EL4452CS datasheet(11/12 Pages) ELANTEC

11 / 12 page

EL4452C

Wideband Variable-Gain Amplifier with Gain of 10

Applications Information

Contd

The output of the EL4452 drives a diode detector

which is compared to VREF by an offset integra-

tor Its output feeds the gain-control input of the

EL4452 The integrator’s output is attenuated by

the2k

X and 27 kX resistors to prevent the op-

amp from overloading the gain-control pin dur-

ing zero input conditions The 510 k

X resistor

provides a pull-down current to the peak level

storage capacitor C1 to allow it to drift negative

when output amplitude reduces Thus the detec-

tor is of fast attack and slow decay design able to

reduce AGC gain rapidly when signal amplitude

suddenly increases

and increases gain slowly

when the input drops out momentarily The val-

ue of C1 determines drop-out reaction rates and

the value of CF affects overall loop time constant

as well as the amount of ripple on the gain-con-

trol line C2 can be used to reduce this ripple fur-

ther although it contributes to loop overshoot

when input amplitude changes suddenly The op-

amp can be any inexpensive low-frequency type

The major problem with diode detectors is their

large and variable forward voltage They require

at leasta2VP-P peak output signal to function

reliably and the forward voltage should be com-

pensated by including a negative VD added to

VREF Even this is only moderately successful

At the expense of bandwidth op-amp circuits can

greatly improve diode rectifiers (see ‘‘An Im-

proved Peak Detector’’ an Elantec application

note) Fortunately the detector will see a con-

stant amplitude of signal if the AGC is operating

correctly

A better-calibrated method is to use a four-quad-

rant multiplier as a square-law detector Here is a

circuit employing the EL4450

4452 – 26

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