7.2.2 Feedback Resistor Selection

Figure 27. Suggested R_F vs Gain

The LMH6574 has a current feedback output buffer with gain determined by external feedback (R_F) and gain set (R_G) resistors. With current feedback amplifiers, the closed loop frequency response is a function of R_F. For a gain of 2 V/V, the recommended value of R_F is 575Ω. For other gains see Figure 27. Generally, lowering R_F from the 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 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 R_F 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 R_F and closed-loop frequency response for current feedback operational amplifiers. The impedance looking into pin 12 is approximately 20Ω. This allows for good bandwidth at gains up to 10 V/V. When used with gains over 10 V/V, the LMH6574 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 (SNOSA39) to provide additional gain.