LMH6629 datasheet(16/22 Pages) NSC

16 / 22 page

30068022

FIGURE 9. Voltage Noise Density vs. Source Resistance

If bias current cancellation is not a requirement, then R

f || Rg

need not equal R

seq. In this case, according to Equation 1,

f||Rg should be as low as possible to minimize noise. Results

similar to Equation 1 are obtained for the inverting configura-

tion of Figure 5 if R

seq is replaced by Rb and Rg is replaced by

g + Rs. With these substitutions, Equation 1 will yield an eni

referred to the non-inverting input. Referring e

ni to the invert-

ing input is easily accomplished by multiplying e

ni by the ratio

of non-inverting to inverting gains (1+R

g/Rf).

NOISE FIGURE

Noise Figure (NF) is a measure of the noise degradation

caused by an amplifier.

Equation 3: General Noise Figure Equation

(3)

Looking at the two parts of the NF expression (inside the log

function) yields:

i/So→ Inverse of the power gain provided by the amplifier

o/Ni→ Total output noise power, including the contribution of

S, divided by the noise power at the input due to RS

To simplify this, consider N

a as the noise power added by the

amplifier (reflected to its input port):

i/So→ 1/G

o/Ni→ G * (Ni+Na) /Ni (where G*(Ni +Na ) = No)

Substituting these two expressions into the NF expression:

Equation 4: Simplified Noise Figure Equation

(4)

The noise figure expression has simplified to depend only on

the ratio of the noise power added by the amplifier at its input

(considering the source resistor to be in place but noiseless

in getting N

a) to the noise power delivered by the source re-

sistor (considering all amplifier elements to be in place but

noiseless in getting N

i).

For a given amplifier with a desired closed loop gain, to min-

imize noise figure:

•

Minimize R

f||Rg

•

Choose the Optimum R

S (ROPT)

OPT is the point at which the NF curve reaches a minimum

and is approximated by:

OPT ≈ en/in

Figure 10 is a plot of NF vs R

S with the circuit of Figure 4 (Rf

= 240Ω, A

V = +10V/V). The NF curves for both Unterminated

T = open) and Terminated systems (RT = RS) are shown.

Table 1 indicates NF for various source resistances including

S = ROPT.

30068079

FIGURE 10. Noise Figure vs. Source Resistance

TABLE 1. Noise Figure for Various R

S (Ω)

NF (Terminated)

(dB)

NF (Unterminated)

(dB)

7.96

3.18

OPT

4.13

OPT = 750Ω)

1.12

OPT = 350Ω)

SINGLE SUPPLY OPERATION

The LMH6629 can be operated with single power supply as

shown in Figure 11. Both the input and output are capacitively

coupled to set the DC operating point.

30068026

FIGURE 11. Single Supply Operation

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