ada4091-2arz-r2 Analog Devices, Inc., ada4091-2arz-r2 Datasheet - Page 13

• Manufacturer Part Number: ada4091-2arz-r2
• Description: Micropower, Single-supply, Rail-to-rail Input/output Operational Amplifier
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADA4091-2 is a single-supply, micropower amplifier
featuring rail-to-rail inputs and outputs. To achieve wide input
and output ranges, this amplifier employs unique input and
output stages. In Figure 37, the input stage comprises two
differential pairs, a PNP pair and an NPN pair. These two
stages do not work in parallel. Instead, only one stage is on
for any given input signal level. The PNP stage (Transistor Q1
and Transistor Q2) is required to ensure that the amplifier
remains in the linear region when the input voltage approaches
and reaches the negative rail. Alternatively, the NPN stage
(Transistor Q5 and Transistor Q6) is needed for input voltages
up to and including the positive rail.
For the majority of the input common-mode range, the PNP
stage is active, as shown in Figure 4. Notice that the bias current
switches direction at approximately 1.5 V below the positive
rail. At voltages below this level, the bias current flows out of
the ADA4091-2, from the PNP input stage. Above this voltage,
however, the bias current enters the device, due to the NPN
stage. The actual mechanism within the amplifier for switching
between the input stages comprises Transistor Q3, Transistor Q4,
and Transistor Q7. As the input common-mode voltage
increases, the emitters of Q1 and Q2 follow that voltage plus
a diode drop. Eventually, the emitters of Q1 and Q2 are high
enough to turn on Q3, which diverts the tail current away from
the PNP input stage, turning it off. Instead, the current is
mirrored through Q4 and Q7 to activate the NPN input stage.
+IN
Q1
Q2
Q4
Q3
Figure 37. Simplified Schematic without Input Protection (see Figure 39)
–IN
Q5 Q6
Q7
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Q8
Q9
Q10
Q11
A common practice in bipolar amplifiers to protect the input
transistors from large differential voltages is to include series
resistors and differential diodes. (See Figure 39 for the full input
protection circuitry.) These diodes turn on whenever the diffe-
rential voltage exceeds approximately 0.6 V. In this condition,
current flows between the input pins, limited only by the two
5 kΩ resistors. Evaluate each circuit carefully to make sure that
the increase in current does not affect performance.
The output stage in the ADA4091-2 device uses a PNP and
an NPN transistor, as do most output stages. However, Q32
and Q33, the output transistors, are actually connected with
their collectors to the output pin to achieve the rail-to-rail
output swing.
As the output voltage approaches either the positive or negative
rail, these transistors begin to saturate. Thus, the final limit
on output voltage is the saturation voltage of these transistors,
which is about 50 mV. The output stage does have inherent gain
arising from the collectors and any external load impedance.
Because of this, the open-loop gain of the op amp is dependent
on the load resistance.
Q12
Q13
Q14
Q15
Q18
Q16
Q17
Q19
ADA4091-2
Q32
Q33