ADA4084-2ARMZ-R7 datasheet(29/36 Pages) AD

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Data Sheet

ADA4084-1/ADA4084-2/ADA4084-4

Rev. I | Page 29 of 36

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

The ADA4084-1/ADA4084-2/ADA4084-4 devices are precision

single-supply, rail-to-rail operational amplifiers. Intended for

portable instrumentation, the ADA4084-1/ADA4084-2/

ADA4084-4 devices combine the attributes of precision, wide

bandwidth, and low noise, making them an ideal choice in

single-supply applications that require both ac and precision dc

performance. Other low supply voltage applications for which

the ADA4084-1/ADA4084-2/ADA4084-4 devices are well suited

include 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.

D101

D100

Figure 106. Equivalent Input Circuit

For example, Figure 106 illustrates a simplified equivalent

circuit for the input stage of the ADA4084-1/ADA4084-2/

ADA4084-4. 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 ADA4084-1/ADA4084-2/

ADA4084-4, 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 ADA4084-1/ADA4084-2/ADA4084-4, which is a modified

compound folded cascade gain stage. It is also in the second

gain stage that 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 ADA4084-1/ADA4084-2/ADA4084-4 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 ADA4084-1/ADA4084-2/ADA4084-4 not only exhibit

different amplitudes, but they also exhibit different polarities. This

effect is best shown in Figure 19, Figure 20, Figure 50, Figure 51,

Figure 81, and Figure 82. It is, therefore, important that the

effective source impedances that are connected to the ADA4084-1/

ADA4084-2/ADA4084-4 inputs be balanced for optimum dc

and ac performance.

To achieve rail-to-rail output, the ADA4084-1/ADA4084-2/

ADA4084-4 output stage design employs a unique topology for

both sourcing and sinking current. This circuit topology is shown

in Figure 107. 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 to the mirror to 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

Figure 107. Equivalent Output Circuit

Thus, the saturation voltage of the output transistors sets the

limit on the ADA4084-1/ADA4084-2/ADA4084-4 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 ADA4084-1/

ADA4084-2/ADA4084-4.

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