EL2186CS datasheet(11/19 Pages) ELANTEC

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EL2186C, EL2286C

250MHz/3mA Current Mode Feedback Amp w/Disable

resistors further exacerbates the problem by further low-

ering the pole frequency.

The EL2186C/EL2286C have been specially designed

to reduce power dissipation in the feedback network by

using large 750

Ω feedback and gain resistors. With the

high bandwidths of these amplifiers, these large resistor

values would normally cause stability problems when

combined with parasitic capacitance, but by internally

canceling the effects of a nominal amount of parasitic

capacitance, the EL2186C/EL2286C remain very stable.

For less experienced users, this feature makes the

EL2186C/EL2286C much more forgiving, and therefore

easier to use than other products not incorporating this

proprietary circuitry.

The experienced user with a large amount of PC board

layout experience may find in rare cases that the

EL2186C/EL2286C have less bandwidth than expected.

In this case, the inverting input may have less parasitic

capacitance than expected

by the internal compensation circuitry of the

EL2186C/EL2286C. The reduction of feedback resistor

values (or the addition of a very small amount of exter-

nal capacitance at the inverting input, e.g. 0.5pF) will

increase bandwidth as desired. Please see the curves for

Frequency Response for Various RF and RG, and Fre-

quency Response for Various CIN-.

Feedback Resistor Values

The EL2186C/EL2286C have been designed and speci-

fied at gains of +1 and +2 with RF = 750Ω. This value of

feedback resistor gives 250MHz of -3dB bandwidth at

AV = +1 with about 2.5dB of peaking, and 180MHz of -

3dB bandwidth at AV = +2 with about 0.1dB of peaking.

Since the EL2186C/EL2286C 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 peak-

ing can be easily modified by varying the value of the

feedback resistor.

Because the EL2186C/EL2286C are current-feedback

amplifiers, their gain-bandwidth product is not a con-

stant for different closed-loop gains. This feature

actually allows the EL2186C/EL2286C to maintain

about the same -3dB bandwidth, regardless of closed-

loop gain. However, as closed-loop gain is increased,

bandwidth decreases slightly while stability increases.

Since the loop stability is improving with higher closed-

loop gains, it becomes possible to reduce the value of RF

below the specified 750

Ω and still retain stability, result-

ing in only a slight loss of bandwidth with increased

closed-loop gain.

Supply Voltage Range and Single-Supply

Operation

The EL2186C/EL2286C have been designed to operate

with supply voltages having a span of greater than 3V,

and less than 12V. In practical terms, this means that the

EL2186C/EL2286C will operate on dual supplies rang-

ing from ±1.5V to ±6V. With a single-supply, the

EL2176C will operate from +3V to +12V.

As supply voltages continue to decrease, it becomes nec-

essary to provide input and output voltage ranges that

can get as close as possible to the supply voltages. The

EL2186C/EL2286C have an input voltage range that

extends to within 1V of either supply. So, for example,

on a single +5V supply, the EL2186C/EL2286C have an

input range which spans from 1V to 4V. The output

range of the EL2186C/EL2286C is also quite large,

extending to within 1V of the supply rail. On a ±5V sup-

ply, the output is therefore capable of swinging from -

4V to +4V. Single-supply output range is even larger

because of the increased negative swing due to the exter-

nal pull-down resistor to ground. On a single +5V

supply, output voltage range is about 0.3V to 4V.

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same fre-

quency response as DC levels are changed at the output.

This is especially difficult when driving a standard video

load of 150

Ω, because of the change in output current

with DC level. Until the EL2186C/EL2286C, good Dif-

ferential Gain could only be achieved by running high

idle currents through the output transistors (to reduce

variations in output impedance). These currents were

typically comparable to the entire 3mA supply current of

each EL2186C/EL2286C amplifier! Special circuitry

has been incorporated in the EL2186C/EL2286C to

reduce the variation of output impedance with current

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