OPA660AP datasheet(9/20 Pages) TI1

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OPA660

BUFFER SECTION—AN OVERVIEW

The buffer section of the OPA660 is an open-loop buffer

consisting of complementary emitter-followers. It uses no

feedback, so its low frequency gain is slightly less than unity

and somewhat dependent on loading. It is designed prima-

rily for interstage buffering. It is not designed for driving

long cables or low impedance loads (although with small

signals, it may be satisfactory for these applications).

TRANSCONDUCTANCE

(OTA) SECTION—AN OVERVIEW

The symbol for the OTA section is similar to a transistor.

Applications circuits for the OTA look and operate much

like transistor circuits—the transistor, too, is a voltage-

controlled current source. Not only does this simplify the

understanding of applications circuits, but it aids the circuit

optimization process. Many of the same intuitive techniques

used with transistor designs apply to OTA circuits as well.

The three terminals of the OTA are labeled B, E, and C. This

calls attention to its similarity to a transistor, yet draws

distinction for clarity.

While it 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 advan-

tages over a discrete transistor. The OTA is self-biased,

simplifying the design process and reducing component

count. The OTA is far more linear than a transistor.

Transconductance of the OTA is constant over a wide range

of collector currents—this implies a fundamental improve-

ment of linearity.

BASIC CONNECTIONS

Figure 2 shows basic connections required for operation.

These connections are not shown in subsequent circuit

diagrams. Power supply bypass capacitors should be located

as close as possible to the device pins. Solid tantalum

capacitors are generally best. See “Circuit Layout” at the end

of the applications discussion and Figure 26 for further

suggestions on layout.

QUIESCENT CURRENT CONTROL PIN

The quiescent current of the OPA660 is set with a resistor,

RQ, connected from pin 1 to V–. It affects the operating

currents of both the buffer and OTA sections. This controls

the bandwidth and AC behavior as well as the

transconductance of the OTA section.

Q = 250Ω sets approximately 20mA total quiescent current at

°C. With a fixed 250Ω resistor, process variations could

cause this current to vary from approximately 16mA to 26mA.

It may be appropriate in some applications to trim this resistor

to achieve the desired quiescent current or AC performance.

Applications circuits generally do not show resistor, RQ,

but it is required for proper operation.

With a fixed RQ resistor, quiescent current increases with

temperature (see typical performance curve, Quiescent Current

vs Temperature). This variation of current with temperature

holds the transconductance, gm, of the OTA relatively con-

stant with temperature (another advantage over a transistor).

It is also possible to vary the quiescent current with a control

signal. The control loop in Figure 3 shows a 1/2 of a REF200

current source used to develop 100mV on R1. The loop

forces 100mV to appear on R

2. Total quiescent current of the

OPA660 is approximately 85 • I1, where I1 is the current

made to flow out of pin 1.

FIGURE 2. Basic Connections.

50k

Ω

100

Ω

–V

425

Ω

1/2

OPA1013

(1)

1/2 REF200

100µA

V+

Ω

Internal

Current Source

Circuitry

85 • I

= 85 • (100µA)

= 20mA

≈

NOTE: (1) Requires input common-mode range and

output swing close to V–, thus the choice of OPA1013.

OPA660

FIGURE 3. Optional Control Loop for Setting Quiescent

Current.

With this control loop, quiescent current will be nearly

constant with temperature. Since this differs from the tem-

perature-dependent behavior of the internal current source,

other temperature-dependent behavior may differ from that

shown in typical performance curves.

The circuit of Figure 3 will control the IQ of the OPA660

somewhat more accurately than with a fixed external resis-

tor, RQ. Otherwise, there is no fundamental advantage to

2.2µF

Solid

Tantalum

–5V

(1)

250

Ω

R = 250

Ω sets roughly

20mA

≈

Solid

Tantalum

+5V (1)

NOTE: (1) V

= ±6V absolute max.

2.2µF

10nF

470pF

10nF

(25

Ω to

200

Ω)

(25

Ω to 200Ω)

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