AD813

REV. B

–13–

General Consideration

The AD813 is a wide bandwidth, triple video amplifier that

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

plifier of quiescent supply current. With its fast acting power

down switch, it is designed to offer outstanding functionality

and 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 AD813 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 & Gain Resistors

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

width of the AD813 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 also affect the band-

width. 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.)

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

)

V

Gain

R

)

BW (MHz)

±15

+1

866

125

+2

681

100

+10

357

60

–1

681

100

–10

357

55

± 5

+1

750

75

+2

649

65

+10

154

40

–1

649

70

–10

154

40

+5

+1

715

60

+2

619

50

+10

154

30

–1

619

50

–10

154

30

+3

+1

681

50

+2

619

40

+10

154

25

–1

619

40

–10

154

20

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.

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 AD813 may be used:

A

G

S

RGrC

f

S R

Gr

C

CL

FIN

T

FIN

T

=

+

++

+

















2

2

2

1

()

()

π

where:

open-loop dominant pole with the

transresistance

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 Supplies

V

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:

f

3 =

f

n

1

where:

f n =













1/2

and:

d

=

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 accurate enough to predict either the phase behavior or the

frequency response peaking of the AD813.