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LMH6715_17 Datasheet, PDF (10/21 Pages) Texas Instruments – Dual Wideband Video Op Amp
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LMH6715
SNOSA10C – MAY 2002 – REVISED APRIL 2013
Frequency Response vs. RF
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Figure 29.
Figure 29 shows the LMH6715's frequency response as RF is varied (RL = 100Ω, AV = +2). This plot shows that
an RF of 200Ω results in peaking and marginal stability. An RF of 300Ω gives near maximal bandwidth and gain
flatness with good stability, but with very light loads (RL > 300Ω) the device may show some peaking. An RF of
500Ω gives excellent stability with good bandwidth and is the recommended value for most applications. Since all
applications are slightly different it is worth some experimentation to find the optimal RF for a given circuit. For
more information see Application Note OA-13 (Literature Number SNOA366) which describes the relationship
between RF and closed-loop frequency response for current feedback operational amplifiers.
When configuring the LMH6715 for gains other than +2V/V, it is usually necessary to adjust the value of the
feedback resistor. The two plots labeled shown in Figure 30 and Figure 31 provide recommended feedback
resistor values for a number of gain selections.
RF vs. Non-Inverting Gain
Figure 30.
Both plots show the value of RF approaching a minimum value (dashed line) at high gains. Reducing the
feedback resistor below this value will result in instability and possibly oscillation. The recommended value of RF
is depicted by the solid line, which begins to increase at higher gains. The reason that a higher RF is required at
higher gains is the need to keep RG from decreasing too far below the output impedance of the input buffer. For
the LMH6715 the output resistance of the input buffer is approximately 160Ω and 50Ω is a practical lower limit for
RG. Due to the limitations on RG the LMH6715 begins to operate in a gain bandwidth limited fashion for gains of
±5V/V or greater.
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