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EL4451

Wideband Variable-Gain Amplifier, Gain of 2

Elantec -> Intersil 

EL4451C

Wideband Variable-Gain Amplifier Gain of 2

Applications Information

The EL4451 is a complete two-quadrant multipli-

er gain control with 70 MHz bandwidth It has

three sets of inputs a differential signal input

VIN a differential gain-controlling input VGAIN

and another differential input which is used to

complete a feedback loop with the output Here is

a typical connection

360X or less Alternatively a small capacitor

across RF can be used to create more of a fre-

quency-compensated divider The value of the ca-

pacitor should scale with the parasitic capaci-

tance at the FB input It is also practical to place

small capacitors across both the feedback and the

gain resistors (whose values maintain the desired

gain) to swamp out parasitics For instance two

10pF capacitors across equal divider resistors for

a maximum gain of 4 will dominate parasitic ef-

fects and allow a higher divider resistance

The REF pin can be used as the output’s ground

reference for DC offsetting of the output or it

can be used to sum in another signal

4451-2

The gain of the feedback divider is

H e RG

RG a RF

The transfer function of the part is

VOUT e AO c (((VINa) b (VINb)) c ((VGAINa) b (VGAINb)) a

(VREF b VFB))

VFB is connected to VOUT through a feedback

network so VFB e H c VOUT AO is the open-

loop gain of the amplifier and is approximately

600 The large value of AO drives

((VINa) b (VINb)) c ((VGAINa) b (VGAINb)) a (VREF b VFB)

x0

Rearranging and substituting for VFB

VOUT e (((VINa) b (VINb)) c ((VGAINa) b (VGAIN)) a VREF) H

or

VOUT e (VIN c VGAIN a VREF) H

Thus the output is equal to the difference of the

VIN’s times the difference of VGAIN’S and offset

by VREF all gained up by the feedback divider

ratio The EL4451 is stable for a direct connec-

tion between VOUT and FB and the divider may

be used for higher output gain although with the

traditional loss of bandwidth

It is important to keep the feedback divider’s im-

pedance at the FB terminal low so that stray ca-

pacitance does not diminish the loop’s phase

margin The pole caused by the parallel imped-

ance of the feedback resistors and stray capaci-

tance should be at least 150 MHz typical strays

of 3 pF thus require a feedback impedance of

Gain-Control Characteristics

The quantity VGAIN in the above equations is

bounded as 0sVGAINs2 even though the exter-

nally applied voltages exceed this range Actual-

ly the gain transfer function around 0 and 2V is

‘‘soft’’ that is the gain does not clip abruptly

below the 0%-VGAIN voltage nor above the

100%-VGAIN level An overdrive of 0 3V must be

applied to VGAIN to obtain truly 0% or 100%

Because the 0%- or 100%- VGAIN levels cannot

be precisely determined they are extrapolated

from two points measured inside the slope of the

gain transfer curve Generally an applied VGAIN

range of b0 5V to a2 5V will assure the full nu-

merical span of 0sVGAINs2

The gain control has a small-signal bandwidth

equal to the VIN channel bandwidth and over-

load recovery resolves in about 20 nsec

Input Connections

The input transistors can be driven from resistive

and capacitive sources but are capable of oscilla-

tion when presented with an inductive input It

takes about 80nH of series inductance to make

the inputs actually oscillate equivalent to four

inches of unshielded wiring or 6 of unterminat-

ed input transmission line The oscillation has a

characteristic frequency of 500 MHz Often plac-

ing one’s finger (via a metal probe) or an oscillo-

scope probe on the input will kill the oscillation

Normal high-frequency construction obviates

any such problems where the input source is rea-

sonably close to the input If this is not possible

one can insert series resistors of around 51X to

de-Q the inputs

7

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