13

EL5197C, EL5197AC

Single 200MHz Fixed Gain Amplifier with Enable

Feedback Resistor Values

The EL5197C has been designed and specified at a gain

Ω. This value of feed-

mately 500

Ω gives 175MHz of bandwidth with 0.2dB of

peaking. Since the EL5197C is a current-feedback

get more bandwidth. As seen in the curve of Frequency

ing can be easily modified by varying the value of the

feedback resistor.

Because the EL5197C is a current-feedback amplifier,

its gain-bandwidth product is not a constant for different

closed-loop gains. This feature actually allows the

EL5197C to maintain about the same -3dB bandwidth.

As gain is increased, bandwidth decreases slightly while

stability increases. Since the loop stability is improving

with higher closed-loop gains, it becomes possible to

Ω 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 EL5197C has been designed to operate with supply

voltages having a span of greater than 5V and less than

10V. In practical terms, this means that the EL5197C

will operate on dual supplies ranging from ±2.5V to

±5V. With single-supply, the EL5197C will operate

from 5V to 10V.

As supply voltages continue to decrease, it becomes nec-

essary to provide input and output voltage ranges that

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

EL5197C has an input range which extends to within 2V

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

EL5197C has an input range which spans ±3V. The out-

put range of the EL5197C is also quite large, extending

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

+4V. Single-supply output range is larger because of the

increased negative swing due to the external pull-down

resistor to ground.

Video Performance

For good video performance, an amplifier is required to

maintain the same output impedance and the same fre-

quency response as DC levels are changed at the output.

This is especially difficult when driving a standard video

load of 150

Ω, because of the change in output current

with DC level. Previously, 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 comparable

to the entire 4mA supply current of each EL5197C

amplifier. Special circuitry has been incorporated in the

EL5197C to reduce the variation of output impedance

with current output. This results in dG and dP specifica-

tions of 0.03% and 0.04°, while driving 150

Ω at a gain

of 2.

Video performance has also been measured with a 500

Ω

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

EL5197C has dG and dP specifications of 0.03% and

0.04°.

Output Drive Capability

In spite of its low 4mA of supply current, the EL5197C

is capable of providing a minimum of ±120mA of output

current. With a minimum of ±120mA of output drive,

the EL5197C is capable of driving 50

Ω loads to both

rails, making it an excellent choice for driving isolation

transformers in telecommunications applications.

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 resis-

tor will decouple the EL5197C from the cable and allow

extensive capacitive drive. However, other applications

may have high capacitive loads without a back-termina-

tion resistor. In these applications, a small series resistor

(usually between 5

Ω and 50Ω) can be placed in series

with the output to eliminate most peaking. The gain

loss which may be created by this additional resistor at

the output. In many cases it is also possible to simply

the peaking.