10

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 EL2170,

EL2270, and EL2470 have 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

Feedback Resistor Values

The EL2170, EL2270, and EL2470 have been designed and

with about 1.5dB of peaking, and 60MHz of -3dB bandwidth

EL2270, and EL2470 are current-feedback amplifiers, it is

bandwidth. As seen in the curve of Frequency Response For

modified by varying the value of the feedback resistor.

Because the EL2170, EL2270, and EL2470 are current-

feedback amplifiers, their gain-bandwidth product is not a

constant for different closed-loop gains. This feature actually

allows the EL2170, EL2270, and EL2470 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 EL2170, EL2270, and EL2470 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

EL2170, EL2270, and EL2470 will operate on dual supplies

ranging from ±1.5V to ±6V. With a single-supply, the EL2170,

EL2270, and EL2470 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

EL2170, EL2270, and EL2470 have an input voltage range

that extends to within 1V of either supply. So, for example, on

a single +5V supply, the EL2170, EL2270, and EL2470 have

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

of the EL2170, EL2270, and EL2470 is also quite large,

extending to within 1V of the supply rail. On a ±5V supply,

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 external pull-down

resistor to ground. On a single +5V supply, output voltage

range is about 0.3V to 4V.

Video Performance

For good video performance, an amplifier is required 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

Ω, because of the change in output current with DC

level. Until the EL2170, EL2270, and EL2470, good

Differential Gain could only be achieved by running high idle

currents through the output transistors (to reduce variations

in output impedance). These currents were typically more

than the entire 1mA supply current of each EL2170, EL2270,

and EL2470 amplifier! Special circuitry has been

incorporated in the EL2170, EL2270, and EL2470 to reduce

the variation of output impedance with current output. This

results in dG and dP specifications of 0.15% and 0.15° while

driving 150

Ω at a gain of +2.

Video performance has also been measured with a 500

Ω

load at a gain of +1. Under these conditions, the EL2170,

EL2270, and EL2470 have dG and dP specifications of

0.01% and 0.02° respectively while driving 500

Ω at A

Output Drive Capability

In spite of its low 1mA of supply current, the EL2170 is

capable of providing a minimum of ±80mA of output current.

Similarly, each amplifier of the EL2270 and the EL2470 is

capable of providing a minimum of ±50mA. These output

drive levels are unprecedented in amplifiers running at these

supply currents. With a minimum ±80mA of output drive, the

EL2170 is capable of driving 50

Ω loads to ±4V, making it an

excellent choice for driving isolation transformers in

telecommunications applications. Similarly, the ±50mA

minimum output drive of each EL2270 and EL2470 amplifier

allows swings of ±2.5V into 50

Ω loads.

EL2170, EL2270, EL2470