ADA4091-2 Analog Devices, ADA4091-2 Datasheet - Page 14

• Manufacturer Part Number: ADA4091-2
• Description: Dual, Precision, Micropower, OVP, Rail-to-Rail Input/Output Operational Amplifier
• Manufacturer: Analog Devices

Specifications of ADA4091-2

• -3db Bandwidth: 1.27MHz
• Slew Rate: 0.46V/µs
• Vos: 35µV
• Ib: 50nA
• # Opamps Per Pkg: 2
• Input Noise (nv/rthz): 25nV/rtHz
• Vcc-vee: 3V to 30V
• Isy Per Amplifier: 200µA
• Packages: CSP,SOIC

ADA4091-2/ADA4091-4
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
+IN
Q1
Q2
Q4
Q3
Figure 47. Simplified Schematic Without Input Protection (see Figure 48)
–IN
Q5 Q6
Q7
Q8
Rev. F | Page 14 of 20
Q9
Q10
Q11
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.
Q12
Q13
Q14
Q15
Q18
Q16
Q17
Q19
Q32
Q33
OUT