OPA861IDBVT datasheet(13/28 Pages) TI

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QUIESCENT CURRENT CONTROL PIN

BASIC APPLICATIONS CIRCUITS

Common-E Amplifier or Forward Amplifier

G +

gm ) RE

(1)

1.25k

Ω

TLV2262

OPA861

1/2 REF200

100

µA

425

Ω

V+

Q Adjust

1 I

OPA861

SBOS338 – AUGUST 2005

With this control loop, quiescent current will be nearly

constant with temperature. Since this method differs

The

quiescent

current

the

transconductance

from the temperature-dependent behavior of the

portion of the OPA861 is set with a resistor, RADJ,

internal current source, other temperature-dependent

connected from pin 1 to –VS. The maximum quiesc-

behavior may differ from that shown in the Typical

ent current is 6mA. RADJ should be set between 50Ω

Characteristics. The circuit of Figure 28 will control

and 1k

Ω for optimal performance of the OTA section.

the IQ of the OPA861 somewhat more accurately than

This range corresponds to the 5mA quiescent current

with a fixed external resistor, RQ. Otherwise, there is

for RADJ = 50Ω, and 1mA for RADJ = 1kΩ. If the IQ

no fundamental advantage to using this more com-

adjust pin is connected to the negative supply, the

plex biasing circuitry. It does, however, demonstrate

quiescent current will be set by the 250

Ω internal

the possibility of signal-controlled quiescent current.

resistor.

This capability may suggest other possibilities such

as AGC, dynamic control of AC behavior, or VCO.

Reducing or increasing the quiescent current for the

OTA section controls the bandwidth and AC behavior

as well as the transconductance. With RADJ = 250Ω,

this sets approximately 5.4mA total quiescent current

Most applications circuits for the OTA section consist

at 25

°C. It may be appropriate in some applications to

of a few basic types, which are best understood by

trim this resistor to achieve the desired quiescent

analogy to a transistor. Used in voltage-mode, the

current or AC performance.

OTA section can operate in three basic operating

Applications circuits generally do not show the

states—common emitter, common base, and com-

resistor RQ, but it is required for proper operation.

mon collector. In the current-mode, the OTA can be

useful for analog computation such as current ampli-

With a fixed RADJ resistor, quiescent current in-

fier, current differentiator, current integrator, and cur-

creases with temperature (see Figure 11 in the

rent summer.

Typical Characteristics section). This variation of

current with temperature holds the transconductance,

gm, of the OTA relatively constant with temperature

(another advantage over a transistor).

Figure 29 compares the common-emitter configur-

ation for a BJT with the common-E amplifier for the

It is also possible to vary the quiescent current with a

OTA section. There are several advantages in using

control signal. The control loop in Figure 28 shows

the OTA section in place of a BJT in this configur-

1/2 of a REF200 current source used to develop

ation. Notably, the OTA does not require any biasing,

100mV on R1. The loop forces 125mV to appear on

and the transconductance gain remains constant over

R2. Total quiescent current of the OPA861 is approxi-

temperature. The output offset voltage is close to 0,

mately 37

× I

1, where I1 is the current made to flow

compared with several volts for the common-emitter

out of pin 1.

amplifier.

The gain is set in a similar manner as for the BJT

equivalent with Equation 1:

Just as transistor circuits often use emitter degener-

ation, OTA circuits may also use degeneration. This

option can be used to reduce the effects that offset

voltage and offset current might otherwise have on

the

operating

point

the

OTA.

The

E-degeneration resistor may be bypassed with a

large capacitor to maintain high AC gain. Other

circumstances may suggest a smaller value capacitor

used to extend or optimize high-frequency perform-

ance.

Figure 28. Optional Control Loop for Setting

Quiescent Current

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