1

2

3

4

5

6

7

8

9

10

0

100

200

300

400

500

600

700

800

|GAIN| (V/V)

NON-INVERTING (AV > 0)

INVERTING (AV < 0)

LMH6738

SNOSAC1E – APRIL 2004 – REVISED MARCH 2013

www.ti.com

Figure 24. Recommended RF vs. Gain

See Figure 24, Recommended RF. 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 RF for a given circuit. In

general a value of RF 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 750

Ω feedback resistor for stable operation.

The LMH6738 was optimized for high speed operation. As shown in Figure 24 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 RFcan

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

configurations. For the LMH6738 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 LMH6738 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 −7 V/V and an optimal value for RF the input impedance is equal to

50

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 (SNOA366) 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 LMH6738 is approximately 30

Ω. 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.

10

Submit Documentation Feedback

Copyright © 2004–2013, Texas Instruments Incorporated

Product Folder Links: LMH6738