11

EL5397C - Preliminary

Triple 200MHz Fixed Gain Amplifier

Applications Information

Product Description

The EL5397C is a current-feedback operational ampli-

fier that offers a wide -3dB bandwidth of 300MHz and a

low supply current of 4mA per amplifier. The EL5397C

works with supply voltages ranging from a single 5V to

10V and they are also capable of swinging to within 1V

of either supply on the output. Because of their current-

feedback topology, the EL5397C does not have the nor-

mal gain-bandwidth product associated with voltage-

feedback operational amplifiers. Instead, its -3dB band-

width to remain relatively constant as closed-loop gain is

increased. This combination of high bandwidth and low

power, together with aggressive pricing make the

EL5397C the ideal choice for many low-power/high-

bandwidth applications such as portable, handheld, or

battery-powered equipment.

For varying bandwidth needs, consider the EL5191C

with 1GHz on a 9mA supply current or the EL5192C

with 600MHz on a 6mA supply current. Versions

include single, dual, and triple amp packages with 5-pin

SOT23, 16-pin QSOP, and 8-pin or 16-pin SO outlines.

Power Supply Bypassing and Printed Circuit

Board Layout

As with any high frequency device, good printed circuit

board layout is necessary for optimum performance.

Low impedance ground plane construction is essential.

Surface mount components are recommended, but if

leaded components are used, lead lengths should be as

short as possible. The power supply pins must be well

bypassed to reduce the risk of oscillation. The combina-

tion of a 4.7µF tantalum capacitor in parallel with a

0.01µF 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) Even

when ground plane construction is used, it should be

removed from the area near the inverting input to mini-

mize 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 additional series inductance. Use of sockets,

particularly for the SO package, should be avoided if

possible. Sockets add parasitic inductance and capaci-

tance which will result in 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 capac-

itance (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 exacerbates the problem by further lowering

the pole frequency (increasing the possibility of

oscillation.)

The EL5397C has been optimized with a 475

Ω feedback

resistor. With the high bandwidth of these amplifiers,

these resistor values might cause stability problems

when combined with parasitic capacitance, thus ground

plane is not recommended around the inverting input pin

of the amplifier.

Feedback Resistor Values

The EL5397C has been designed and specified at a gain

mately 500

Ω gives 175MHz of bandwidth with 0.2dB of

peaking. Since the EL5397C is a current-feedback

get more bandwidth. As seen in the curve of Frequency

ing can be easily modified by varying the value of the

feedback resistor.

Because the EL5397C is a current-feedback amplifier,

its gain-bandwidth product is not a constant for different

closed-loop gains. This feature actually allows the

EL5397C to maintain about the same -3dB bandwidth.

As gain is increased, bandwidth decreases slightly while