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|GAIN| (V/V)

INVERTING (AV < 0)

NON-INVERTING (AV > 0)

LMH6733

www.ti.com

SNOSAW0D – JANUARY 2007 – REVISED MAY 2013

Figure 36. Recommended RF 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 RF for a given circuit. In general a value of RF 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 324

Ω feedback resistor for stable operation.

The LMH6733 has been optimized for high speed operation. As shown in Figure 36 the suggested value for RF

decreases for higher gains. Due to the impedance of the input buffer there is a practical limit for how small RF

can go, based on the lowest practical value of RG. This limitation applies to both inverting and non-inverting

configurations. For the LMH6733 the input resistance of the inverting input is approximately 30

Ω and 20Ω is a

practical (but not hard and fast) lower limit for RG. The LMH6733 begins to operate in a gain bandwidth limited

fashion in the region where RG is nearly equal to the input buffer impedance. Note that the amplifier will operate

with RG 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 RG || RT (RT is optional). The value of

RG is RF /gain. Thus for an inverting gain of −5 V/V and an optimal value for RF the input impedance is equal to

55

Ω. Using a termination resistor this can be brought down to match a 25Ω source; however, a 150Ω source

cannot be matched. To match a 150

Ω 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 RF and closed-loop

frequency response for current feedback operational amplifiers. The value for the inverting input impedance for

the LMH6733 is approximately 30

Ω. 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 RF can be substantially reduced from the value

recommended in the RF vs. Gain charts. The benefit of reducing RF 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 RF will likely result in device

instability and is not recommended.

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