AD812

REV. B

–12–

General Considerations

The AD812 is a wide bandwidth, dual video amplifier which

offers a high level of performance on less than 5.5 mA per am-

plifier of quiescent supply current. It is designed to offer out-

standing performance at closed-loop inverting or noninverting

gains of one or greater.

Built on a low cost, complementary bipolar process, and achiev-

ing bandwidth in excess of 100 MHz, differential gain and phase

errors of better than 0.1% and 0.1

° (into 150 Ω), and output

current greater than 40 mA, the AD812 is an exceptionally

efficient video amplifier. Using a conventional current feedback

architecture, its high performance is achieved through careful

attention to design details.

Choice of Feedback and Gain Resistors

Because it is a current feedback amplifier, the closed-loop band-

width of the AD812 depends on the value of the feedback resis-

tor. The bandwidth also depends on the supply voltage. In

addition, attenuation of the open-loop response when driving

load resistors less than about 250

Ω will affect the bandwidth.

Table I contains data showing typical bandwidths at different

supply voltages for some useful closed-loop gains when driving a

load of 150

Ω. (Bandwidths will be about 20% greater for load

resistances above a few hundred ohms.)

The choice of feedback resistor is not critical unless it is impor-

tant to maintain the widest, flattest frequency response. The

resistors recommended in the table are those (metal film values)

that will result in the widest 0.1 dB bandwidth. In those appli-

cations where the best control of the bandwidth is desired, 1%

metal film resistors are adequate. Wider bandwidths can be

attained by reducing the magnitude of the feedback resistor (at

the expense of increased peaking), while peaking can be reduced

by increasing the magnitude of the feedback resistor.

Table I. –3 dB Bandwidth vs. Closed-Loop Gain and

Ω)

V

Gain

R

)

BW (MHz)

±15

+1

866

145

+2

715

100

+10

357

65

–1

715

100

–10

357

60

± 5

+1

750

90

+2

681

65

+10

154

45

–1

715

70

–10

154

45

+5

+1

750

60

+2

681

50

+10

154

35

–1

715

50

–10

154

35

+3

+1

750

50

+2

681

40

+10

154

30

–1

715

40

–10

154

25

To estimate the –3 dB bandwidth for closed-loop gains or feed-

back resistors not listed in the above table, the following two

pole model for the AD812 many be used:

A

G

S

RGrC

f

S R

Gr

C

CL

FIN

T

FIN

T

=

+

()

















++

2

2

2

1

π

where:

G

dominant pole with the tranresistance

= frequency of second (nondominant) pole

S

= 2

πj f

Appropriate values for the model parameters at different supply

voltages are listed in Table II. Reasonable approximations for

these values at supply voltages not found in the table can be

obtained by a simple linear interpolation between those tabu-

lated values which “bracket” the desired condition.

Table II. Two-Pole Model Parameters at Various

Supply Voltages

V

S

r

)C

f

±15

85

2.5

150

±5

90

3.8

125

+5

105

4.8

105

+3

115

5.5

95

As discussed in many amplifier and electronics textbooks (such

as Roberge’s Operational Amplifiers: Theory and Practice), the

–3 dB bandwidth for the 2-pole model can be obtained as:

where:

f

f

RGr

C

N

FIN

T

=

+

()

















2

12

/

and:

1/2

This model will predict –3 dB bandwidth within about 10 to

15% of the correct value when the load is 150

Ω. However, it is

not an accurate enough to predict either the phase behavior or

the frequency response peaking of the AD812.

Printed Circuit Board Layout Guidelines

As with all wideband amplifiers, printed circuit board parasitics

can affect the overall closed-loop performance. Most important

for controlling the 0.1 dB bandwidth are stray capacitances at

the output and inverting input nodes. Increasing the space between

signal lines and ground plane will minimize the coupling. Also,

signal lines connecting the feedback and gain resistors should be

kept short enough that their associated inductance does not

cause high frequency gain errors.