Application Note
S2079
Drivers for GaAs FET Switches and Digital Attenuators
Rev. V5
Built-in Drivers
Some of M/A-COM's newer switches and attenuators
feature simplified control using CMOS (0 V, 2.7 V) or TTL
(0 V, 5 V) logic, with no need for negative control
voltages.
The Appendix to this application note lists some popular
M/A-COM switches. The SW-277, SW-349, SW-394,
and SW-399 include level shifting components for
compatibility with positive CMOS or TTL control voltages,
but these switches still require complementary control
logic. The SW-335, SW65-0xxx series and related
switches incorporate a CMOS driver circuit in the same
package, along with the GaAs switching elements, for
true single line control. Many future switches from M/A-
COM will likely incorporate driver circuitry and switching
elements together in small, low cost plastic packages.
The AT-226, AT-264, and AT-242 digital attenuators
feature internal level shifting to provide control with a
single CMOS input line for each attenuation bit. The
AT65-0xxx series miniature digital attenuator modules
incorporate CMOS driver circuitry to accomplish this.
Another consideration in design with switches and
attenuators is the elimination of crosstalk that can arise
from RF leakage onto control lines.
Most board
designers take care of this by adding capacitance to
ground on the control lines, shunting any RF energy to
ground. The SW-109 and SWD-119 output buffers can
drive load capacitance up to 25 pF.
Other Circuits as Drivers
You can use TTL and CMOS logic IC's to drive GaAs
FET switches and attenuators. An ideal driver would run
from a single supply voltage, consume little current, and
introduce very little switching delay.
One driver technique that works well floats the channel of
the FET's on the MMIC switch above ground potential
through the addition of pull-up resistors and DC blocking
and bypass capacitors. As shown in
Figure 3,
the circuit
takes a voltage of 0 VDC, applied to either control port,
and shifts it to -5 VDC at the attached FET gates to turn
them off. A voltage of +5 VDC shifts to 0 VDC at the
FET gates to turn them on.
RF Common
SWD-109 & SWD-119 Drivers
M/A-COM's SWD-109 and quad-channel SWD-119
provide the complementary control voltages necessary
for driving GaAs FET switches and digital attenuators
using a single control input per bit. Both the SWD-109
and SWD-119 incorporate buffering stages so that the
drivers will switch with either standard TTL or CMOS
logic level input. The devices employ standard CMOS
analog fabrication techniques for low power
consumption.
The devices consist of input buffers, inverters to
generate complementary logic values, voltage
translators, and output buffers, all designed to allow the
designer the flexibility to optimize switch and attenuator
performance.
To design a board with RF switches and attenuators,
consider that modulation of the source-drain resistance
in the FET's by input RF can lead to output compression
and intermodulation distortion. Although GaAs FET
switches and attenuators will operate well with nominal 0
V and -5 V for control, careful selection of the control
voltages in the ranges of - 8 V < V
FEToff
< - 5 V, and 0 V <
V
FETon
< 2 V can improve the maximum RF level (P1dB).
With proper selection of positive and negative supply
voltage, the SWD-109 and SWD-119 can both provide
output control voltages in these ranges.
RF1
Q2
C1
R1
+5 VDC
Q3
C4
Q1
Q4
RF2
C5
R2
+5V
C2
Control
"A"
Control
"B"
R3
+5V
C3
TTL Control
VCC
+5 VDC
GND
CD54HCT04
C6
Control A Control B
TTL Low
TTL High
TTL High
TTL Low
RF Common
to RF 1
ON
OFF
RF Common
to RF 2
OFF
ON
Figure 3: GaAs SPDT Switch with CMOS Driver
2
Visit www.macomtech.com for additional data sheets and product information.
•
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Tel: 800.366.2266 •
Europe
Tel: +353.21.244.6400
•
India
Tel: +91.80.4155721
•
China
Tel: +86.21.2407.1588
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.
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