13

®

OPA620

DISTORTION

The OPA620’s harmonic distortion characteristics into a

50

Ω load are shown vs frequency and power output in the

Typical Performance Curves. Distortion can be further im-

proved by increasing the load resistance as illustrated in

Figure 8. Remember to include the contribution of the

feedback resistance when calculating the effective load

resistance seen by the amplifier.

FIGURE 8. 10MHz Harmonic Distortion vs Load Resistance.

G = +1V/V

V = 2Vp-p

O

10MHz HARMONIC DISTORTION

vs LOAD RESISTANCE

–40

–50

–60

–70

–80

–90

0

100

200

300

400

500

Load Resistance ( )

Ω

G = +2V/V

3f

2f

Two-tone third-order intermodulation distortion (IM) is an

important parameter for many RF amplifier applications.

Figure 9 shows the OPA620’s two-tone third-order IM

intercept vs frequency. For these measurements, tones were

spaced 1MHz apart. This curve is particularly useful for

determining the magnitude of the third-order IM products as

a function of frequency, load resistance, and gain. For

example, assume that the application requires the OPA620

to operate in a gain of +2V/V and drive 2Vp-p (4dBm for

each tone) into 50

Ω at a frequency of 10MHz. Referring to

Figure 9 we find that the intercept point is +40dBm. The

magnitude of the third-order IM products can now be easily

calculated from the expression:

P

O

= output level/tone, dBm/tone

Third IMD = third-order intermodulation ratio

below each output tone, dB

order IMD = 2(40 – 10) = 72dB below either 4dBm tone.

The OPA620’s low IMD makes the device an excellent

choice for a variety of RF signal processing applications.

FIGURE 9. 2-Tone, 3rd Order Intermodulation Intercept vs

Frequency.

0

10203040

5060708090

100

10

15

20

25

30

35

40

45

50

55

G = +1V/V

R

L

P

OUT

P

OUT

250

Ω

250

Ω

R

L

60

–

–

+

+

G = +2V/V

G = +1V/V

Frequency (MHz)

R = 50

L

Ω

R = 100

L

Ω

G = +2V/V

R = 50

L

Ω

R = 100

L

Ω

R = 400

L

Ω

R = 400

L

Ω

INTERCEPT vs FREQUENCY

NOISE FIGURE

The OPA620’s voltage and current noise spectral densities

are specified in the Typical Performance Curves. For RF

applications, however, Noise Figure (NF) is often the

preferred noise specification since it allows system noise

performance to be more easily calculated. The OPA620’s

Noise Figure vs Source Resistance is shown in Figure 10.

NOISE FIGURE vs SOURCE RESISTANCE

25

20

15

10

5

0

10

100

1k

10k

100k

Source Resistance ( )

Ω

NF

dB

= 10log 1 +

e

n

2

4kTR

S

FIGURE 10. Noise Figure vs Source Resistance.