EL2180CEL2280CEL2480C
250 MHz3 mA Current Mode Feedback Amplifiers
Applications Information
Contd
The experienced user with a large amount of PC
board layout experience may find in rare cases
that the EL2180CEL2280CEL2480C have less
bandwidth than expected
The reduction of feedback resistor values (or the
addition of a very small amount of external ca-
pacitance at the inverting input eg 05 pF) will
increase bandwidth as desired
Please see the
curves for Frequency Response for Various RF
and RG and Frequency Response for Various
CINb
Feedback Resistor Values
The EL2180CEL2280CEL2480C have been de-
signed and specified at gains of a1 and a2 with
RF e 750X in PDIP and SOIC packages and RF
e
560
X in SOT23-5 package These values of
feedback resistors give 250 MHz of b3 dB band-
width at AV ea1 with about 25 dB of peaking
and 180 MHz of b3 dB bandwidth at AV ea2
with about 01 dB of peaking The SOT23-5 pack-
age is characterized with a smaller value of feed-
back resistor for a given bandwidth to compen-
sate for lower parasitics within both the package
itself and the printed circuit board where it will
be
placed
Since
the
EL2180CEL2280C
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 Fre-
quency Response For Various RF and RG band-
width and peaking can be easily modified by
varying the value of the feedback resistor
Because the EL2180CEL2280CEL2480C are
current-feedback
amplifiers
their
gain-band-
width product is not a constant for different
closed-loop gains This feature actually allows
the EL2180CEL2280CEL2480C to maintain
about the same b3 dB bandwidth regardless of
closed-loop gain However as closed-loop gain is
increased bandwidth decreases slightly while sta-
bility increases Since the loop stability is im-
proving with higher closed-loop gains it becomes
possible to reduce the value of RF below the spec-
ified 560
X and 750X and still retain stability re-
sulting in only a slight loss of bandwidth with
increased closed-loop gain
Supply Voltage Range and Single-
Supply Operation
The EL2180CEL2280CEL2480C have been de-
signed to operate with supply voltages having a
span of greater than 3V and less than 12V In
practical terms this means that the EL2180C
EL2280CEL2480C will operate on dual supplies
ranging from g15V to g6V With a single-sup-
ply the EL2180CEL2280CEL2480C will oper-
ate from a3V to a12V
As supply voltages continue to decrease it be-
comes necessary to provide input and output
voltage ranges that can get as close as possible to
the supply voltages
The EL2180CEL2280C
EL2480C have an input voltage range that ex-
tends to within 1V of either supply So for exam-
ple
on a single a5V supply
the EL2180C
EL2280CEL2480C have an input range which
spans from 1V to 4V The output range of the
EL2180CEL2280CEL2480C is also quite large
extending to within 1V of the supply rail On a
g
5V supply the output is therefore capable of
swinging from b4V to a4V Single-supply out-
put range is even larger because of the increased
negative swing due to the external pull-down re-
sistor to ground On a single a5V supply output
voltage range is about 03V to 4V
Video Performance
For good video performance an amplifier is re-
quired 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
X be-
cause of the change in output current with DC
level
Until the EL2180CEL2280CEL2480C
good Differential Gain could only be achieved by
running high idle currents through the output
transistors (to reduce variations in output imped-
ance) These currents were typically comparable
to the entire 3 mA supply current of each
EL2180CEL2280CEL2480C amplifier Special
circuitry has been incorporated in the EL2180C
EL2280CEL2480C to reduce the variation of
output impedance with current output This re-
sults in dG and dP specifications of 005% and
005 while driving 150
X at a gain of a2
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