EL4452CS datasheet(9/12 Pages) ELANTEC

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EL4452C

Wideband Variable-Gain Amplifier with Gain of 10

Applications Information

Contd

and outputs will have to have their levels shifted

above ground to accommodate the lack of nega-

tive supply

The power dissipation of the EL4452 increases

with power supply voltage and this must be

compatible with the package chosen This is a

close estimate for the dissipation of a circuit

PDe2cVScIS maxa(VSbVO)cVO RPAR

where IS max is the maximum supply current

VS is the g supply voltage (assumed

equal)

VO is the output voltage

RPAR is the parallel of all resistors loading

the output

For instance the EL4452 draws a maximum of

18mA With light loading RPAR

x% and the

dissipation with g5V supplies is 180 mW The

maximum supply voltage that the device can run

on for a given PD and other parameters is

VS maxe(PDaVO2 RPAR)(2ISaVO RPAR)

The maximum dissipation a package can offer is

PD max e (TJ maxbTA max)

iJA

Where

TJ max is the maximum die tempera-

ture 150 C for reliability less to re-

tain optimum electrical performance

TA max is the ambient temperature

70 C for commercial and 85 C for in-

dustrial range

iJA is the thermal resistance of the

mounted

package

obtained

from

data sheet dissipation curves

The more difficult case is the SO-14 package

With a maximum die temperature of 150 C and a

maximum ambient temperature of 85 C the 65 C

temperature rise and package thermal resistance

of 120 CW gives a dissipation of 542 mW at

85 C This allows the full maximum operating

supply voltage unloaded but reduced if loaded

Output Loading

The output stage of the EL4452 is very powerful

It can typically source 80 mA and sink 120 mA

Of course this is too much current to sustain and

the part will eventually be destroyed by excessive

dissipation or by metal traces on the die opening

The metal traces are completely reliable while de-

livering the 30 mA continuous output given in

the Absolute Maximum Ratings table in this

data sheet or higher purely transient currents

Gain changes only 02% from no load to a 100

load Heavy resistive loading will degrade fre-

quency response and distortion for loads k100

Capacitive loads will cause peaking in the fre-

quency response If capacitive loads must be driv-

en a small-valued series resistor can be used to

isolate it 12

X to 51X should suffice A 22X series

resistor will limit peaking to 1 dB with even a

220 pF load

AGC Circuits

The basic AGC (automatic gain control) loop is

this

4452 – 24

Basic AGC Loop

A multiplier scales the input signal and provides

necessary gain and buffers the signal presented

to the output load a level detector (shown sche-

matically here as a diode) converts some measure

of the output signal amplitude to a DC level a

low-pass filter attenuates any signal ripple pres-

ent on that DC level and an amplifier compares

that level to a reference and amplifies the error to

create a gain-control voltage for the multiplier

The circuitry is a servo that attempts to keep the

output amplitude constant by continuously ad-

justing the multiplier’s gain control input

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