M521 Datasheet, PDF (1/3 Pages) M/A-COM Technology Solutions, Inc. – Positive Voltage Control of GaAs MMIC Control Devices

Application Note
M521
Positive Voltage Control of
GaAs MMIC Control Devices
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.
Rev. V3
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 1.
Positive Voltage Control Configuration of a
Single Attenuator Bit
1
Visit www.macomtech.com for additional data sheets and product information.
M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make
changes to the product(s) or information contained herein without notice.
Figure 2a.
Positive Voltage Control Configuration
of the AT-210
• North America Tel: [phone] • Europe Tel: [phone]
• India Tel: [phone]
• China Tel: [phone]