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LMH6723 Datasheet, PDF (15/28 Pages) National Semiconductor (TI) – Single/Dual/Quad 370 MHz 1 mA Current Feedback Op Amp
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LMH6723, LMH6724
SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014
Feedback Resistor Selection (continued)
For more information see Application Note OA-13 which describes the relationship between RF and closed-loop
frequency response for current feedback operational amplifiers. The value for the inverting input impedance for
the LMH6723/LMH6724 is approximately 500 Ω. The LMH6723/LMH6724 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.
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0
1 2 3 4 5 6 7 8 9 10
GAIN (V/V)
Figure 32. RF vs. Non-Inverting Gain
Figure 32 and Figure 33 show the value of RF versus gain. A higher RF is required at higher gains to keep RG
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 the input resistance of the inverting input is
approximately 500Ω and 100Ω is a practical lower limit for RG. The LMH6723/LMH6724 begins to operate in a
gain bandwidth limited fashion in the region where RF must be increased for higher gains. Note that the amplifier
will operate with RG values well below 100 Ω; 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 RG || RT. For most sources this limits the
maximum inverting gain since RF is determined by the desired gain as shown in Figure 33. The value of RG is
then RF/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 50-Ω or 75-Ω source; however, a 150Ω source cannot be matched
without a severe compromise in RF.
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GAIN (-V/V)
Figure 33. RF vs. Inverting Gain
Copyright © 2003–2014, Texas Instruments Incorporated
Product Folder Links: LMH6723 LMH6724
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