EL4451 datasheet(9/12 Pages) ELANTEC

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EL4451C

Wideband Variable-Gain Amplifier Gain of 2

Applications Information

Contd

Leveling Circuits

Often a variable-gain control is used to normalize

an input signal to a standard amplitude from a

modest range of possible input amplitude A good

example is in video systems where an untermi-

nated cable will yield a twice-sized standard vid-

eo amplitude and an erroneously twice-terminat-

ed cable gives a 23-sized input

Here is a g6 dB range preamplifier

Linearized Leveling Amplifier

4451 – 30

In this arrangement the EL4451 outputs a mix-

ture of the signal routed through the multiplier

and the REF terminal The multiplier port pro-

duces the most distortion and needs to handle a

fraction of an oversized video input whereas the

REF port is just like an op-amp input summing

into the output Thus for oversized inputs the

gain will be decreased and the majority of the

signal is routed through the linear REF terminal

For undersized inputs the gain is increased and

the multiplier’s contribution added to the output

Here are some component values for two designs

Attenuation

3dB

Ratio

Bandwidth

200

X 400X 300X 100X 200X

47 MHz

400

X 400X 500X 100X 200X

28 MHz

EL4451 Leveler Circuit

Attenuation Ratio e 15

4451 – 31

EL4451 Leveler Circuit

Attenuation Ratio e 2

4451 – 32

With the higher attenuation ratio the multiplier

sees a smaller input amplitude and distorts less

however the higher output gain reduces circuit

bandwidth As seen in the next curves the peak

differential gain error is 047% for the attenua-

tion ratio of 15 but only 027% with the gain of

2 constants To maintain bandwidth an external

op amp can be used instead of the RF -RG divid-

er to boost the EL4451’s output by the attenua-

tion ratio

Sinewave Oscillators

Generating a stable low distortion sinewave has

long been a difficult task Because a linear oscil-

lator’s output tends to grow or diminish continu-

ously either a clipping circuit or automatic gain

control (AGC) is needed Clipping circuits gener-

ate severe distortion which needs subsequent fil-

tering and AGC’s can be complicated

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