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/µs 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 IQ adjust resistor (RADJ).

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

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