THEORY OF OPERATION
The AD8031/AD8032 are single and dual versions of high
speed, low power, voltage feedback amplifiers featuring an
innovative architecture that maximizes the dynamic range
capability on the inputs and outputs. The linear input common-
mode range exceeds either supply voltage by 200 mV, and the
amplifiers show no phase reversal up to 500 mV beyond supply.
The output swings to within 20 mV of either supply when
driving a light load; 300 mV when driving up to 5 mA.
Fabricated on Analog Devices, Inc. eXtra Fast Complementary
Bipolar (XFCB) process, the amplifier provides an impressive
80 Hz bandwidth when used as a follower and a 30 V/μs slew
rate at only 800 μA supply current. Careful design allows the
amplifier to operate with a supply voltage as low as 2.7 V.
INPUT STAGE OPERATION
A simplified schematic of the input stage appears in Figure 43.
For common-mode voltages up to 1.1 V within the positive
supply (0 V to 3.9 V on a single 5 V supply), tail current I2
flows through the PNP differential pair, Q13 and Q17. Q5 is cut
off; no bias current is routed to the parallel NPN differential
pair, Q2 and Q3. As the common-mode voltage is driven within
1.1 V of the positive supply, Q5 turns on and routes the tail
current away from the PNP pair and to the NPN pair. During
this transition region, the input current of the amplifier changes
magnitude and direction. Reusing the same tail current ensures
that the input stage has the same transconductance, which
determines the gain and bandwidth of the amplifier, in both
regions of operation.
Switching to the NPN pair as the common-mode voltage is
driven beyond 1 V within the positive supply allows the amplifier
to provide useful operation for signals at either end of the
supply voltage range and eliminates the possibility of phase
reversal for input signals up to 500 mV beyond either power
supply. Offset voltage also changes to reflect the offset of the
input pair in control. The transition region is small, approximately
180 mV. These sudden changes in the dc parameters of the
input stage can produce glitches that adversely affect distortion.
OVERDRIVING THE INPUT STAGE
Sustained input differential voltages greater than 3.4 V should
be avoided as the input transistors can be damaged. Input clamp
diodes are recommended if the possibility of this condition
The voltages at the collectors of the input pairs are set to
200 mV from the power supply rails. This allows the amplifier
to remain in linear operation for input voltages up to 500 mV
beyond the supply voltages. Driving the input common-mode
voltage beyond that point will forward bias the collector junction of
the input transistor, resulting in phase reversal. Sustaining this
condition for any length of time should be avoided because it is
easy to exceed the maximum allowed input differential voltage
when the amplifier is in phase reversal.
Figure 43. Simplified Schematic of AD8031 Input Stage
Rev. C | Page 13 of 20