OP191GSZ datasheet(17/24 Pages) AD

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OP191/OP291/OP491

Rev. D | Page 17 of 24

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.

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.

8µA

5kΩ

–IN

Q5 Q6

Q11

Q10

Q13

Q15

Q14

Q12

Q16

Q17

Q18

Q19

Q20

Q21

Q24

Q23

Q22

Q27

Q26

Q30

Q31

Q28

Q25

Q29

Q32

VOUT

Q33

10pF

+IN

Figure 61. Simplified Schematic

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