ADA4091-2/ADA4091-4

Rev. E | Page 14 of 20

THEORY OF OPERATION

The ADA4091 family 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. Alter-

natively, 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-x 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-x 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 op amp 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)