EL2386CS 데이터 시트보기 (PDF) - Elantec -> Intersil

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EL2386CS

250 MHz Triple Current Feedback Amp w/Disable

Elantec -> Intersil 

EL2386C

250 MHz Triple Current Feedback Amp w Disable

Applications Information Contd

Capacitance at the Inverting Input

Any manufacturer’s high-speed voltage- or cur-

rent-feedback amplifier can be affected by stray

capacitance at the inverting input For inverting

gains this parasitic capacitance has little effect

because the inverting input is a virtual ground

but for non-inverting gains this capacitance (in

conjunction with the feedback and gain resistors)

creates a pole in the feedback path of the amplifi-

er This pole if low enough in frequency has the

same destabilizing effect as a zero in the forward

open-loop response The use of large value feed-

back and gain resistors further exacerbates the

problem by further lowering the pole frequency

The experienced user with a large amount of PC

board layout experience may find in rare cases

that the EL2386C has less bandwidth than ex-

pected In this case the inverting input may have

less parasitic capacitance than expected The re-

duction of feedback resistor values (or the addi-

tion of a very small amount of external capaci-

tance at the inverting input e g 0 5 pF) will

increase bandwidth as desired Please see the

curves for Frequency Response for Various RF

and RG and Frequency Response for Various

CIN-

Feedback Resistor Values

The EL2386C has been designed and specified at

gains of a1 and a2 with RF e 750X This value

of feedback resistor gives 250 MHz of b3 dB

bandwidth at AV e a1 with about 2 5 dB of

peaking and 180 MHz of b3 dB bandwidth at

AV e a2 with about 0 1 dB of peaking Since

the EL2386C is a current-feedback amplifier it is

also possible to change the value of RF to get

more bandwidth As seen in the curve of Fre-

quency Response For Various RF and RG band-

width and peaking can be easily modified by

varying the value of the feedback resistor

Because the EL2386C is a current-feedback am-

plifier its gain-bandwidth product is not a con-

stant for different closed-loop gains This feature

actually allows the EL2386C to maintain about

the same b3 dB bandwidth regardless of closed-

loop gain However as closed-loop gain is in-

creased 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 750X 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 EL2386C has 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 EL2386C will operate on dual supplies

ranging from g1 5V to g6V With a single-sup-

ply the EL2386C will operate 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 EL2386C has an input

voltage range that extends to within 1V of either

supply So for example on a single a5V supply

the EL2386C has an input range which spans

from 1V to 4V The output range of the EL2386C

is also quite large extending to within 1V of the

supply rail On a g5V supply the output is

therefore capable of swinging from b4V to a4V

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

a5V supply output voltage range is about 0 3V

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 150X be-

cause of the change in output current with DC

level Until the EL2386C good Differential Gain

could only be achieved by running high idle cur-

rents through the output transistors (to reduce

variations in output impedance) These currents

were typically comparable to the entire 3 mA

supply current of each EL2386C amplifier Spe-

10

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