ADA4084-2

Data Sheet

Rev. A | Page 20 of 24

APPLICATIONS INFORMATION

FUNCTIONAL DESCRIPTION

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

tional amplifier. Intended for portable instrumentation, the

ADA4084-2 combines the attributes of precision, wide band-

width, 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 telecommunications. To combine all

of these attributes with rail-to-rail input/output operation, novel

circuit design techniques are used.

D2

D101

D100

D5

D4

D1

Q1

Q4

Q3

Q2

R4

R1

R2

R3

Figure 73. ADA4084-2 Equivalent Input Circuit

For example, Figure 73 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 7, Figure 8,

Figure 30, Figure 31, Figure 53, and Figure 54. 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 74. 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

MIRROR

R5

R6

R7 C2

C1

Figure 74. 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.