OP291_15 Datasheet, PDF (17/24 Pages) Analog Devices – Micropower Single-Supply Rail-to-Rail Input/Output Op Amps

THEORY OF OPERATION
The OP191/OP291/OP491 are single-supply, micropower
amplifiers featuring rail-to-rail inputs and outputs. To achieve
wide input and output ranges, these amplifiers employ unique
input and output stages. In Figure 61 , 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. On the other hand, 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 is shown in Figure 12. 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 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
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.
OP191/OP291/OP491
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 turn on whenever the differential voltage exceeds
approximately 0.6 V. In this condition, current flows between
the input pins, limited only by the two 5 kΩ resistors. 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 in OP191 devices 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 amplifier is
dependent on the load resistance.
5kΩ
+IN
8µA
Q3
Q1 Q2
–IN
5kΩ
Q22
Q26
Q32
Q23 Q27
Q5 Q6
Q8
Q10 Q12
Q20
Q16
Q17
Q14
Q21
Q30
10pF
Q31
V_OUT
Q9
Q11 Q13
Q15
Q24 Q28
Q4
Q7
Q18
Q19
Q25
Q29
Q33
Figure 61. Simplified Schematic
Rev. E | Page 17 of 24