EL2276CSZ datasheet(10/14 Pages) RENESAS

10 / 14 page

E L2276

FN7054 Rev 1.00

Page 10 of 14

May 6, 2005

Feedback Resistor Values

The EL2276 has been designed and specified at gains of +1

and +2 with RF = 1.0k. This value of feedback resistor

gives 70MHz of -3dB bandwidth at AV = +1 with about 1.5dB

of peaking, and 60MHz of -3dB bandwidth at AV = +2 with

about 0.5dB of peaking. Since the EL2276 is current-

feedback amplifier, it is also possible to change the value of

RF to get more bandwidth. As seen in the curve of

Frequency Response For Various RF and RG, bandwidth

and peaking can be easily modified by varying the value of

the feedback resistor.

Because the EL2276 is a current-feedback amplifier, the

gain-bandwidth product is not a constant for different closed-

loop gains. This feature actually allows the EL2276 to

maintain about the same -3dB bandwidth, regardless of

closed-loop gain. However, as closed-loop gain is increased,

bandwidth decreases slightly while stability increases.

Since the loop stability is improving with higher closed-loop

gains, it becomes possible to reduce the value of RF below

the specified 1.0k

 and still retain stability, resulting in only a

slight loss of bandwidth with increased closed-loop gain.

Supply Voltage Range and Single-Supply

Operation

The EL2276 has been designed to operate with supply

voltages having a span of greater than 3V, and less than

12V. In practical terms, this means that the EL2276 will

operate on dual supplies ranging from ±1.5V to ±6V.

As supply voltages continue to decrease, it becomes

necessary to provide input and output voltage ranges that

can get as close as possible to the supply voltages. The

EL2276 has an input voltage range that extends to within 1V

of either supply. So, for example, on a single +5V supply, the

EL2276 has an input range which spans from 1V to 4V. The

output range of the EL2276 is also quite large, extending to

within 1V of the supply rail. On a ±5V supply, the output is

therefore capable of swinging from -4V to +4V. Single-

supply output range is even larger because of the increased

negative swing due to the external pull-down resistor to

ground. On a single +5V supply, output voltage range is

about 0.3V to 4V.

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same

frequency response as DC levels are changed at the output.

This is especially difficult when driving a standard video load

of 150

, because of the change in output current with DC

level. Until the EL2276, good Differential Gain could only be

achieved by running high idle currents through the output

transistors (to reduce variations in output impedance).

These currents were typically in excess of the entire 1mA

supply current of the EL2276 amplifier! Special circuitry has

been incorporated in the EL2276 to reduce the variation of

output impedance with current output. This results in dG and

dP specifications of 0.15% and 0.15° while driving 150

 at a

gain of +2.

Video Performance has also been measured with a 500



load at a gain of +1. Under these conditions, the EL2276 has

dG and dP specifications of 0.01% and 0.02° respectively

while driving 500

 at AV = +1.

Output Drive Capability

Each amplifier of the EL2276 is capable of providing a

minimum of ±50mA. These output drive levels are

unprecedented in amplifiers running at these supply

currents. The ±50mA minimum output drive of each EL2276

amplifier allows swings of ±2.5V into 50

 loads.

Driving Cables and Capacitive Loads

When used as a cable driver, double termination is always

recommended for reflection-free performance. For those

applications, the back-termination series resistor will

decouple the EL2276 from the cable and allow extensive

capacitive drive. However, other applications may have high

capacitive loads without a back-termination resistor. In these

applications, a small series resistor (usually between 5

 and

) can be placed in series with the output to eliminate

most peaking. The gain resistor (RG) can then be chosen to

make up for any gain loss which may be created by this

additional resistor at the output. In many cases it is also

possible to simply increase the value of the feedback

resistor (RF) to reduce the peaking.

Current Limiting

The EL2276 has no internal current-limiting circuitry. If any

output is shorted, it is possible to exceed the Absolute

Maximum Ratings for output current or power dissipation,

potentially resulting in the destruction of the device.

Power Dissipation

With the high output drive capability of the EL2276, it is

possible to exceed the 150°C Absolute Maximum junction

temperature under certain very high load current conditions.

Generally speaking, when RL falls below about 25, it is

important to calculate the maximum junction temperature

(TJmax) for the application to determine if power-supply

voltages, load conditions, or package type need to be

modified for the EL2276 to remain in the safe operating area.

These parameters are calculated as follows:

TJMAX = TMAX + (JA * n * PDMAX)

where:

TMAX=Maximum Ambient Temperature

JA =Thermal Resistance of the Package

n=Number of Amplifiers in the Package

PDMAX=Maximum Power Dissipation of each Amplifier in

the Package

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