Positive Voltage Control of

GaAs MMIC Control Devices

Rev. V3

Application Note

M521

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SW-239 with +5 V Supply

Figure 2b.

Positive Voltage Control Configuration

of the SW-239

Applications

This type of voltage floating technique can be used

in most control component applications. A typical

switch and digital attenuator application are pre-

sented in Figure 2. Each application is similar in

that the source of the FET’s are biased at +5 V and

the gates can be controlled by 0 and +5 V.

In all cases, the ground pins which have a source

connection must be AC coupled with the shortest

possible path to ground.

The AT-210 is shown as an example of a digital

attenuator which is floated. Pins 10 through 15 are

normally grounded, but in this case are floated to

the +5 V supply and AC coupled to ground. One of

the RF connections must also be tied to +5 V to

ensure the source of the series FET’s are floated.

As mentioned above, all efforts to minimize induc-

tance to ground must be taken to ensure the full

attenuation range of the device. A truth table for

the digital attenuator in the floated configuration is

shown in Table 1.

Attn

(dB)

VC4 VC4 VC3 VC3 VC2 VC2 VC1 VC1

0

1

0

1

0

1

0

1

Ref.

1

0

0

1

0

1

0

1

1

0

1

1

0

0

1

0

1

2

1

0

1

0

0

1

0

1

3

0

1

0

1

1

0

0

1

4

1

0

0

1

1

0

0

1

5

0

1

1

0

1

0

0

1

6

1

0

1

0

1

0

0

1

7

0

1

0

1

0

1

1

0

8

1

0

0

1

0

1

1

0

9

0

1

1

0

0

1

1

0

10

1

0

1

0

0

1

1

0

11

0

1

0

1

1

0

1

0

12

1

0

0

1

1

0

1

0

13

0

1

1

0

1

0

1

0

14

1

0

1

0

1

0

1

0

15

Control Inputs

Table 1.

AT-210 Truth Table (+5 V Control)

The SW-239 is shown as an example of a SPDT

switch which is floated using this technique. Pins

2, 7, and 8 are normally ground and must be

floated with +5 V as well as being AC coupled to

ground. A +5 V supply is also required at the RF

common to ensure that the source of the series

FET’s are floated. The resistors R1 and R3 will

result in slower switching speed, but will minimize

coupling of RF signals along the +5 V line. A truth

table for this device in the floated configuration is

given in Table 2.

Control A

Control B

RF1

RF2

0 V

+5 V

ON

OFF

+5 V

0 V

OFF

ON

Table 2.

SW-239 Truth Table