1

10

100

1000

FREQUENCY (MHz)

-6

-5

-4

-3

-2

-1

0

1

2

3

4

STANDARD CIRCUIT

(FIGURE 2)

ALTERNATE CIRCUIT

(FIGURE 4)

2

3

4

6

1

+

-

+5V

-5V

6.8 µF

.01 µF

6.8 µF

.01 µF

0.1 µF

LMH6704

LMH6704

www.ti.com

SNOSAD0C – FEBRUARY 2005 – REVISED MARCH 2013

Figure 20. Alternate Unity Gain Configuration

UNITY GAIN COMPENSATION

With a current feedback Selectable Gain Buffer like the LMH6704, the feedback resistor is a compromise

between the value needed for stability at unity gain and the optimized value needed at a gain of two. In standard

adjusted to match the required gain. Since the feedback resistor is integrated in the LMH6704, it is not possible

to adjust it’s value. However, we can employ the circuit shown in Figure 20. This circuit modifies the noise gain of

the amplifier to eliminate the peaking associated with using the circuit shown in Figure 18. The frequency

response is shown in Figure 21. The decreased peaking does come at a price as the output referred voltage

noise density increases by a factor of 1.1.

Figure 21. Unity Gain Frequency Response

OUTPUT VOLTAGE NOISE

Open-loop operational amplifiers specify three input referred noise parameters: input voltage noise, non-inverting

input current noise, and inverting input current noise. These specifications are used to calculate the total voltage

noise produced at the output of the amplifier. The LMH6704 is a closed loop amplifier with internal resistors, thus

only the non-inverting input current noise flows through external components. All other noise sources are internal

to the part. There are four possible values for the noise at the output depending on the gain configuration as

shown in Table 2. For more information on calculating noise in current feedback amplifiers see Application Notes

OA-12 and AN104 available at www.ti.com.

The total noise voltage at the output can be calculated using Equation 1:

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