lm6182 National Semiconductor Corporation, lm6182 Datasheet - Page 16

• Manufacturer Part Number: lm6182
• Description: Dual 100 Ma Output, 100 Mhz Current Feedback Amplifier
• Manufacturer: National Semiconductor Corporation

www.national.com
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 LM6182, 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 similiar bandwidths for both
configurations.
FEEDBACK RESISTOR SELECTION: R
Selecting the feedback resistor, R
compensating the LM6182. For general applications the
LM6182 will maintain specified performance with an 820
feedback resistor. The closed-loop bandwidth of the LM6182
depends on the feedback resistance, R
not R
demonstrated in Figure 2 .
FIGURE 1. Variation of Closed-Loop Gain from −1 to −5
f
, is varied to adjust for the desired closed-loop gain as
Yields Similar Responses.
1A. A
1B. A
V
V
= −1
= −5
f
, is a dominant factor in
f
. Therefore, Rs, and
f
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16
Although this R
plications, 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 LM6182 and reduce overshoot in the time domain
response. Conversely, smaller feedback resistance values
than 820
bandwidth at high closed-loop gains, due to 2nd order ef-
fects. For example Figure 4A and Figure 4B illustrate reduc-
ing R
in an amplifier configured for a closed-loop gain of +25.
Adjusted to Obtain the Desired Closed-Loop Gain, A
FIGURE 2. R
FIGURE 3. Increase Compensation by Increasing
f
to 500 to establish the desired small signal response
can be used to compensate for the reduction of
f
value will provide good results for most ap-
f
Sets Amplifier Bandwidth and R
3B. R
3A. R
R
f
f
, A
f
while maintaining the same
f
V
= 1640
= 820
= +2
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s
is
V
.