8

package chosen. This is a close estimate for the dissipation

of a circuit:

where

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

supplies is 180mW. The maximum supply voltage that the

The maximum dissipation a package can offer is:

Where

reliability, less to retain optimum electrical performance

and 85°C for industrial range

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 100

Ω load. Heavy

resistive loading will degrade frequency response and video

distortion for loads < 100

Ω.

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. 12

Ω to 51Ω should

suffice. A 22

Ω 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:

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

RATIO

-3dB

BANDWIDTH

1.5

200

Ω 400Ω 300Ω 100Ω 200Ω

47MHz

2

400

Ω 400Ω 500Ω 100Ω 200Ω

28MHz

FIGURE 1. LINEARIZED LEVELING AMPLIFIER

EL4451