LMH6723 Datasheet, PDF (14/28 Pages) National Semiconductor (TI) – Single/Dual/Quad 370 MHz 1 mA Current Feedback Op Amp

LMH6723, LMH6724
SNOSA83I – AUGUST 2003 – REVISED AUGUST 2014
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7.4 Feedback Resistor Selection
One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency
response independent of gain by using appropriate values for the feedback resistor (R_F). The Electrical
Characteristics and Typical Performance plots were generated with an R_F of 1200Ω, a gain of +2V/V and ±5V or
±2.5V power supplies (unless otherwise specified). Generally, lowering R_F from its recommended value will peak
the frequency response and extend the bandwidth; however, increasing the value of R_F will cause the frequency
response to roll off faster. Reducing the value of R_F too far below it's recommended value will cause overshoot,
ringing, and eventually, oscillation.
2
1
R_F = 800:
0
-1
R_F = 1200:
-2
-3
R_F = 2000:
-4
-5
-6
V_S = ±2.5V
-7 _{VOUT = 1VPP}
-8
0.1
1
10
100 1000
FREQUENCY (MHz)
Figure 31. Frequency Response vs. R_F
Figure 31 shows the LMH6723/LMH6724'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 experimenting 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 requires a 2000-Ω feedback
resistor for stable operation. For other gains see the charts Figure 32 and Figure 33. These charts provide a
good place to start when selecting the best feedback resistor value for a variety of gain settings.
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