LMH6720MA Datasheet, PDF (10/29 Pages) Texas Instruments – LMH6714/ LMH6720/ LMH6722/ LMH6722Q Wideband Video Op Amp; Single, Single with Shutdown and Quad

LMH6714, LMH6720
LMH6722, LMH6722-Q1
SNOSA39G – NOVEMBER 2002 – REVISED APRIL 2013
APPLICATION SECTION
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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 specify an R_F of 300Ω, 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.
1
0
-1
R_F = 147:
-2
R_F = 300:
-3
-4
R_F = 400:
-5
R_F = 600:
-6
-7 ^{AV = 2V/V}
V_OUT = 0.5V_PP
-8
1
10
100
FREQUENCY (MHz)
1000
Figure 31. Frequency Response vs. R_F
Figure 31 shows the LMH6714/LMH6720/LMH6722's frequency response as R_F is varied (R_L = 100Ω, A_V = +2).
This plot shows that an R_F of 147Ω results in peaking. An R_F of 300Ω gives near maximal bandwidth and gain
flatness with good stability. An R_F of 400Ω gives excellent stability with only a small bandwidth penalty. Since all
applications are slightly different it is worth some experimentation to find the optimal R_F for a given circuit. 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 LMH6714/LMH6720/LMH6722 requires a 600Ω feedback resistor for stable
operation.
For more information see Application Note OA-13 (SNOA366) 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 LMH6714/LMH6720/LMH6722 is approximately 180Ω. The LMH6714/LMH6720/LMH6722 is
designed for optimum performance at gains of +1 to +6 V/V and −1 to −5V/V. When using gains of ±7V/V or
more the low values of R_G required will make inverting input impedances very low.
When configuring the LMH6714/LMH6720/LMH6722 for gains other than +2V/V, it is usually necessary to adjust
the value of the feedback resistor. Figure 32 and Figure 33 provide recommended feedback resistor values for a
number of gain selections.
700
600
500
400
300
200
100
0
1 2 3 4 5 6 7 8 9 10
GAIN (V/V)
Figure 32. R_F vs. Non-Inverting Gain
10
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