ADA4084-2ARMZ-R7 datasheet(22/28 Pages) AD

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ADA4084-2

Data Sheet

Rev. C | Page 22 of 28

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

The ADA4084-2 is a precision single-supply, rail-to-rail operational

amplifier. Intended for portable instrumentation, the ADA4084-2

combines the attributes of precision, wide bandwidth, and low noise

to make it an ideal choice in single-supply applications that require

both ac and precision dc performance. Other low supply voltage

applications for which the ADA4084-2 is well suited are active filters,

audio microphone preamplifiers, power supply control, and tele-

communications. To combine all of these attributes with rail-to-rail

input/output operation, novel circuit design techniques are used.

D101

D100

Figure 78. ADA4084-2 Equivalent Input Circuit

For example, Figure 78 illustrates a simplified equivalent circuit for

the input stage of the ADA4084-2. It comprises a PNP differential

pair, Q1 and Q2, and an NPN differential pair, Q3 and Q4,

operating concurrently. Diode D100 and Diode D101 serve to

clamp the applied differential input voltage to the ADA4084-2,

thereby protecting the input transistors against Zener breakdown

of the emitter-base junctions. Input stage voltage gains are kept

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

output voltages are connected to the second stage of the ADA4084-2,

which is a modified 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 ADA4084-2 are the arithmetic sum of the base

currents in Q1 and Q4 and in Q2 and Q3. As a result of this design

approach, the input bias currents in the ADA4084-2 not only

exhibit different amplitudes; they also exhibit different polarities.

This effect is best illustrated by Figure 9, Figure 10, Figure 34,

Figure 35, Figure 59, and Figure 60. It is therefore important

that the effective source impedances connected to the ADA4084-2

inputs be balanced for optimum dc and ac performance.

To achieve rail-to-rail output, the ADA4084-2 output stage

design employs a unique topology for both sourcing and sinking

current. This circuit topology is illustrated in Figure 79. 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, Q13 provides the base current drive to Q19

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

signal path via Q18 → mirror → Q24 provides the base current

drive for Q23 to conduct (source) current. Both transistors

provide output current until they are forced into saturation.

Q24

Q21

D20

Q13

Q18

Q19

Q23

VEE

VOUT

VCC

VBIAS

MIRROR

R7 C2

Figure 79. ADA4084-2 Equivalent Output Circuit

Thus, the saturation voltage of the output transistors sets the

limit on the ADA4084-2 maximum output voltage swing. Output

short-circuit current limiting is determined by the maximum

signal current into the base of Q13 from the second gain stage.

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 on the total load resistance at the

output of the ADA4084-2.

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