EL4451 查看數據表（PDF） - Intersil

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EL4451

Wideband Variable-Gain Amplifier, Gain of 2

Intersil 

EL4451 Leveler Circuit

Attenuation Ratio = 1.5

EL4451

Here is the EL4451 used as an oscillator with simple AGC:

EL4451 Leveler Circuit

Attenuation Ratio = 2

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 0.47% for the

attenuation ratio of 1.5, but only 0.27% with the gain of 2

constants. To maintain bandwidth, an external op amp can

be used instead of the RF - RG divider to boost the EL4451’s

output by the attenuation ratio.

Sinewave Oscillators

Generating a stable, low distortion sinewave has long been a

difficult task. Because a linear oscillator’s output tends to

grow or diminish continuously, either a clipping circuit or

automatic gain control (AGC) is needed. Clipping circuits

generate severe distortion which needs subsequent filtering,

and AGCs can be complicated.

FIGURE 2. LOW-DISTORTION SINEWAVE OSCILLATOR

The oscillation frequency is set by the resonance of a series-

tuned circuit, which may be an L-C combination or a crystal.

At resonance, the series impedance of the tuned circuit

drops and its phase lag is 0°, so the EL4451 needs a gain

just over unity to sustain oscillation. The VGAIN- terminal is

initially at -0.7V and the VGAIN+ terminal at about +2.1V,

setting the maximum gain in the EL4451. At such high gain,

the loop oscillates and output amplitude grows until D1

rectifies more positive voltage at VGAIN-, ultimately reducing

gain until a stable 0.5Vrms output is produced.

Using a 2MHz crystal, output distortion was -53dBc, or

0.22%. Sideband modulation was only 14Hz wide at -90dBc,

limited by the filter of the spectrum analyzer used.

The circuit works up to 30MHz. A parallel-tuned circuit can

replace the 510Ω resistor and the 510Ω resistor moved in

place of the series-tuned element to allow grounding of the

tuned components.

Filters

The EL4451 can be connected to act as a voltage-variable

integrator as shown:

EL4451 CONNECTED AS VARIABLE INTEGRATOR

9

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