LM6181AMJSLASH883 datasheet(16/23 Pages) NSC

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Typical Applications

CURRENT FEEDBACK TOPOLOGY

For a conventional voltage feedback amplifier the resulting

small-signal bandwidth is inversely proportional to the de-

sired gain to a first order approximation based on the

gain-bandwidth concept. In contrast, the current feedback

amplifier topology, such as the LM6181, transcends this limi-

tation to offer a signal bandwidth that is relatively indepen-

dent of the closed-loop gain.

Figure 1a and Figure 1b illus-

trate that for closed loop gains of −1 and −5 the resulting

pulse fidelity suggests quite similar bandwidths for both

configurations.

The closed-loop bandwidth of the LM6181 depends on the

feedback resistance, R

f. Therefore, RS and not Rf, must be

varied to adjust for the desired closed-loop gain as in

Figure

POWER SUPPLY BYPASSING AND LAYOUT

CONSIDERATIONS

A fundamental requirement for high-speed amplifier design

is adequate bypassing of the power supply. It is critical to

maintain a wideband low-impedance to ground at the ampli-

fiers supply pins to insure the fidelity of high speed amplifier

transient signals. 10 µF tantalum and 0.1 µF ceramic bypass

capacitors are recommended for each supply pin. The by-

pass capacitors should be placed as close to the amplifier

pins as possible (0.5" or less).

FEEDBACK RESISTOR SELECTION: R

Selecting the feedback resistor, R

f, is a dominant factor in

compensating the LM6181. For general applications the

LM6181 will maintain specified performance with an 820

Ω

feedback resistor. Although this value will provide good re-

sults for most applications, it may be advantageous to adjust

this value slightly. Consider, for instance, the effect on pulse

responses with two different configurations where both the

closed-loop gains are 2 and the feedback resistors are 820

Ω

and 1640

Ω, respectively. Figure 3a and Figure 3b illustrate

the effect of increasing R

while maintaining the same

closed-loop gain — the amplifier bandwidth decreases. Ac-

cordingly, larger feedback resistors can be used to slow

down the LM6181 (see −3 dB bandwidth vs R

ftypical curves)

and reduce overshoot in the time domain response. Con-

versely, smaller feedback resistance values than 820

Ω can

be used to compensate for the reduction of bandwidth at

high closed loop gains, due to 2nd order effects. For ex-

ample

Figure 4 illustrates reducing R

f to 500Ω to establish

the desired small signal response in an amplifier configured

for a closed loop gain of 25.

DS011328-12

DS011328-13

FIGURE 1. 1a, 1b: Variation of Closed Loop Gain

from −1 to −5 Yields Similar Responses

DS011328-14

FIGURE 2. R

S Is Adjusted to Obtain

the Desired Closed Loop Gain, A

VCL

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