®

OPA640

11

DISTORTION

The OPA640’s Harmonic Distortion characteristics vs fre-

quency and power output are shown in the Typical Perfor-

mance Curves. Distortion can be further improved by in-

creasing the load resistance. Refer to Figure 6. Remember to

include the contribution of the feedback resistance when

calculating the effective load resistance seen by the ampli-

fier.

FIGURE 6. 5MHz Harmonic Distortion vs Load Resistance.

The third-order intercept point is an important parameter for

many RF amplifier applications. Figure 7 shows the

OPA640’s single tone, third-order intercept vs frequency.

This curve is particularly useful for determining the magni-

tude of the third harmonic as a function of frequency, load

resistance, and gain. For example, assume that the applica-

tion requires the OPA640 to operate in a gain of +1V/V and

drive 2Vp-p into 50

Ω at a frequency of 10MHz. Referring to

Figure 11 we find that the intercept point is +47dBm. The

magnitude of the third harmonic can now be easily calcu-

lated from the expression:

P

O

= output level, dBm

O

= 47dBm, and the third

Harmonic = 2(47 – 10) = 74dB below the fundamental tone.

The OPA640’s low distortion makes the device an excellent

choice for a variety of RF signal processing applications.

The value for the two-tone, third-order intercept is typically

8dB lower than the single tone value.

FIGURE 7. Single-Tone , 3rd-Order Intercept Point vs Fre-

quency.

NOTE: Feedback Resistance used was 402

Ω.

NOISE FIGURE

The OPA640 voltage and current noise spectral densities are

specified in the Typical Performance Curves. For RF appli-

cations, however, Noise Figure (NF) is often the preferred

noise specification since it allows system noise performance

to be more easily calculated. The OPA640’s Noise Figure vs

Source Resistance is shown in Figure 8.

FIGURE 8. Noise Figure vs Source Resistance.

–70

–80

–90

–100

10

Load Resistance (

Ω)

20

50

100

200

500

1k

2f

O

3f

O

Source Resistance (

Ω)

10

100

1k

10k

25

20

15

10

5

0

100k

NF = 10 LOG 1 +

e

n

2

4KTR

S

+60

+50

+40

+30

+20

Frequency (Hz)

10M

100M

1M