EL5197C, EL5197AC
Single 200MHz Fixed Gain Amplifier with Enable
Feedback Resistor Values
Video Performance
The EL5197C has been designed and specified at a gain
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 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 150Ω at a gain
of 2.
of +2 with R approximately 500Ω. This value of feed-
F
back resistor gives 200MHz of -3dB bandwidth at A =2
V
with 2dB of peaking. With A =-2, an R of approxi-
V
F
mately 500Ω gives 175MHz of bandwidth with 0.2dB of
peaking. Since the EL5197C is a current-feedback
amplifier, it is also possible to change the value of R to
F
get more bandwidth. As seen in the curve of Frequency
Response for Various R and R , bandwidth and peak-
F
G
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
Video performance has also been measured with a 500Ω
load at a gain of +1. Under these conditions, the
EL5197C has dG and dP specifications of 0.03% and
0.04°.
reduce the value of R below the specified 475Ω and
F
still retain stability, resulting in only a slight loss of
bandwidth with increased closed-loop gain.
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 50Ω loads to both
rails, making it an excellent choice for driving isolation
transformers in telecommunications applications.
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.
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 5Ω and 50Ω) can be placed in series
with the output to eliminate most peaking. The gain
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
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.
resistor (R ) can then be chosen to make up for any gain
G
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 (R ) to reduce
F
the peaking.
13
ELANTEC [ ELANTEC SEMICONDUCTOR ]
