Datasheet
APPLICATION INFORMATION
BUFFER SECTION — AN OVERVIEW
TRANSCONDUCTANCE (OTA) SECTION — AN
1
3
2
C
E
B
C
E
B
V
IN1
I
OUT
V
IN2
V
IN1
I
OUT
V
IN2
CCII+
Z
Diamond Transistor
Voltage−Controlled Current Source
Transconductor
Macro Transistor
Current Conveyor II+
OPA860
www.ti.com
....................................................................................................................................................... SBOS331C – JUNE 2005 – REVISED AUGUST 2008
The OPA860 combines a high-performance buffer
with a transconductance section. This
transconductance section is discussed in the OTA
The buffer section of the OPA860 is an 1600MHz,
(Operational Transconductance Amplifier) section of
4000V/ µ s closed-loop buffer that can be used as a
this data sheet. Over the years and depending on the
building block for AGC amplifiers, LED driver circuit,
writer, the OTA section of an op amp has been
integrator for fast pulse, fast control loop amplifiers,
referred to as a Diamond Transistor,
and control amplifiers for capacitive sensors and
Voltage-Controlled Current source, Transconductor,
active filters. The Buffer section does not share the
Macro Transistor, or positive second-generation
bias circuit of the OTA section; thus, it is not affected
current conveyor (CCII+). Corresponding symbols for
by changes in the I
Q
adjust resistor (R
ADJ
).
these terms are shown in Figure 44 .
OVERVIEW
The symbol for the OTA section is similar to a
transistor (see Figure 44 ). Applications circuits for the
OTA look and operate much like transistor
circuits — the transistor is also a voltage-controlled
current 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
Figure 44. Symbols and Terms
negative B-to-E input voltage. The OTA offers many
advantages over a discrete transistor. The OTA is
Regardless of its depiction, the OTA section has a
self-biased, simplifying the design process and
high-input impedance (B input), a low-input/output
reducing component count. In addition, the OTA is far
impedance (E input), and a high impedance current
more linear than a transistor. Transconductance of
source output (C output).
the OTA is constant over a wide range of collector
currents — this feature implies a fundamental
improvement of linearity.
Copyright © 2005 – 2008, Texas Instruments Incorporated Submit Documentation Feedback 13
Product Folder Link(s): OPA860