AD8042

Rev. D | Page 12 of 16

OVERDRIVE RECOVERY

Overdrive of an amplifier occurs when the output and/or input

range are exceeded. The amplifier must recover from this

overdrive condition. As shown in Figure 35, the AD8042

recovers within 30 ns from negative overdrive and within 25 ns

from positive overdrive.

5.0V

2.5V

0V

G = +2

50ns

1V

Figure 35. Overdrive Recovery

CIRCUIT DESCRIPTION

The AD8042 is fabricated on Analog Devices proprietary eXtra-

Fast Complementary Bipolar (XFCB) process, which enables

the construction of PNP and NPN transistors with similar fTs in

the 2 GHz to 4 GHz region. The process is dielectrically isolated

to eliminate the parasitic and latch-up problems caused by

junction isolation. These features allow the construction of high

frequency, low distortion amplifiers with low supply currents.

This design uses a differential output input stage to maximize

bandwidth and headroom (see Figure 36). The smaller signal

swings required on the first stage outputs (nodes S1P, S1N)

reduce the effect of nonlinear currents due to junction

capacitances and improve the distortion performance. With this

design, harmonic distortion of better than −77 dB @ 1 MHz

supply is achieved.

SIN

R21

R3

Q11

Q3

I10

R26

R39

Q5

Q4

Q40

I7

R2

R15

Q13

Q17

R5

C7

Q2

SIP

Q22

Q7

Q21

Q24

R23 R27

I2

I3

I1

Q51

Q25

Q50

Q39

Q47

Q27

Q31

Q23

I9

I5

I8

Q36

Q8

C3

C9

Figure 36. Simplified Schematic

The AD8042’s rail-to-rail output range is provided by a

complementary common-emitter output stage. High output

drive capability is provided by injecting all output stage

predriver currents directly into the bases of the output devices

Q8 and Q36. Biasing of Q8 and Q36 is accomplished by I8 and

I5, along with a common-mode feedback loop (not shown).

This circuit topology allows the AD8042 to drive 40 mA of

output current with the outputs within 0.5 V of the supply rails.

On the input side, the device can handle voltages from 0.2 V

below the negative rail to within 1.2 V of the positive rail.

Exceeding these values does not cause phase reversal; however,

the input ESD devices do begin to conduct if the input voltages

exceed the rails by greater than 0.5 V.

DRIVING CAPACITIVE LOADS

The capacitive load drive of the AD8042 can be increased by

adding a low valued resistor in series with the load. Figure 37

shows the effects of a series resistor on capacitive drive for

varying voltage gains. As the closed-loop gain is increased, the

larger phase margin allows for larger capacitive loads with less

overshoot. Adding a series resistor with lower closed-loop gains

accomplishes the same effect. For large capacitive loads, the

frequency response of the amplifier is dominated by the roll-off

of the series resistor and capacitive load.

1000

100

10

15

24

3

CLOSED-LOOP GAIN (V/V)

200mV STEP WITH 90% OVERSHOOT

Figure 37. Capacitive Load Drive vs. Closed-Loop Gain