Application Information

GENERAL AMPLIFIER FUNCTION

oltage feedback amplifiers have a small-signal bandwidth that

is a function of the closed-loop gain. Conversely, the

LME49871 current feedback amplifier features a small-signal

bandwidth that is relatively independent of the closed-loop

gain. This is shown in Figure 1 where the LME49871’s gain

is –1,–2, –5 and –10. Like all current feedback amplifiers, the

LME49871’s closed-loop bandwidth is a function of the feed-

back resistance value. Therefore, R

the desired closed-loop gain.

POWER SUPPLY BYPASSING AND LAYOUT

CONSIDERATIONS

Properly placed and correctly valued supply bypassing is es-

sential for optimized high-speed amplifier operation. The sup-

ply bypassing must maintain a wideband, low-impedance

capacitive connection between the amplifier’s supply pin and

ground. This helps preserve high speed signal and fast tran-

sient fidelity. The bypassing is easily accomplished using a

parallel combination of a 10

μF tantalum and a 0.1μF ceramic

capacitors for each power supply pin. The bypass capacitors

should be placed as close to the amplifier power supply pins

as possible.

FEEDBACK RESISTOR SELECTION (R

The value of the R

the LME49871. For general applications, the LME49871 will

maintain specified performance with an 1.2k

Ω feedback re-

sistor. Although this value will provide good results for most

applications, it may be advantageous to adjust this value

slightly for best pulse response optimized for the desired

bandwidth. In addition to reducing bandwidth, increasing the

feedback resistor value also reduces overshoot in the time

domain response.

300426p0

FIGURE 1. Bandwidth as a function of gain

SLEW RATE CONSIDERATIONS

A current feedback amplifier’s slew rate characteristics are

different than that of voltage feedback amplifiers. A voltage

feedback amplifier’s slew rate limiting or non-linear amplifier

behavior is dominated by the finite availability of the first stage

tail current charging the second stage voltage amplifier’s

compensation capacitor. Conversely, a current feedback

amplifier’s slew rate is not constant. Transient current at the

inverting input determines slew rate for both inverting and

non-inverting gains. The non-inverting configuration slew rate

is also determined by input stage limitations. Accordingly,

variations of slew rates occur for different circuit topologies.

DRIVING CAPACITIVE LOADS

The LME49871 can drive significantly higher capacitive loads

than many current feedback amplifiers. Although the

LME49871 can directly drive as much as 100pF without os-

cillating, the resulting response will be a function of the feed-

back resistor value.

CAPACITIVE FEEDBACK

It is quite common to place a small lead-compensation ca-

pacitor in parallel with a voltage feedback amplifier’s feedback

resistance, R

peaking in the frequency domain and damps the transient re-

sponse. Whereas this yields the expected results when used

with voltage feedback amplifiers, this technique must not be

used with current feedback amplifiers. The dynamic

impedance of capacitors in the feedback loop reduces the

amplifier’s stability. Instead, reduced peaking in the frequency

response and bandwidth limiting can be accomplished by

adding an RC circuit to the amplifier’s input.

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