LMH6715 Datasheet, PDF (8/12 Pages) National Semiconductor (TI) – Dual Wideband Video Op Amp

Application Section
FEEDBACK RESISTOR SELECTION
One of the key benefits of a current feedback operational
amplifier is the ability to maintain optimum frequency re-
sponse independent of gain by using appropriate values for
the feedback resistor (R_F). The Electrical Characteristics and
Typical Performance plots specify an R_F of 500Ω, a gain of
+2V/V and ±5V power supplies (unless otherwise specified).
Generally, lowering R_F from it’s recommended value will
peak the frequency response and extend the bandwidth
while 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.
Frequency Response vs. R_F
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FIGURE 1. Non-Inverting Configuration with Power
Supply Bypassing
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FIGURE 2. Inverting Configuration with Power Supply
Bypassing
Application Introduction
Offered in an 8-pin package for reduced space and cost, the
wideband LMH6715 dual current-feedback op amp provides
closely matched DC and AC electrical performance charac-
teristics making the part an ideal choice for wideband signal
processing. Applications such as broadcast quality video
systems, IQ amplifiers, filter blocks, high speed peak detec-
tors, integrators and transimedance amplifiers will all find
superior performance in the LMH6715 dual op amp.
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The plot labeled “Frequency Response vs. R_F” shows the
LMH6715’s frequency response as R_F is varied (R_L = 100Ω,
A_V = +2). This plot shows that an R_F of 200Ω results in
peaking and marginal stability. An R_F of 300Ω gives near
maximal bandwidth and gain flatness with good stability, but
with very light loads (R_L > 300Ω) the device may show some
peaking. An R_F of 500Ω gives excellent stability with good
bandwidth and is the recommended value for most applica-
tions. Since all applications are slightly different it is worth
some experimentation to find the optimal R_F for a given
circuit. 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.
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 “R_F vs. Non-inverting Gain”
and “R_F vs. Inverting Gain” provide recommended feedback
resistor values for a number of gain selections.
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