EL5191C
1GHz Current Feedback Amplifier
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 250Ω and
still retain stability, resulting in only a slight loss of
bandwidth with increased closed-loop gain.
Video performance has also been measured with a 500Ω
load at a gain of +1. Under these conditions, the
EL5191C has dG and dP specifications of 0.02% and
0.02°, respectively.
Output Drive Capability
Supply Voltage Range and Single-Supply
Operation
In spite of its low 9mA of supply current, the EL5191C
is capable of providing a minimum of ±95mA of output
current. With a minimum of ±95mA of output drive, the
EL5191C is capable of driving 50Ω loads to both rails,
making it an excellent choice for driving isolation trans-
formers in telecommunications applications.
The EL5191C 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 EL5191C
will operate on dual supplies ranging from ±2.5V to
±5V. With single-supply, the EL5191C will operate
from 5V to 10V.
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 EL5191C 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 5Ω and 50Ω) 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.
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
EL5191C has an input range which extends to within 2V
of either supply. So, for example, on ±5V supplies, the
EL5191C has an input range which spans ±3V. The out-
put range of the EL5191C 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 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 150Ω, 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 9mA supply current of each EL5191C
amplifier. Special circuitry has been incorporated in the
EL5191C to reduce the variation of output impedance
with current output. This results in dG and dP specifica-
tions of 0.035% and 0.04°, while driving 150Ω at a gain
of 2.
Current Limiting
The EL5191C has no internal current-limiting circuitry.
If the output is shorted, it is possible to exceed the Abso-
lute Maximum Rating for output current or power
dissipation, potentially resulting in the destruction of the
device.
Power Dissipation
With the high output drive capability of the EL5191C, it
is possible to exceed the 125°C Absolute Maximum
junction temperature under certain very high load cur-
rent conditions. Generally speaking when RL falls below
about 25Ω, 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 EL5191C to
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ELANTEC [ ELANTEC SEMICONDUCTOR ]
