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Applications Information

Product Description

The EL2270 is a current-feedback operational amplifier that

offers a wide -3dB bandwidth of 70MHz and a low supply

current of 1mA per amplifier. This product also features high

output current drive. The EL2270 can output 55mA per

amplifier and works with supply voltages ranging from a

single 3V to ±6V, and is also capable of swinging to within 1V

of either supply on the input and the output. Because of its

current-feedback topology, the EL2270 does not have the

normal gain-bandwidth product associated with voltage-

feedback operational amplifiers. This allows its -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 EL2270

the ideal choice for many low-power/high-bandwidth

applications such as portable computing, HDSL, and video

processing.

For applications where board space is extremely critical. The

EL2270 is available in industry standard pinouts in SO

package.

For single and dual applications with disable, consider the

EL2176 (8-pin single) and EL2276 (14-pin dual). If higher

speed is required, refer to the EL2180/EL2186 family which

provides singles, duals, triples, and quads with 250MHz of

bandwidth while consuming 3mA of supply current per

amplifier.

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 supply pins must

be well bypassed to reduce the risk of oscillation. The

combination of a 4.7µF tantalum capacitor in parallel with a

0.1µF capacitor has been shown to work well when placed at

each supply pin.

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-feedback

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 capacitance (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 lowering the pole frequency.

The experienced user with a large amount of PC board

layout experience may find in rare cases that the EL2270 has

less bandwidth than expected. The reduction of feedback

resistor values (or the addition 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

Feedback Resistor Values

The EL2270 has been designed and specified at gains of +1

about 0.5dB of peaking. Since the EL2270 is a current-

feedback amplifier, it is also possible to change the value of

and peaking can be easily modified by varying the value of

the feedback resistor.

Because the EL2270 is a current-feedback amplifier, its gain-

bandwidth product is not a constant for different closed-loop

gains. This feature actually allows the EL2270 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

specified 1k

Ω and still retain stability, resulting in only a slight

loss of bandwidth with increased closed-loop gain.

Supply Voltage Range and Single-Supply

Operation

The EL2270 has 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 EL2270 will

operate on dual supplies ranging from ±1.5V to ±6V. With a

single-supply, the EL2270 will operate from +3V to +12V.

As supply voltages continue to decrease, it becomes

necessary to provide input and output voltage ranges that

can get as close as possible to the supply voltages. The

EL2270 has an input voltage range that extends to within 1V

of either supply. So, for example, on a single +5V supply, the

EL2270 has an input range which spans from 1V to 4V. The

output range of the EL2270 is also quite large, extending to

within 1V of the supply rail. On a ±5V supply, the output is

EL2270