Datasheet

1 2 3 4 5 6 7 8 9 10
0
500
1000
1500
2000
2500
R
F
(:)
GAIN (V/V)
LMH6723, LMH6724, LMH6725
SNOSA83H –AUGUST 2003–REVISED APRIL 2013 www.ti.com
Figure 37 shows the LMH6723/LMH6724/LMH6725's frequency response as R
F
is varied (R
L
= 100Ω, A
V
= +2).
This plot shows that an R
F
of 800Ω results in peaking. An R
F
of 1200Ω gives near maximal bandwidth and gain
flatness with good stability. Since each application is slightly different it is worth some experimentation to find the
optimal R
F
for a given circuit. In general a value of R
F
that produces ~0.1 dB of peaking is the best compromise
between stability and maximal bandwidth. Note that it is not possible to use a current feedback amplifier with the
output shorted directly to the inverting input. The buffer configuration of the LMH6723/LMH6724/LMH6725
requires a 2000Ω feedback resistor for stable operation. For other gains see the charts Figure 38 and Figure 39.
These charts provide a good place to start when selecting the best feedback resistor value for a variety of gain
settings.
For more information see Application Note OA-13 which describes the relationship between R
F
and closed-loop
frequency response for current feedback operational amplifiers. The value for the inverting input impedance for
the LMH6723/LMH6724/LMH6725 is approximately 500Ω. The LMH6723/LMH6724/LMH6725 is designed for
optimum performance at gains of +1 to +5V/V and −1 to −4V/V. Higher gain configurations are still useful;
however, the bandwidth will fall as gain is increased, much like a typical voltage feedback amplifier.
Figure 38. RF vs. Non-Inverting Gain
Figure 38 and Figure 39 show the value of R
F
versus gain. A higher R
F
is required at higher gains to keep R
G
from decreasing too far below the input impedance of the inverting input. This limitation applies to both inverting
and non-inverting configurations. For the LMH6723/LMH6724/LMH6725 the input resistance of the inverting input
is approximately 500Ω and 100Ω is a practical lower limit for R
G
. The LMH6723/LMH6724/LMH6725 begins to
operate in a gain bandwidth limited fashion in the region where R
F
must be increased for higher gains. Note that
the amplifier will operate with R
G
values well below 100Ω; however, results will be substantially different than
predicted from ideal models. In particular, the voltage potential between the Inverting and Non-Inverting inputs
cannot be expected to remain small.
For inverting configurations the impedance seen by the source is R
G
|| R
T
. For most sources this limits the
maximum inverting gain since R
F
is determined by the desired gain as shown in Figure 39. The value of R
G
is
then R
F
/Gain. Thus for an inverting gain of −4 V/V the input impedance is equal to 100Ω. Using a termination
resistor, this can be brought down to match a 50Ω or 75Ω source; however, a 150Ω source cannot be matched
without a severe compromise in R
F
.
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