LM6181AIM-8 National Semiconductor, LM6181AIM-8 Datasheet - Page 16

• Manufacturer Part Number: LM6181AIM-8
• Description: 100 mA/ 100 MHz Current Feedback Amplifier
• Manufacturer: National Semiconductor

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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
varied to adjust for the desired closed-loop gain as in Figure
2 .
FIGURE 1. 1a, 1b: Variation of Closed Loop Gain
from −1 to −5 Yields Similar Responses
f
. Therefore, R
1b
1a
S
and not R
f
, must be
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16
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
compensating the LM6181. For general applications the
LM6181 will maintain specified performance with an 820
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
and 1640 , respectively. Figure 3a and Figure 3b illustrate
the effect of increasing R
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
and reduce overshoot in the time domain response. Con-
versely, smaller feedback resistance values than 820
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
the desired small signal response in an amplifier configured
for a closed loop gain of 25.
the Desired Closed Loop Gain, A
FIGURE 2. R
S
Is Adjusted to Obtain
f
while maintaining the same
f
, is a dominant factor in
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f
to 500
f
f
typical curves)
VCL
to establish
can