OP484FS datasheet(14/24 Pages) AD

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OP184/OP284/OP484

Rev. D | Page 14 of 24

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

To achieve rail-to-rail output, the OP284 output stage design

employs a unique topology for both sourcing and sinking

current. This circuit topology is illustrated in

The OP184/OP284/OP484 are precision single-supply, rail-to-rail

operational amplifiers. Intended for the portable instrumentation

marketplace, the OPx84 family of devices combine the attributes

of precision, wide bandwidth, and low noise to make them a

superb choice in single-supply applications that require both ac

and precision dc performance. Other low supply voltage

applications for which the OP284 is well suited are active filters,

audio microphone preamplifiers, power supply control, and

telecommunications. To combine all of these attributes with

rail-to-rail input/output operation, novel circuit design techniques

are used.

Figure 45. The

output stage is voltage-driven from the second gain stage. The

signal path through the output stage is inverting; that is, for

positive input signals, Q1 provides the base current drive to Q6

so that it conducts (sinks) current. For negative input signals,

the signal path via Q1→Q2→D1→Q4→Q3 provides the base

current drive for Q5 to conduct (source) current. Both

amplifiers provide output current until they are forced into

saturation, which occurs at approximately 20 mV from the

negative supply rail and 100 mV from the positive supply rail.

VPOS

VNEG

VOUT

INPUT FROM

SECOND GAIN

STAGE

VPOS

V01

V02

–IN

VNEG

+IN

–

3kΩ

4kΩ

Figure 44. OP284 Equivalent Input Circuit

Figure 45. OP284 Equivalent Output Circuit

Thus, the saturation voltage of the output transistors sets the

limit on the OP284 maximum output voltage swing. Output

short-circuit current limiting is determined by the maximum

signal current into the base of Q1 from the second gain stage.

Under output short-circuit conditions, this input current level

is approximately 100 μA. With transistor current gains around 200,

the short-circuit current limits are typically 20 mA. The output

stage also exhibits voltage gain. This is accomplished by the use

of common-emitter amplifiers, and, as a result, the voltage gain

of the output stage (thus, the open-loop gain of the device)

exhibits a dependence to the total load resistance at the output

of the OP284.

For example, Figure 44 illustrates a simplified equivalent circuit

for the input stage of the OP184/OP284/OP484. It comprises

an NPN differential pair, Q1→Q2, and a PNP differential pair,

Q3→Q4, operating concurrently. Diode Network D1→Diode

Network D2 serves to clamp the applied differential input

voltage to the OP284, thereby protecting the input transistors

against avalanche damage. Input stage voltage gains are kept low

for input rail-to-rail operation. The two pairs of differential

output voltages are connected to the OP284’s second stage,

which is a compound folded cascade gain stage. It is also in the

second gain stage, where the two pairs of differential output

voltages are combined into a single-ended, output signal voltage

used to drive the output stage. A key issue in the input stage is

the behavior of the input bias currents over the input common-

mode voltage range. Input bias currents in the OP284 are the

arithmetic sum of the base currents in Q1→Q3 and in Q2→Q4.

As a result of this design approach, the input bias currents in

the OP284 not only exhibit different amplitudes; they also

exhibit different polarities. This effect is best illustrated by

INPUT OVERVOLTAGE PROTECTION

As with any semiconductor device, if conditions exist where the

applied input voltages to the device exceed either supply voltage,

the input overvoltage I-V characteristic of the device must be

considered. When an overvoltage occurs, the amplifier could be

damaged, depending on the magnitude of the applied voltage

and the magnitude of the fault current. Figure 46 illustrates the

overvoltage I-V characteristic of the OP284. This graph was

generated with the supply pins connected to GND and a curve

tracer’s collector output drive connected to the input.

Figure 10. It is, therefore, of paramount importance that the

effective source impedances connected to the OP284 inputs

be balanced for optimum dc and ac performance.

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