OP191/OP291/OP491

REV. 0

–12–

exceeds approximately 0.6 V. In this condition, current will

flow between the input pins, limited only by the two 5 k

Ω

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 k

Ω 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 k

Ω resistor.

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 k

Ω 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

Figure 55. Simplified Schematic

Q1 Q2

8

µA

5k

Q3

5k

–IN

Q5 Q6

Q11

Q10

Q8

Q7

Q4

Q13

Q15

Q14

Q12

Q9

Q16

Q17

Q18

Q19

Q20

Q21

Q24

Q23

Q22

Q27

Q26

Q30

Q31

Q28

Q25

Q29

Q32

V

OUT

Q33

10pF

+IN