EL2186C, EL2286C
250MHz/3mA Current Mode Feedback Amp w/Disable
resistors further exacerbates the problem by further low-
ering the pole frequency.
loop gain. However, as closed-loop gain is increased,
bandwidth decreases slightly while stability increases.
The EL2186C/EL2286C have been specially designed
to reduce power dissipation in the feedback network by
using large 750W feedback and gain resistors. With the
high bandwidths of these amplifiers, these large resistor
values would normally cause stability problems when
combined with parasitic capacitance, but by internally
canceling the effects of a nominal amount of parasitic
capacitance, the EL2186C/EL2286C remain very stable.
For less experienced users, this feature makes the
EL2186C/EL2286C much more forgiving, and therefore
easier to use than other products not incorporating this
proprietary circuitry.
Since the loop stability is improving with higher closed-
loop gains, it becomes possible to reduce the value of RF
below the specified 750W and still retain stability, result-
ing in only a slight loss of bandwidth with increased
closed-loop gain.
Supply Voltage Range and Single-Supply
Operation
The EL2186C/EL2286C 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
EL2186C/EL2286C will operate on dual supplies rang-
ing from ±1.5V to ±6V. With a single-supply, the
EL2176C will operate from +3V to +12V.
The experienced user with a large amount of PC board
layout experience may find in rare cases that the
EL2186C/EL2286C have less bandwidth than expected.
In this case, the inverting input may have less parasitic
capacitance than expected
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
EL2186C/EL2286C have an input voltage range that
extends to within 1V of either supply. So, for example,
on a single +5V supply, the EL2186C/EL2286C have an
input range which spans from 1V to 4V. The output
range of the EL2186C/EL2286C 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.
by the internal compensation circuitry of the
EL2186C/EL2286C. The reduction of feedback resistor
values (or the addition of a very small amount of exter-
nal capacitance at the inverting input, e.g. 0.5pF) will
increase bandwidth as desired. Please see the curves for
Frequency Response for Various RF and RG, and Fre-
quency Response for Various CIN-
.
Feedback Resistor Values
The EL2186C/EL2286C have been designed and speci-
fied at gains of +1 and +2 with RF = 750W. This value of
feedback resistor gives 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.
Since the EL2186C/EL2286C 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 EL2186C/EL2286C, good Dif-
ferential 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 3mA supply current of
each EL2186C/EL2286C amplifier! Special circuitry
has been incorporated in the EL2186C/EL2286C to
reduce the variation of output impedance with current
Because the EL2186C/EL2286C are current-feedback
amplifiers, their gain-bandwidth product is not a con-
stant for different closed-loop gains. This feature
actually allows the EL2186C/EL2286C to maintain
about the same -3dB bandwidth, regardless of closed-
11
ELANTEC [ ELANTEC SEMICONDUCTOR ]
