LMH6733 Datasheet, PDF (13/25 Pages) Texas Instruments – Single Supply, 1.0 GHz, Triple Operational Amplifier

LMH6733
www.ti.com
SNOSAW0D – JANUARY 2007 – REVISED MAY 2013
450
400
350
INVERTING (A_V < 0)
300
250
200
NON-INVERTING (A_V > 0)
150
100
50
0
1 2 3 4 5 6 7 8 9 10
|GAIN| (V/V)
Figure 36. Recommended R_F vs. Gain
See Figure 36 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 about 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 LMH6733 requires a 324Ω feedback resistor for stable operation.
The LMH6733 has been optimized for high speed operation. As shown in Figure 36 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_F
can go, based on the lowest practical value of R_G. This limitation applies to both inverting and non-inverting
configurations. For the LMH6733 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 LMH6733 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 −5 V/V and an optimal value for R_F the input impedance is equal to
55Ω. 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 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 LMH6733 is approximately 30Ω. The LMH6733 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
The choice of reactive components requires much attention when using any current feedback operational
amplifier as an active filter. 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 and OA-26 for more information on Active Filter applications for Current Feedback Op
Amps.
When using the LMH6733 as a low pass filter the value of R_F can be substantially reduced from the value
recommended in the R_F vs. Gain charts. The benefit of reducing R_F is increased gain at higher frequencies,
which improves attenuation in the stop band. Stability problems are avoided because in the stop band additional
device bandwidth is used to cancel the input signal rather than amplify it. The benefit of this change depends on
the particulars of the circuit design. With a high pass filter configuration reducing R_F will likely result in device
instability and is not recommended.
Copyright © 2007–2013, Texas Instruments Incorporated
Product Folder Links: LMH6733
Submit Documentation Feedback
13