AD8067

Rev. 0 | Page 15 of 24

THEORY OF OPERATION

FREQUENCY – MHz

90

0

10

20

30

40

50

60

70

80

–10

120

–150

–120

–90

–60

–30

0

30

60

90

–180

0.01

0.1

1

10

100

1k

PHASE

GAIN

The AD8067 is a low noise, wideband, voltage feedback operational

amplifier that combines a precision JFET input stage with Analog

Devices’ dielectrically isolated eXtra Fast Complementary Bipolar

(XFCB) process BJTs. Operating supply voltages range from 5 V

to 24 V. The amplifier features a patented rail-to-rail output stage

capable of driving within 0.25 V of either power supply while

sourcing or sinking 30 mA. The JFET input, composed of

N-channel devices, has a common-mode input range that includes

the negative supply rail and extends to 3 V below the positive

supply. In addition, the potential for phase reversal behavior has

been eliminated for all input voltages within the power supplies.

The combination of low noise, dc precision, and high bandwidth

makes the AD8067 uniquely suited for wideband, very high input

impedance, high gain buffer applications. It will also prove useful

in wideband transimpedance applications, such as a photodiode

interface, that require very low input currents and dc precision.

Figure 41. Open-Loop Frequency Response

Basic Frequency Response

The bandwidth formula only holds true when the phase margin of

the application approaches 90°, which it will in high gain config-

urations. The bandwidth of the AD8067 used in a G = +10 buffer

is 54 MHz, considerably faster than the 30 MHz predicted by the

closed loop –3 dB frequency equation. This extended bandwidth is

due to the phase margin being at 60° instead of 90°. Gains lower

than +10 will show an increased amount of peaking, as shown in

Figure 4. For gains lower than +7, use the AD8065, a unity gain

stable JFET input op amp with a unity gain bandwidth of 145 MHz,

or refer to the Applications section for using the AD8067 in a gain

of 2 configuration.

The AD8067’s typical open-loop response (see Figure 41) shows a

phase margin of 60° at a gain of +10. Typical configurations for

noninverting and inverting voltage gain applications are shown in

Figure 40 and Figure 42.

The closed-loop frequency response of a basic noninverting gain

configuration can be approximated using the equation:

() ()

G

F

G

R

R

R

GBP

Frequency

dB

–

Loop

Closed

+

×

=

3

Gain

BW (MHz)

10

110

1

54

20

49.9

1

15

50

20

1

6

100

10

1

3

1

+

=

G

R

Gain

DC

GBP is the gain bandwidth product of the amplifier. Typical GBP

for the AD8067 is 300 MHz. See Table 1 for recommended values

1

R

R

Gain

Noise

ion

Configurat

ng

Noninverti

G

F

+

=

AD8067

+

–

+

–

10

µF

0.1

µF

10

µF

0.1

µF

SIGNAL

SOURCE

+

+

AD8067

+

–

+

–

FOR BEST PERFORMANCE,

10

µF

0.1

µF

10

µF

+

0.1

µF

SIGNAL

SOURCE

+

Figure 40. Noninverting Gain Configuration

Figure 42. Inverting Gain Configuration