EL2170CS datasheet(11/16 Pages) ELANTEC

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EL2170CEL2270CEL2470C

70 MHz1 mA Current Mode Feedback Amplifiers

Applications Information

Contd

The experienced user with a large amount of PC

board layout experience may find in rare cases

that the EL2170CEL2270CEL2470C have less

bandwidth than expected The reduction of feed-

back resistor values (or the addition of a very

small amount of external capacitance at the in-

verting input e g 05 pF) will increase band-

width as desired Please see the curves for Fre-

quency Response for Various RF and RG and

Frequency Response for Various CINb

Feedback Resistor Values

The EL2170CEL2270CEL2470C have been de-

signed and specified at gains of a1 and a2 with

RF e 1kX This value of feedback resistor gives

70 MHz of b3 dB bandwidth at AV ea1 with

about 15 dB of peaking and 60 MHz of b3dB

bandwidth at AV ea2 with about 05 dB of

peaking Since the EL2170CEL2270CEL2470C

are current-feedback amplifiers it is also possible

to change the value of RF to get more bandwidth

As seen in the curve of Frequency Response For

Various RF and RG bandwidth and peaking can

be easily modified by varying the value of the

feedback resistor

Because the EL2170CEL2270CEL2470C are

current-feedback

amplifiers

their

gain-band-

width product is not a constant for different

closed-loop gains This feature actually allows

the EL2170CEL2270CEL2470C to maintain

about the same b3 dB bandwidth regardless of

closed-loop gain However as closed-loop gain is

increased bandwidth decreases slightly while sta-

bility increases Since the loop stability is im-

proving with higher closed-loop gains it becomes

possible to reduce the value of RF below the spec-

ified 1 k

X and still retain stability resulting in

only a slight loss of bandwidth with increased

closed-loop gain

Supply Voltage Range and Single-

Supply Operation

The EL2170CEL2270CEL2470C have been de-

signed to operate with supply voltages having a

span of greater than 3V and less than 12V In

practical terms this means that the EL2170C

EL2270CEL2470C will operate on dual supplies

ranging from g15V to g6V With a single-sup-

ply the EL2170CEL2270CEL2470C will oper-

ate from a3V to a12V

As supply voltages continue to decrease it be-

comes necessary to provide input and output

voltage ranges that can get as close as possible to

the supply voltages

The EL2170CEL2270C

EL2470C have an input voltage range that ex-

tends to within 1V of either supply So for exam-

ple

on a single a5V supply

the EL2170C

EL2270CEL2470C have an input range which

spans from 1V to 4V The output range of the

EL2170CEL2270CEL2470C is also quite large

extending to within 1V of the supply rail On a

5V supply the output is therefore capable of

swinging from b4V to a4V Single-supply out-

put range is even larger because of the increased

negative swing due to the external pull-down re-

sistor to ground On a single a5V supply output

voltage range is about 03V to 4V

Video Performance

For good video performance an amplifier is re-

quired to maintain the same output impedance

and the same frequency response as DC levels are

changed at the output This is especially difficult

when driving a standard video load of 150

X be-

cause of the change in output current with DC

level

Until the EL2170CEL2270CEL2470C

good Differential Gain could only be achieved by

running high idle currents through the output

transistors (to reduce variations in output imped-

ance) These currents were typically more than

the entire 1 mA supply current of each EL2170C

EL2270CEL2470C amplifier

Special circuitry

has

been

incorporated

the

EL2170C

EL2270CEL2470C to reduce the variation of

output impedance with current output This re-

sults in dG and dP specifications of 015% and

015 while driving 150

X at a gain of a2

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