LMH6738 Datasheet, PDF (10/20 Pages) National Semiconductor (TI) – Very Wideband, Low Distortion Triple Op Amp

LMH6738
SNOSAC1E – APRIL 2004 – REVISED MARCH 2013
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800
700
600
NON-INVERTING (A_V > 0)
500
400
300
INVERTING (A_V < 0)
200
100
0
1 2 3 4 5 6 7 8 9 10
|GAIN| (V/V)
Figure 24. Recommended R_F vs. Gain
See Figure 24, Recommended R_F. vs Gain for selecting a feedback resistor value for gains of ±1 to ±10. Since
each application is slightly different it is worth some experimentation to find the optimal R_F for a given circuit. In
general a value of R_F that produces ~.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 LMH6738 requires a 750Ω feedback resistor for stable operation.
The LMH6738 was optimized for high speed operation. As shown in Figure 24 the suggested value for R_F
decreases for higher gains. Due to the impedance of the input buffer there is a practical limit for how small R_Fcan
go, based on the lowest practical value of R_G. This limitation applies to both inverting and non inverting
configurations. For the LMH6738 the input resistance of the inverting input is approximately 30Ω and 20Ω is a
practical (but not hard and fast) lower limit for R_G. The LMH6738 begins to operate in a gain bandwidth limited
fashion in the region where R_G is nearly equal to the input buffer impedance. Note that the amplifier will operate
with R_G values well below 20Ω, however results may 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.
Inverting gain applications that require impedance matched inputs may limit gain flexibility somewhat (especially
if maximum bandwidth is required). The impedance seen by the source is R_G || R_T (R_T is optional). The value of
R_G is R_F /Gain. Thus for an inverting gain of −7 V/V and an optimal value for R_F the input impedance is equal to
50Ω. Using a termination resistor this can be brought down to match a 25Ω source, however, a 150Ω source
cannot be matched. To match a 150Ω source would require using a 1050Ω feedback resistor and would result in
reduced bandwidth.
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 LMH6738 is approximately 30Ω. The LMH6738 is designed for optimum performance at gains
of +1 to +10 V/V and −1 to −9 V/V. Higher gain configurations are still useful, however, the bandwidth will fall as
gain is increased, much like a typical voltage feedback amplifier.
ACTIVE FILTER
When using any current feedback Operational Amplifier as an active filter it is necessary to be careful using
reactive components in the feedback loop. Reducing the feedback impedance, especially at higher frequencies,
will almost certainly cause stability problems. Likewise capacitance on the inverting input should be avoided. See
Application Notes OA-07 (SNOA365) and OA-26 (SNOA387) for more information on Active Filter applications for
Current Feedback Op Amps.
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