AD8031/AD8032

Rev. C | Page 13 of 20

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.

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.

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.

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

exists.

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.

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.

Q3

Q2

Q13

Q17

Q6

Q8

Q10

4

Q14

4

1

1

Q7

Q15

1

Q11

4

1

4

Q16

Q18

Q4

Q5

Q9

OUTPUT STAGE,

COMMON-MODE

FEEDBACK

R4

2kΩ

R2

2kΩ

R1

2kΩ

I3

25µA

I4

25µA

R3

2kΩ

I1

5µA

I2

90µA

1.1V

R5

50kΩ

R6

850Ω

R7

850Ω

R8

850Ω

R9

850Ω

Figure 43. Simplified Schematic of AD8031 Input Stage