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EL4450CS

Wideband Four-Quadrant Multiplier

Elantec -> Intersil 

EL4450C

Wideband Four-Quadrant Multiplier

Applications Information

The EL4450 is a complete four-quadrant multiplier with

90 MHz bandwidth. It has three sets of inputs; a differ-

ential multiplying X-input, a differential multiplying Y-

input, and another differential input which is used to

complete a feedback loop with the output. Here is a typ-

ical connection:

Figure 1.

The gain of the feedback divider is H, and H = RG/(RG +

RF). The transfer function of the part is

VOUT = AO × (1/2 × ((VINX+–VINX-) × (VINY+–VINY-)) + (VREF–VFB)).

VFB is connected to VOUT through a feedback network,

so VFB = H*VOUT. AO is the open-loop gain of the

amplifier, and is about 600. The large value of AO drives

(1/2 × ((VINX+–VINX-) × (VINY+–VINY-)) + (VREF–VFB))→0.

Rearranging and substituting for VREF

VOUT = (1/2 × ((VINX+–VINX-) × (VINY+–VINY-)) +VREF)/H, or

VOUT = (XY/2 + VREF)/H

Thus the output is equal to one-half the product of X and

Y inputs and offset by VREF, all gained up by the feed-

back divider ratio. The EL4450 is stable for a direct

connection between VOUT and FB, and the feedback

divider may be used for higher output gain, although

with the traditional loss of bandwidth.

It is important to keep the feedback divider’s impedance

at the FB terminal low so that stray capacitance does not

diminish the loop’s phase margin. The pole caused by

the parallel impedance of the feedback resistors and

stray capacitance should be at least 150 MHz; typical

strays

of 3 pF thus require a feedback impedance of 360¾ or

less, Alternatively, a small capacitor across RF can be

used to create more of a frequency-compensated divider.

The value of the capacitor should scale with the parasitic

capacitance at the FB input. It is also practical to place

small capacitors across both the feedback resistors

(whose values maintain the desired gain) to swamp out

parasitics. For instance, two 10 pF capacitors across

equal divider resistors for a maximum gain of 1 will

dominate parasitic effects and allow a higher divider

resistance.

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

ence, or for DC offsetting of the output, or it can be used

to sum in another signal.

Input Connections

The input transistors can be driven from resistive and

capacitive sources, but are capable of oscillation when

presented with an inductive input. It takes about 80 nH

of series inductance to make the inputs actually oscillate,

equivalent to four inches of unshielded wiring or about 6

of unterminated input transmission line. The oscillation

has a characteristic frequency of 500 MHz. Placing

one’s finger (via a metal probe) or an oscilloscope probe

on the input will kill the oscillation. Normal high-fre-

quency construction obviates any such problems, where

the input source is reasonably close to the input. If this is

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