every dB these resistor values are decreased, the voltage
noise at the output of the OP275 is reduced by one dB.
For example, with 10 kW resistors, the output noise floor
drops to 104 dBV (typical) at 0 dB gain -- a 6 dB re-
duction in noise because 10 kW is 1/2 of (6 dB lower
than) 20 kW.
crease the noise level by 6 dB, while reducing distortion
at maximum voltage levels. Furthermore, if maximum
signal levels are higher (or lower) than the traditional
10 V rms, these resistors should be scaled to accommo-
date the actual voltages prevalent in the circuit. Since the
2180 handles signals as currents, these ICs can even op-
erate with signal levels far exceeding the 2180's supply
rails, provided appropriately large resistors are used.
around the internal opamp (essentially Q1/D1 and
Q3/D3) are fixed, low values for the input resistor will
require more closed-loop gain from the opamp. Since
the open-loop gain naturally falls off at high frequencies,
asking for too much gain will lead to increased
should be kept to 10 kW or above.
ances of less than 60 kW at high frequencies. For most
audio applications, this will present no problem
general-purpose opamps, this is not well controlled. Any
dc input currents will cause dc in the output which will
be modulated by gain; this may cause audible thumps. If
the input is dc coupled, dc input currents may be gener-
ated due to the input offset voltage of the 2180 itself, or
due to offsets in stages preceeding the 2180. Therefore,
capacitive coupling is almost mandatory for quality au-
dio applications. Choose a capacitor which will give ac-
ceptable low frequency performance for the application.
such a case, a single coupling capacitor may be located
next to pin 1 rather than multiple capacitors at the
driven ends of the summing resistors. However, take
care that the capacitor does not pick up stray signals.
may be converted to a voltage (see Figures 2 & 14).
Choose the external opamp for good audio performance.
The feedback resistor should be chosen based on the de-
sired current-to-voltage conversion constant. Since the
input resistor determines the voltage-to-current conver-
sion at the input, the familiar ratio of R
the 2180 is set for 0 dB current gain. Since the VCA per-
forms best at settings near unity gain, use the input and
feedback resistors to provide design-center gain or loss,
VCA. Without it, this capacitance will destabilize most
opamps. The capacitance at pin 8 is typically 15 pf.
practice to include a small (~1 mf) electrolytic or
(~0.1 mf) ceramic capacitor close to the VCA IC on the
PCB. Performance is not particularly dependent on sup-
termined by the sum of the input and output currents
through the core and voltage bias generator. Reducing
signal currents may help accommodate low supply volt-
ages. THAT Corporation intends to publish an applica-
tion note covering operation on low supply voltages.
Please inquire for its availability.
+18 V is the nominal limit.
connected to a current source I
supply the sum of the input and output signal currents,
plus the bias to run the rest) of the IC. The minimum
value for this current is 570 mA over the sum of the re-
quired signal currents. Usually, I
Higher bias levels are of limited value, largely because
the core transistors become ineffective at logging and
antilogging at currents over 1 mA.
Since this pin connects to a (high impedance) current
supply, not a voltage supply, bypassing at pin 5 is not
opamp is connected here, as are various portions of the
internal bias network. It may not be used as an addi-
tional input pin.
tive voltage causes loss, negative voltage causes gain. As
described on Page 5, the current gain of the VCA is unity
when pin 3 is at 0 V with respect to pin 2, and varies
with voltage at approximately -6.1 mV/dB, at room tem-
this approach is shown in Figure 14. E
of control can sometimes save an inverter in the control
path. In order to maintain the wafer level adjustment
which minimizes THD, leave pin 4 open.
which case, the control sensitivities of each port are
summed), or through two different control signals.
There is no reason why both control ports cannot be
transistors. The accuracy of the logging and antilogging
is dependent on the E
transistors will follow the collector currents, of course.
Since the collector currents are signal-related, the base
currents are therefore also signal-related. Should the
source impedance of the control voltage(s) be large, the
signal-related base currents will cause signal-related
voltages to appear at the control ports, which will inter-
fere with precise logging and antilogging, in turn causing
infinite source impedance at pin 4. (Pin 4 should be left
open.) To realize all the performance designed into a
2180, keep the source impedance of the control voltage
driver well under 50 W.
ance of an opamp typically rises at high frequencies be-
cause open loop gain falls off as frequency increases. A
typical opamp's output impedance is therefore inductive
at high frequencies. Excessive inductance in the control
port source impedance can cause the VCA to oscillate in-
ternally. In such cases, a 100 W resistor in series with a
1.5 nf capacitor from the control port to ground will
usually suffice to prevent the instability.
well known, this includes not only active devices such as
opamps and transistors, but extends to the choice of im-
pedance levels as well. High value resistors have higher in-
herent thermal noise, and the noise performance of an
otherwise quiet circuit can be easily spoiled by the wrong
choice of impedance levels.
age-control circuitry. The 2180 Series VCAs act like multi-
pliers: when no signal is present at the signal input, noise at
the control input is rejected. So, when measuring noise (in
the absence of signal as most everyone does), even very
noisy control circuitry often goes unnoticed. However, noise
at the control port of these parts will cause noise modula-
tion of the signal. This can become significant if care is not
taken to drive the control ports with quiet signals.
where the shot noise in the core transistors reaches a mini-
mum with no signal, and increases with the square root of
the instantaneous signal current. However, in an optimum
circuit, the noise floor rises only to -94 dBV with a
50 mA rms signal at unity gain -- 4 dB of noise modulation.
By contrast, if a unity-gain connected, non-inverting 5534
opamp is used to directly drive the control port, the noise
floor will rise to 92 dBV -- 6 dB of noise modulation.
One useful technique is to process control voltages at a
multiple of the eventual control constant (e.g., 61 mV/dB
-- ten times higher than the VCA requires), and then at-
tenuate the control signal just before the final drive am-
plifier. With careful attention to impedance levels,
relatively noisy opamps may be used for all but the final
signals within the signal path. As with noise in the con-
trol path, signal pickup in the control path can ad-
versely effect the performance of an otherwise good VCA.
Because it is a multiplier, the 2180 produces second
harmonic distortion if the audio signal itself is present at
the control port. Only a small voltage at the control port
is required: as little as 10 mV of signal can increase dis-
tortion to over 0.01%. This can frequently be seen at
high frequencies, where capacitive coupling between the
signal and control paths can cause stray signal pickup.
nique is to temporarily bypass the control port to
ground via a modest-sized capacitor (e.g., 10 mF). If the
distortion diminishes, signal pickup in the control path
is the likely cause.
the amount of gain or loss commanded. The constant of
proportionality is 0.33% of the decibel gain commanded,
per degree Celsius, referenced to 27°C (300°K). This
means that at 0 dB gain, there is no change in gain with
temperature. However, at -122 mV, the gain will be
+20 dB at room temperature, but will be 20.66 dB at a
temperature 10°C lower.
may be compensated by a resistor embedded in the con-
trol voltage path whose value varies with temperature at
the same rate of 0.33%/°C. Such parts are available from
RCD Components, Inc, 3301 Bedford St., Manchester,
NH, USA [(603) 669-0054], and KOA/Speer Electronics,
PO Box 547, Bradford, PA, 16701 USA [(814)362-5536].
and application. Our engineering staff includes designers
who have decades of experience in applying our parts.
Please feel free to contact us to discuss your applications