Datasheet

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.
D2
D101
D100
D5 D4
D1
Q1
Q4 Q3
Q2
08237-073
R4
R1 R2
R3
F
igure 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
V
EE
V
OUT
V
CC
V
BIAS
MIRROR
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R5
R6
R7
C2
C1
F
igure 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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