S2079 Datasheet, PDF (1/4 Pages) M/A-COM Technology Solutions, Inc. – Drivers for GaAs FET Switches and Digital Attenuators

Application Note
S2079
Drivers for GaAs FET Switches and Digital Attenuators
Rev. V5
Introduction
Many of M/A-COM's GaAs FET switches and digital
attenuators cannot operate directly with simple TTL or
CMOS logic, but instead require external circuits to
provide appropriate control voltages. This application
note, an update of M539, Drivers for GaAs FET MMIC
Switches and Digital Attenuators, provides information
on M/A-COM's SW-109 and SWD-119 drivers and
other commercially available digital logic IC's for control
of switches and digital attenuators.
Figure 2 shows a 3-bit digital attenuator. Applying the
correct bias voltage and its complement to any stage
switches the pad for that stage into the RF signal path.
RF1
Q1
RF Common
Q2
Q3
RF2
Q4
GaAs FET’s
GaAs MMIC control devices such as switches and
digital attenuators typically employ Field Effect
Transistors (FET’s). The most common FET is the n-
channel depletion mode device, which has low source-
to-drain resistance in the absence of a gate bias, and
allows a current IDSS to flow. With the application of a
negative gate bias voltage, the electric field below the
gate causes the conduction channel to narrow,
increasing the source-to-drain resistance. The gate
voltage that creates a high enough resistance to reduce
the source-to-drain current to (typically) 1 - 2 percent of
IDSS is known as the pinch-off voltage. For M/A-COM
FET’s,. the pinch-off voltage is typically –2.5 volts. If
the transistor is biased at the extremes, (0 V and –5 V
typically), on and off switching results, providing the
basis for both GaAs MMIC switches and digital
attenuators.
Switch Circuit Topology
In switches, FET's are arranged in both series and
shunt configurations. The series FET's provide a
through-path for the on state, while the shunt FET's
provide isolation for the off state. The operation of the
switch requires that series FET's and shunt FET's
associated with each switch state have opposite (or
complementary) conduction states and therefore
opposite (or complementary) gate biases. For
example, Figure 1 illustrates the operation of a typical
dual control SPST GaAs MMIC switch. If the RF to
RF1 path is on and the RF to RF2 path is off, then
FET's Q2 and Q4 are biased on, while Q1 and Q3 are
biased off.
Digital attenuators use series/shunt stages with circuit
components that form fixed attenuator pads,
corresponding to digital attenuation bits, switched in or
out of the transmission path, either individually or in
combination. Switches require complementary bias
voltages for each state, while digital attenuators require
complementary bias voltage to activate each bit.
Control
"A"
Control
"B"
Dual Control Switch Truth Table
Control A Control B RF Common RF Common
to RF1
to RF2
-5 V
0V
On
Off
0V
-5 V
Off
On
Typical complementary logic control voltages:
Logic low
Logic high
0 V to –2 V @ 20 μA max.
-5 V to 40 μA typ. To –8 V @ 200 μ
A max.
Figure 1: Typical Dual Control Switch (SW-239, etc)
VC1
VC1
RF 1
16 dB
Pad
VC2 VC2
8 dB
Pad
VC3 VC3
4 dB
Pad
RF 2
Figure 2: Digital Attenuator Based on Switched Pads
(AT-230)
1
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