ADA4091-2/ADA4091-4

Data Sheet

Rev. H | Page 14 of 20

THEORY OF OPERATION

The ADA4091-2/ADA4091-4 is a single-supply, micropower

amplifier featuring rail-to-rail inputs and outputs. To achieve

wide input and output ranges, these amplifiers employ unique

input and output stages.

INPUT STAGE

In Figure 47, the input stage comprises two differential pairs, a

PNP pair (PNP input stage) and an NPN pair (NPN input stage).

These input stages do not work in parallel. Instead, only one stage

is on for any given input common-mode 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. Alternatively, 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 shown in Figure 7, Figure 21, and Figure 31.

Notice that the bias current switches direction at approximately

1.5 V below the positive rail. At voltages below this level, the bias

current flows out of the ADA4091-2/ADA4091-4 input, from the

PNP input stage. Above this voltage, however, the bias current

enters the device, due to 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 tail current away from the PNP input

stage, turning it off. The tail current of the PNP pair is diverted

to the Q4/Q7 current mirror to activate the NPN input stage.

A common practice in bipolar amplifiers to protect the input

transistors from large differential voltages is to include series

resistors and differential diodes. See Figure 48 for the full input

protection circuitry. These diodes turn on whenever the diffe-

rential voltage exceeds approximately 0.6 V. In this condition,

current flows between the input pins, limited only by the two

5 kΩ resistors. Evaluate each application carefully to make sure

that the increase in current does not affect performance.

OUTPUT STAGE

The output stage in the ADA4091-2/ADA4091-4 device uses a

PNP and an NPN transistor, as do most output stages. However,

Q32 and Q33, the output transistors, connect 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 has inherent gain arising

from the transistor output impedance, as well as any external load

impedance; consequently, the open-loop gain of the operational

amplifier is dependent on the load resistance and decreases when

the output voltage is close to either rail.

Q1

Q3

–IN

Q5

Q6

Q11

Q10

Q8

Q7

Q4

Q13

Q15

Q14

Q12

Q9

Q16

Q17

Q18

Q19

Q32

OUT

Q33

+IN

Q2

Figure 47. Simplified Schematic Without Input Protection (see Figure 48)