EL4451 Просмотр технического описания (PDF) - Intersil

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EL4451

Wideband Variable-Gain Amplifier, Gain of 2

Intersil 

EL4451

package chosen. This is a close estimate for the dissipation

of a circuit:

PD = 2 × VS × IS, max + (VS - VO) × VO/RPAR

where

IS, max is the maximum supply current

VS is the ± supply voltage (assumed equal)

VO is the output voltage

RPAR is the parallel of all resistors loading the output

For instance, the EL4451 draws a maximum of 18mA. With

light loading, RPAR →∞ and the dissipation with ±5V

supplies is 180mW. The maximum supply voltage that the

device can run on for a given PD and other parameters is:

VS, max = (PD + VO2/RPAR) / (2IS + VO/RPAR)

The maximum dissipation a package can offer is:

PD, max = (TJ, max-TA, max) / θJA

Where

TJ,max is the maximum die temperature, 150°C for

reliability, less to retain optimum electrical performance

TA,max is the ambient temperature, 70°C for commercial

and 85°C for industrial range

θJAis 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°C/W gives a dissipation

of 542mW at 85°C. This allows the full maximum operating

supply voltage unloaded, but reduced if loaded.

Output Loading

The output stage of the EL4451 is very powerful. It typically

can source 80mA and sink 120mA. 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

delivering the 30mA continuous output given in the Absolute

Maximum Ratings table in this data sheet, or higher purely

transient currents.

Gain changes only 0.2% from no load to 100Ω load. Heavy

resistive loading will degrade frequency response and video

distortion for loads < 100Ω.

Capacitive loads will cause peaking in the frequency

response. If capacitive loads must be driven, a small-valued

series resistor can be used to isolate it. 12Ω to 51Ω should

suffice. A 22Ω series resistor will limit peaking to 2.5dB with

even a 220pF load.

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 unterminated cable will yield a twice-

sized standard video amplitude, and an erroneously twice-

terminated cable gives a 2/3-sized input.

Here is a ±6dB range preamplifier:

FIGURE 1. LINEARIZED LEVELING AMPLIFIER

In this arrangement, the EL4451 outputs a mixture of the

signal routed through the multiplier and the REF terminal.

The multiplier port produces 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

RF RG R1 R2 R3 BANDWIDTH

1.5

200Ω 400Ω 300Ω 100Ω 200Ω 47MHz

2

400Ω 400Ω 500Ω 100Ω 200Ω 28MHz

8

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