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LMH6574

SNCS103E – NOVEMBER 2004 – REVISED AUGUST 2018

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Feature Description (continued)

The LMH6574 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 575Ω. For other gains see Figure 27. 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 12 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 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.

7.2.3 Other Applications

The LMH6574 could support a multi antenna receiver with up to four separate antennas. Monitoring the signal

strength of all 4 antennas and connecting the strongest signal to the final IF stage would provide effective spacial

diversity.

For direction finding, the LMH6574 could be used to provide high speed sampling of four separate antennas to a

single DSP which would use the information to calculate the direction of the received signal.