344 Chapter 12
with different “genders” of transistors—NPN and
PNP—and to provide a class A-B bias scheme to deal
with the crossover region between the two. This made it
possible to generate a sort of bipolar log and antilog. A
block diagram of a Blackmer VCA is shown in Fig.
12-45.
Briefly, the circuit functions as follows. An ac input
signal current IIN flows in pin 1, the input pin. An
internal operational transconductance amplifier (OTA)
maintains pin 1 at virtual ground potential by driving
the emitters of Q 1 and (through the Voltage Bias Gener-
ator) Q 3. Q 3 /D 3 and Q 1 /D 1 act to log the input current,
producing a voltage (V 3 ) that represents the bipolar
logarithm of the input current. (The voltage at the junc-
tion of D 1 and D 2 is the same as V 3 , but shifted by four
forward Vbe drops.)
Pin 8, the output, is usually connected to a virtual
ground. As a result, Q 2 /D 2 and Q 4 /D 4 take the bipolar
antilog of V 3 , creating an output current flowing to the
virtual ground, which is a precise replica of the input
current. If pin 2 (EC+) and pin 3 (EC) are held at
ground, the output current will equal the input current.
For pin 2 positive or pin 3 negative, the output 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.
The log portion of the VCA, D 1 /Q 1 and D 3 /Q 3 , and
the antilog stages, D 2 /Q 2 and D 4 /Q 4 in Fig. 12-45,
require both the NPN and the PNP transistors to be
closely matched to maintain low distortion. As well, all
the devices (including the bias network) must be at the
same temperature. Integration solves the matching and
temperature problems, but conventional “junc-
tion-isolated” integration is notorious for offering
poor-performing PNP devices. Frey and others avoided
this problem by basing their designs exclusively on
NPN devices for the critical multiplier stage.
Blackmer’s design required “good” PNPs as well as
NPNs.
One way to obtain precisely matched PNP transistors
that provide discrete transistor performance is to use an
IC fabrication technology known as dielectric isola-
tion. THAT Corporation uses dielectric isolation to
fabricate integrated PNP transistors that equal or exceed
the performance of NPNs. With dielectric isolation, the
bottom layers of the devices are available early in the
process, so both N- and P-type collectors are possible.
Furthermore, each transistor is electrically insulated
from the substrate and all other devices by an oxide
layer, which enables discrete transistor performance
with the matching and temperature characteristics only
available in monolithic form.
In Fig. 12-45, it can also be seen that the Blackmer
VCA has two EC inputs having opposite control
response—EC+ and EC. This unique characteristic
allows both control inputs to be used simultaneously.
Individually, gain is exponentially proportional to the
voltage at pin 2, and exponentially proportional to the
negative of the voltage at pin 3. When both are used
simultaneously, gain is exponentially proportional to the
difference in voltage between pins 2 and 3. Overall,
because of the exponential characteristic, the control
voltage sets gain linearly in decibels at 6 mV/dB.
Fig. 12-46 shows a typical VCA application based
on a THAT2180 IC. The audio input to the VCA is a
current; an input resistor converts the input voltage to a
current. The VCA output is also a current. An op-amp
and its feedback resistor serve to convert the VCA’s
current output back to a voltage.
As with the basic topologies from Gilbert, Dow,
Curtis, and other transconductance cells, the current
input/output Blackmer VCA can be used as a variable
conductance to tune oscillators, filters, and the like. An
example of a VCA being used to control a first-order
state-variable filter is shown in Fig. 12-47 with the
response plot in Fig. 12-48.
When combined with audio level detectors, VCAs
can be used to form a wide range of dynamics proces-
sors, including compressors, limiters, gates, duckers,Figure 12-45. THAT 2180 equivalent schematic. Courtesy
THAT Corporation.
Q 1Q 3 Q 4Q 2Icell Iadj
548312D 1 D 2IN OUTSYMEc–D 3 D 4Ec+25V–+Voltage
Bias
Generator
IIN