OP291GP View Datasheet(PDF) - Analog Devices
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OP291GP Datasheet PDF : 20 Pages
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OP191/OP291/OP491
FUNCTIONAL DESCRIPTION
The OP191/OP291/OP491 are single-supply, micropower
amplifiers featuring rail-to-rail inputs and outputs. In order to
achieve wide input and output ranges, these amplifiers employ
unique input and output stages. As the simplified schematic
shows (Figure 2), the input stage is actually comprised of two
differential pairs, a PNP pair and an NPN pair. These two stages
do not actually work in parallel. Instead, only one or the other
stage is on for any given input signal level. The PNP stage (tran-
sistors Q1 and Q2) is required to ensure that the amplifier remains
in the linear region when the input voltage approaches and
reaches the negative rail. On the other hand, the NPN stage
(transistors Q5 and 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 is evidenced by examining the graph of Input
Bias Current vs. Common-Mode Voltage. Notice that the bias
current switches direction at approximately 1.2 V to 1.3 V below
the positive rail. At voltages below this, the bias current flows
out of the OP291, indicating a PNP input stage. Above this
voltage, however, the bias current enters the device, revealing the
NPN stage. The actual mechanism within the amplifier for
switching between the input stages is comprised of the transistors
Q3, Q4, and 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
Q3 on. This diverts the 8 µA of 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.
Notice that the input stage includes 5 kΩ series resistors and
differential diodes, a common practice in bipolar amplifiers to
protect the input transistors from large differential voltages.
These diodes will turn on whenever the differential voltage
exceeds approximately 0.6 V. In this condition, current will flow
between the input pins, limited only by the two 5 kΩ resistors.
Being aware of this characteristic is important in circuits where
the amplifier may be operated open-loop, such as a comparator.
Evaluate each circuit carefully to make sure that the increase in
current does not affect the performance.
The output stage of the OP191 family uses a PNP and an NPN
transistor as do most output stages; however, the output tran-
sistors, Q32 and Q33, are actually connected with their collec-
tors 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 amplifier is dependent
on the load resistance.
Input Overvoltage Protection
As with any semiconductor device, whenever the condition
exists for the input to exceed either supply voltage, attention
needs to be paid to the input overvoltage characteristic. When
an overvoltage occurs, the amplifier could be damaged depend-
ing on the voltage level and the magnitude of the fault current.
Figure 3 shows the characteristic for the OP191 family. This
graph was generated with the power supplies at ground and a
curve tracer connected to the input. As can be seen, when the
input voltage exceeds either supply by more than 0.6 V, internal
pn-junctions energize, allowing current to flow from the input to
the supplies. As described above, the OP291/OP491 does have
5 kΩ resistors in series with each input, which helps limit the
current. Calculating the slope of the current versus voltage in
the graph confirms the 5 kΩ resistor.
5k⍀
+IN
8A
Q3
Q1 Q2
–IN
5k⍀
Q5 Q6
Q8
Q9
Q10 Q12
Q14
Q11 Q13
Q15
Q22
Q26
Q16
Q17
Q23
Q20
Q27
Q30
Q32
10pF
Q21
Q31
VOUT
Q24 Q28
Q4
Q7
Q18
Q19
Q25
Q29
Q33
Figure 2. Simplified Schematic
–12–
REV. A
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