EL2180C/EL2280C/EL2480C-Preliminary
250MHz / 3mA Current Mode Feedback Amplifiers
Applications Information
supply pins must be well bypassed to reduce the risk of
Product Description
oscillation. The combination of a 4.7µF tantalum capac-
itor in parallel with a 0.1µF capacitor has been shown to
work well when placed at each supply pin.
The EL2180C/EL2280C/EL2480C are current-feedback
operational amplifiers that offer a wide -3dB bandwidth
of 250MHz and a low supply current of 3mA per ampli-
fier. All of these products also feature high output
current drive. The EL2180C can output 100mA, while
the EL2280C and the EL2480C can output 55mA per
amplifier. The EL2180C/EL2280C/EL2480C work with
supply voltages ranging from a single 3V to ±6V and
they are also capable of swinging to within 1V of either
supply on the input and the output. Because of their cur-
rent-feedback topology, the EL2180C/EL2280C/
EL2480C do not have the normal gain-bandwidth prod-
uct associated with voltage-feedback operational
amplifiers. This allows their -3dB bandwidth to remain
relatively constant as closed-loop gain is increased. This
combination of high bandwidth and low power, together
with aggressive pricing make the EL2180C/EL2280C/
EL2480C the ideal choice for many low-power/high-
bandwidth applications such as portable computing,
HDSL, and video processing.
For good AC performance, parasitic capacitance should
be kept to a minimum especially at the inverting input
(see the Capacitance at the Inverting Input section).
Ground plane construction should be used, but it should
be removed from the area near the inverting input to
minimize any stray capacitance at that node. Carbon or
Metal-Film resistors are acceptable with the Metal-Film
resistors giving slightly less peaking and bandwidth
because of their additional series inductance. Use of
sockets, particularly for the SO package, should be
avoided if possible. Sockets add parasitic inductance and
capacitance which will result in some additional peaking
and overshoot.
Capacitance at the Inverting Input
Any manufacturer's high-speed voltage- or current-feed-
back amplifier can be affected by stray capacitance at
the inverting input. For inverting gains this parasitic
capacitance has little effect because the inverting input is
a virtual ground, but for non-inverting gains this capaci-
tance (in conjunction with the feedback and gain
resistors) creates a pole in the feedback path of the
amplifier. This pole, if low enough in frequency, has the
same destabilizing effect as a zero in the forward open-
loop response. The use of large value feedback and gain
resistors further exacerbates the problem by further low-
ering the pole frequency.
For applications where board space is extremely critical,
the EL2180C is available in the tiny 5-pin SOT23 pack-
age, which has a footprint 28% the size of an 8-pin SO.
The EL2480C is available in the 24-pin LPP package,
offering board space savings and better power dissipa-
tion compared to the SO and PDIP packages. The
EL2180C/EL2280C/EL2480C are each also available in
industry-standard pinouts in PDIP and SO packages.
For single, dual, and triple applications with disable,
consider the EL2186C (8-pin single), EL2286C (14-pin
dual), and EL2386C (16-pin triple). If lower power is
required, refer to the EL2170C/EL2176C family which
provides singles, duals, and quads with 70MHz of band-
width while consuming 1mA of supply current per
amplifier.
The experienced user with a large amount of PC board
layout experience may find in rare cases that the
EL2180C/EL2280C/EL2480C have less bandwidth than
expected.
The reduction of feedback resistor values (or the addi-
tion of a very small amount of external 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 Frequency
Response for Various CIN-.
Power Supply Bypassing and Printed Circuit
Board Layout
As with any high-frequency device, good printed circuit
board layout is necessary for optimum performance.
Ground plane construction is highly recommended.
Lead lengths should be as short as possible. The power
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