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LMH6570

www.ti.com

SNCS104D – APRIL 2005 – REVISED DECEMBER 2014

Typical Application (continued)

7.2.2 Feedback Resistor Selection

Figure 26. Suggested RF vs. Gain

The LMH6570 has a current feedback output buffer with gain determined by external feedback (RF) and gain set

(RG) resistors. With current feedback amplifiers, the closed loop frequency response is a function of RF. For a

gain of 2 V/V, the recommended value of RF is 576 Ω. For other gains, see Figure 26. Generally, lowering RF

from the recommended value will peak the frequency response and extend the bandwidth while increasing the

value of RF will cause the frequency response to roll off faster. Reducing the value of RF too far below the

recommended value will cause overshoot, ringing and, eventually, oscillation.

Since all applications are slightly different, it is worth some experimentation to find the optimal RF for a given

circuit. For more information see Current Feedback Loop Gain Analysis and Performance Enhancement,

Application Note OA-13 (SNOA366), which describes the relationship between RF and closed-loop frequency

response for current feedback operational amplifiers. The impedance looking into pin 8 is approximately 20

Ω.

This allows for good bandwidth at gains up to 10 V/V. When used with gains over 10 V/V, the LMH6570 will

exhibit a “gain bandwidth product” similar to a typical voltage feedback amplifier. For gains of over 10 V/V

consider selecting a high performance video amplifier like the LMH6720 to provide additional gain.

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