LMH6733 Datasheet, PDF (11/16 Pages) Texas Instruments – Single Supply, 1.0 GHz, Triple Operational Amplifier

Application Information
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FIGURE 1. Recommended Non-Inverting Gain Circuit
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FIGURE 2. Recommended Inverting Gain Circuit
GENERAL INFORMATION
The LMH6733 is a high speed current feedback amplifier, op-
timized for very high speed and low distortion. The LMH6733
has no internal ground reference so single or split supply con-
figurations are both equally useful.
FEEDBACK RESISTOR SELECTION
One of the key benefits of a current feedback operational am-
plifier is the ability to maintain optimum frequency response
independent of gain by using the appropriate values for the
feedback resistor (R_F). The Electrical Characteristics and
Typical Performance plots specify an R_F of 340Ω, a gain of
+2 V/V and ±2.5V power supplies (unless otherwise speci-
fied). Generally, lowering R_F from its 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 its
recommended value will cause overshoot, ringing and, even-
tually, oscillation.
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FIGURE 3. Recommended R_F vs. Gain
See Figure 3 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 maxi-
mal bandwidth. Note that it is not possible to use a current
feedback amplifier with the output shorted directly to the in-
verting input. The buffer configuration of the LMH6733 re-
quires a 324Ω feedback resistor for stable operation.
The LMH6733 has been optimized for high speed operation.
As shown in Figure 3 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 sub-
stantially different than predicted from ideal models. In par-
ticular 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 maxi-
mum 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 de-
scribes the relationship between R_F and closed-loop frequen-
cy 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 band-
width will fall as gain is increased, much like a typical voltage
feedback amplifier.
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