Specifications and information are subject to change without notice

WJ Communications, Inc

• Phone 1-800-WJ1-4401 • FAX: 408-577-6621 • e-mail: sales@wj.com • Web site: www.wj.com

Page 2 of 7

January 2006

VG025

High Linearity Variable Gain Amplifier

Product Information

The Communications Edge

TM

Application Circuit Configurations

Circuit Board Material: .014” FR-4, 4 layers, .062” total thickness

The WJ VG025 variable gain amplifier can operate over a very

broad range of frequencies (50 - 2200MHz), but needs specific

matching circuits for specific bands of interest. At the maximum

gain state, reasonable matching is only available for about ±10% of

the reference frequency. The amplifier operates with a typical

current of 150 mA at +5 V while the attenuator current can be varied

from 0 to 30 mA, while maintaining constant OIP3 and P1dB. The

RF matching of the VG025 for the entire frequency range can be

accomplished with the systematic adjustment of only a few parts.

The RF matching is not influenced by different attenuator drive

methods provided there is adequate decoupling of the attenuator bias.

This reference application circuit uses voltage applied between 0

and 4.5V onto the Vctrl as shown on the schematic above. The R2 is

a current limiting resistor to help linearize the drive of the attenuator

and give better attenuator current control.

To properly design the VG025 for an application, pick the frequency

of interest.

Choose blocking capacitor values which give RF

impedance of less than 3 ohms. Choose the RF chokes for the

largest inductance while still having resonant frequency about 30%

greater than the Reference frequency (this allows for good isolation

and inductor variation). Next choose L3 and C9 for interstage

matching. C9 is only needed for lower frequencies and determines

the low frequency roll off of the gain. L3 will have dominant

control over the input and output return losses at maximum gain

state (0 mA gain control pin current). C2 is needed to resonate the

package parasitics to achieve the maximum attenuation values with

attenuator current. With attenuator control pin current of 26 mA, C2

can be chosen to provide maximum attenuation. See the chart below

for suggested component values and predicted performance at

various reference frequencies.

Component values can be

interpolated for reference frequencies not listed.

For lower frequencies the lumped element values can be used and

the layout with unplaced component pads does not greatly affect the

RF performance of the circuit. For frequencies greater than 500

MHz, component size and trace length have more influence on

circuit performance. Smaller components and shorter trace lengths

reduce the affects of the external component parasitics and

interaction with the multichip module parasitics.

Reference Frequency

MHz

240

800

1850

1960

2140

C1, C5

pF

1000

56

56

56

56

C2

pF

0 Ω

27

3.6

3.6

3.6

C3, C4

μF

.01

.01

.01

.01

.01

C6

pF

DNP

DNP

DNP

DNP

DNP

C9

pF

1.2

DNP

DNP

DNP

DNP

L1, L2

nH

220

22

12

12

12

L3

nH

47

8.2

DNP

DNP

DNP

L4

nH

DNP

82

33

33

33

R1

Ω

DNP

DNP

DNP

DNP

DNP

R2

Ω

120

120

120

120

120

Attenuation Range

dB

20

21

12

14

13

Maximum Gain

dB

15.4

15

8.6

8.4

8.4

Input Return Loss

dB

11

6.3

11

12

11

Output Return Loss

dB

11

7.6

7.7

6.5

5.6

Output P1dB

dBm

21.8

21.3

22

22

22

Output IP3

dBm

42

39

40

40

40

Noise Figure

dB

4.5

5.3

7.9

8.1

8.2

DNP = Do Not Place (Component is not used in the design)