LMH6720MA datasheet(10/29 Pages) TI1

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100

200

300

400

500

600

700

GAIN (V/V)

100

1000

FREQUENCY (MHz)

-8

-7

-6

-5

-4

-3

-2

-1

RF = 400:

RF = 600:

RF = 147:

RF = 300:

AV = 2V/V

VOUT = 0.5VPP

LMH6714, LMH6720

LMH6722, LMH6722-Q1

SNOSA39G – NOVEMBER 2002 – REVISED APRIL 2013

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APPLICATION SECTION

FEEDBACK RESISTOR SELECTION

One of the key benefits of a current feedback operational amplifier is the ability to maintain optimum frequency

response independent of gain by using appropriate values for the feedback resistor (RF). The Electrical

Characteristics and Typical Performance plots specify an RF of 300Ω, a gain of +2V/V and ±5V power supplies

(unless otherwise specified). Generally, lowering RF from it's 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 it's recommended value will cause overshoot, ringing and,

eventually, oscillation.

Figure 31. Frequency Response vs. RF

Figure 31 shows the LMH6714/LMH6720/LMH6722's frequency response as RF is varied (RL = 100Ω, AV = +2).

This plot shows that an RF of 147Ω results in peaking. An RF of 300Ω gives near maximal bandwidth and gain

flatness with good stability. An RF of 400Ω gives excellent stability with only a small bandwidth penalty. Since all

applications are slightly different it is worth some experimentation to find the optimal RF for a given circuit. 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 LMH6714/LMH6720/LMH6722 requires a 600

Ω feedback resistor for stable

operation.

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 LMH6714/LMH6720/LMH6722 is approximately 180

Ω. The LMH6714/LMH6720/LMH6722 is

designed for optimum performance at gains of +1 to +6 V/V and

−1 to −5V/V. When using gains of ±7V/V or

more the low values of RG required will make inverting input impedances very low.

When configuring the LMH6714/LMH6720/LMH6722 for gains other than +2V/V, it is usually necessary to adjust

the value of the feedback resistor. Figure 32 and Figure 33 provide recommended feedback resistor values for a

number of gain selections.

Figure 32. RF vs. Non-Inverting Gain

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