OPA861
www.ti.com
SBOS338–AUGUST 2005
APPLICATION INFORMATION
The
OPA861
is
a
versatile
monolithic
for
TRANSCONDUCTANCE (OTA) SECTION—AN
OVERVIEW
transconductance
amplifier
designed
wide-bandwidth systems, including high-performance
video, RF, and IF circuitry. The operation of the
OPA861 is discussed in the OTA (Operational
Transconductance Amplifier) section of this data
sheet. Over the years and depending on the writer,
the OTA section of an op amp has been referred to
as a Diamond Transistor, Voltage-Controlled Current
source, Transconductor, Macro Transistor, or positive
second-generation current conveyor (CCII+). Corre-
sponding symbols for these terms are shown in
Figure 26.
The symbol for the OTA section is similar to a
transistor (see Figure 26). Applications circuits for the
OTA look and operate much like transistor cir-
cuits—the transistor is also a voltage-controlled cur-
rent source. Not only does this characteristic simplify
the understanding of application circuits, it aids the
circuit optimization process as well. Many of the
same intuitive techniques used with transistor designs
apply to OTA circuits. The three terminals of the OTA
are labeled B, E, and C. This labeling calls attention
to its similarity to a transistor, yet draws distinction for
clarity. While the OTA is similar to a transistor, one
essential difference is the sense of the C-output
current: it flows out the C terminal for positive B-to-E
input voltage and in the C terminal for negative B-to-E
input voltage. The OTA offers many advantages over
a discrete transistor. The OTA is self-biased, simplify-
ing the design process and reducing component
count. In addition, the OTA is far more linear than a
transistor. Transconductance of the OTA is constant
over a wide range of collector currents—this feature
implies a fundamental improvement of linearity.
C
3
VIN1
B
1
IOUT
VIN2
2
E
Diamond
Transistor
Transconductor
(used here)
Voltage−Controlled
Current Source
C
VIN1
VIN2
B
Z
IOUT
CCII+
BASIC CONNECTIONS
E
Current Conveyor II+
Macro Transistor
Figure 27 shows basic connections required for
operation. These connections are not shown in sub-
sequent circuit diagrams. Power-supply bypass ca-
pacitors should be located as close as possible to the
device pins. Solid tantalum capacitors are generally
best.
Figure 26. Symbols and Terms
Regardless of its depiction, the OTA section has a
high-input impedance (B-input), a low-input/output
impedance (E-input), and a high-impedance current
source output (C-output).
Ω
RQ = 250 , roughly sets IQ = 5.4mA.
RC
1
8
+5V(1)
µ
0.1
F
+VS
RADJ
2
3
4
7
6
5
RS
Ω
250
+
Ω
Ω
)
(25 to 200
µ
2.2
F
VIN
Solid Tantalum
−
VS
(1)
−
5V
µ
0.1
F
µ
2.2 F
+
Solid
Tantalum
±
= 6.5V absolute maximum.
NOTE: (1) VS
Figure 27. Basic Connections
12
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