EL2180C/EL2280C/EL2480C-Preliminary
250MHz / 3mA Current Mode Feedback Amplifiers
example, on a single +5V supply, the EL2180C/
EL2280C/EL2480C have an input range which spans
from 1V to 4V. The output range of the
EL2180C/EL2280C/EL2480C is also quite large,
extending to within 1V of the supply rail. On a ±5V sup-
ply, 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 exter-
nal pull-down resistor to ground. On a single +5V
supply, output voltage range is about 0.3V to 4V.
Feedback Resistor Values
The EL2180C/EL2280C/EL2480C have been designed
and specified at gains of +1 and +2 with RF = 750W in
PDIP and SO packages and RF = 560W in 5-pin SOT23
package. These values of feedback resistors give
250MHz of -3dB bandwidth at AV = +1 with about
2.5dB of peaking, and 180MHz of -3dB bandwidth at
AV = +2 with about 0.1dB of peaking. The 5-pin SOT23
package is characterized with a smaller value of feed-
back resistor, for a given bandwidth, to compensate for
lower parasitics within both the package itself and the
printed circuit board where it will be placed. Since the
EL2180C/EL2280C/EL2480C are current-feedback
amplifiers, 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 peak-
ing can be easily modified by varying the value of the
feedback resistor.
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 150W, because of the change in output current
with DC level. Until the EL2180C/EL2280C/EL2480C,
good Differential Gain could only be achieved by run-
ning high idle currents through the output transistors (to
reduce variations in output impedance). These currents
were typically comparable to the entire 3mA supply cur-
rent of each EL2180C/EL2280C/EL2480C amplifier!
Special circuitry has been incorporated in the
EL2180C/EL2280C/EL2480C to reduce the variation of
output impedance with current output. This results in dG
and dP specifications of 0.05% and 0.05° while driving
150W at a gain of +2.
Because the EL2180C/EL2280C/EL2480C are current-
feedback amplifiers, their gain-bandwidth product is not
a constant for different closed-loop gains. This feature
actually allows the EL2180C/EL2280C/EL2480C 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 560W and 750W 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 500W
load at a gain of +1. Under these conditions, the
EL2180C/EL2280C/EL2480C have dG and dP specifi-
cations of 0.01% and 0.01° respectively while driving
500W at AV = +1.
Supply Voltage Range and Single-Supply
Operation
The EL2180C/EL2280C/EL2480C have 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 EL2180C/EL2280C/EL2480C will oper-
ate on dual supplies ranging from ±1.5V to ±6V. With a
single-supply, the EL2180C/EL2280C/EL2480C will
operate from +3V to +12V.
Output Drive Capability
In spite of its low 3mA of supply current, the EL2180C
is capable of providing a minimum of ±80mA of output
current. Similarly, each amplifier of the EL2280C and
the EL2480C is capable of providing a minimum of
±50mA. These output drive levels are unprecedented in
amplifiers running at these supply currents. With a min-
imum ±80mA of output drive, the EL2180C is capable
of driving 50W loads to ±4V, making it an excellent
choice for driving isolation transformers in telecommu-
nications applications. Similarly, the ±50mA minimum
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
EL2180C/EL2280C/EL2480C have an input voltage
range that extends to within 1V of either supply. So, for
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