S2079 MA-COM [M/A-COM Technology Solutions, Inc.], S2079 Datasheet - Page 2

• Manufacturer Part Number: S2079
• Description: Drivers for GaAs FET Switches and Digital Attenuators
• Manufacturer: MA-COM [M/A-COM Technology Solutions, Inc.]

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Drivers for GaAs FET Switches and Digital Attenuators
Application Note
S2079
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.
and SW-399 include level shifting components for
compatibility with positive CMOS or TTL control voltages,
but these switches still require complementary control
logic.
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.
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
consumption.
The devices consist of input buffers, inverters to
generate
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.
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
V
With proper selection of positive and negative supply
voltage, the SWD-109 and SWD-119 can both provide
output control voltages in these ranges.
FETon
< 2 V can improve the maximum RF level (P1dB).
The SW-335, SW65-0xxx series and related
fabrication
complementary
The SW-277, SW-349, SW-394,
techniques
logic
FEToff
Although GaAs FET
for
values,
< - 5 V, and 0 V <
low
voltage
power
Another consideration in design with switches and
attenuators is the elimination of crosstalk that can arise
from RF leakage onto control lines.
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.
Control A Control B
Figure 3: GaAs SPDT Switch with CMOS Driver
TTL High
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TTL Low
+5V
RF1
C5
R2
C2
+5 VDC
TTL Control
TTL High
Q1
TTL Low
C6
Control
"A"
VCC
RF Common
Q2
RF Common
CD54HCT04
to RF 1
OFF
ON
C1
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R1
Q3
GND
+5 VDC
Control
"B"
RF Common
Q4
Most board
to RF 2
OFF
ON
C3
R3
C4
Rev. V5
RF2
+5V