Datasheet

1
2
3
4
8
7
6
5
R
(25 to200 )W W
S
R
C
0.1 Fm
R
ADJ
250W
-5V
(1)
V
IN
+
2.2 Fm
0.1 Fm
Solid
Tantalum
+5V
(1)
+
2.2 Fm
SolidTantalum
R =250 ,roughlysetsIW
Q Q
=5.4mA.
NOTE:(1)V = 6.5Vabsolutemaximum.
S
±
+V
S
-V
S
R
E
1
3
2
C
E
B
C
E
B
V
IN1
I
OUT
V
IN2
V
IN1
I
OUT
V
IN2
CCII+
Z
Macro Transistor
Current Conveyor II+
Voltage−Controlled
Current Source
Transconductor
(used here)
Diamond
Transistor
OPA861
SBOS338G –AUGUST 2005–REVISED MAY 2013 www.ti.com
APPLICATION INFORMATION
The OPA861 is a versatile monolithic
TRANSCONDUCTANCE (OTA) SECTION—AN
transconductance amplifier designed for wide-
OVERVIEW
bandwidth systems, including high-performance
video, RF, and IF circuitry. The operation of the
The symbol for the OTA section is similar to a
OPA861 is discussed in the OTA (Operational
transistor (see Figure 29). Applications circuits for the
Transconductance Amplifier) section of this data
OTA look and operate much like transistor
sheet. Over the years and depending on the writer,
circuits—the transistor is also a voltage-controlled
the OTA section of an op amp has been referred to
current source. Not only does this characteristic
as a Diamond Transistor, Voltage-Controlled Current
simplify the understanding of application circuits, it
source, Transconductor, Macro Transistor, or positive
aids the circuit optimization process as well. Many of
second-generation current conveyor (CCII+).
the same intuitive techniques used with transistor
Corresponding symbols for these terms are shown in
designs apply to OTA circuits. The three terminals of
Figure 29.
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, simplifying 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.
BASIC CONNECTIONS
Figure 30 shows basic connections required for
operation. These connections are not shown in
subsequent circuit diagrams. Power-supply bypass
Figure 29. Symbols and Terms
capacitors should be located as close as possible to
the device pins. Solid tantalum capacitors are
Regardless of its depiction, the OTA section has a
generally best.
high-input impedance (B-input), a low-input/output
impedance (E-input), and a high-impedance current
source output (C-output).
Figure 30. Basic Connections
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