Positive Voltage Control of

GaAs MMIC Control Devices

Rev. V3

Application Note

M521

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Discussion

GaAs control devices have seen a tremendous

growth in recent years with M/A-COM being the

market leader in this area. The fact that these de-

vices are negative voltage controlled devices has

precluded the use of standard low cost CMOS driv-

ers and has forced users to develop hybrid driver

circuits to accomplish the driver function. This ap-

plication note discusses a configuration which al-

lows the user to drive GaAs control devices with

standard CMOS logic gates. Although specific ex-

amples are presented, this application is generic

and can be applied to any GaAs control device.

Design Approach

Since the GaAs FET requires a 0 V to -5 V relative

voltage between the source and the gate, an alter-

native to using negative control voltage is to ele-

vate or float the DC voltage at the source of the

FET to +5 V and use a 0 V to +5 V control voltage.

This would produce the required voltage differential

between the source and gate to 0 V and -5 V. In

many circuit applications, the source of the FET’s

are required to be RF ground. This grounding is

very critical to circuit performance requiring the 5 V

supply which is applied to the source be AC cou-

pled to ground. Figure 1 shows a typical schematic

of this topology for a single bit of a digital attenu-

ator.

Figure 1.

Positive Voltage Control Configuration of a

Single Attenuator Bit

Design Considerations

One area of concern in RF design is the adequate

grounding of the devices. If good ground is not

maintained, the performance of the device at the

higher attenuation / isolation states will be de-

graded and will not meet the expected specifica-

tion. When floating the source of a FET which is

required to have a ground connection, the quality of

the AC coupling capacitor and the PCB layout be-

comes extremely critical. The coupling capacitor

must be a high-Q capacitor such as the ATC100A

series supplied by American Technical Ceramics or

equivalent. For operation at lower frequencies a

higher value of capacitor must be chosen. Any

increase in inductance or path length will degrade

the performance of the device so the designer must

exercise good judgment in the placement of com-

ponents and the number of via holes used to pick

up ground. The best solution would be to utilize a

coplanar layout where the AC coupling capacitor

can be soldered directly to ground. If a microstrip

layout is used, the designer must use as many via

holes as possible and place them in very close

proximity to the capacitor.

AT-210 with +5 V Supply

Figure 2a.

Positive Voltage Control Configuration

of the AT-210