OP191 Datasheet, PDF (12/20 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

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 55), 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
(transistors 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 volts to 1.3 volts
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
transistors, Q32 and Q33, 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 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 56 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
8µA
Q3
Q1 Q2
Q4
–IN
5k
Q5 Q6
Q8
Q9
Q10 Q12
Q14
Q11
Q13
Q15
Q16
Q17
Q22
Q26
Q23 Q27
Q20
Q30
Q32
10pF
Q21
Q31
V_OUT
Q24 Q28
Q18
Q19
Q25
Q29
Q33
Q7
Figure 55. Simplified Schematic
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