APN2015.pdf

GaAs FETs as Control Devices
APN2015
-5 V
Gallium arsenide MESFETs are being used in RF control
device applications as switches and attenuators. They are
very easily adapted to monolithic circuit form, dissipate
essentially no power and can easily be designed into
broadband circuits.
The RF signal flows from source to drain, while the RF
isolated gate is the voltage control. The high impedance
“off state” is attained by applying a DC voltage on the gate
more negative than the “pinch-off” voltage (VP). In this
condition the source-drain channel is “pinched off.” The
capacitance is typically 0.25 pF per mm of gate
periphery (see Figure 1A). The “on” state occurs when
zero DC bias is applied to the gate (see Figure 1B). The
channel from source to drain is “open” and represents a
2.5–3.5 Ω resistance per mm of gate periphery.
A configuration of FETs used in series and shunt normally
produce the optimum switch or attenuator performance in
monolithic circuits (see Figure 2). The only DC current that
flows is the leakage of the gate-source and gate-drain
reverse biased junctions (when negative voltage is applied
to gate). Typical current drain is < 25 µA @ -5 V. This
leakage is a function of the wafer fabrication process, the
device periphery, and magnitude of applied voltage. For
isolation of gate voltage control, 2.5–5 kΩ resistor is
incorporated monolithically in the gate. Since no further
external bias circuitry is required, the switch is inherently
broadband.
The power handling of the switches is primarily limited by
the current handling capability, which is related to the IDSS
of the FET. The IDSS is a function of the gate periphery
which then determines the source-drain capacitance.
The input series FET in switches have a nominal 1 dB
compression of 1 W/mm.
5 kΩ
Gate
Source
Drain
N
SI
Figure 1A. MESFET Control Device in
High Impedance State (“Off” State)
0V
5 kΩ
Gate
Source
Drain
N
SI
Figure 1B. MESFET Control Device in
Low Impedance State (“On” State)
Voltage variable attenuators (VVAs) use the channel
resistance of the FET as the actual resistance of the circuit
components. The resistance is a nonlinear relationship
with control voltage as shown in Figure 4 (FET with a
“pinch-off” voltage of 1.5 V). Figure 5 shows a TEE VVA
that uses two series FETs and one shunt FET.
Digital attenuators use FETs configured as “T” or “PI” pads
to achieve a given attenuation value (Figure 3). For the low
loss state V2 is set to 0 V and V1 is set to -5 V. For the
attenuation state V2 is set to -5 V and V1 is set to 0 V.
Typically digital attenuators are composed of multiple bit
values (e.g. 2, 4, 8, 16 dB AD220-25).
Skyworks Solutions, Inc. [781] 376-3000 • Fax [781] 376-3100 • Email [email protected] • www.skyworksinc.com
Specifications subject to change without notice. 9/03A
1
GaAs FETs as Control Devices
APN2015
RF Path
RF Path
5K
Insertion Loss
Isolation
DC
“On” 0 V
“On” 0 V
5K
Isolation
Insertion Loss
DC “Off” -5 V
“Off” -5 V
Shunt FET
Series FET
Figure 2. Shunt/Series FET
V2
RF1
RF2
Fixed Pad
Fixed Pad
V1
Figure 3. Single Bit Configuration for Digital Attenuator
Resistive Path
1 mm FET
DC
“0” V ~ 3.0 Ω
-1 V ~ 10 Ω
-2 V ~ 30 Ω
-3 V ~ 300 Ω
-3.5 V ~ 1000 Ω
-5 V ~ 10 kΩ
Figure 4. FET as a Variable Resistor
2
Figure 5. Voltage Variable Attenuator (VVA) in
Tee Configuration
Skyworks Solutions, Inc. [781] 376-3000 • Fax [781] 376-3100 • Email [email protected] • www.skyworksinc.com
Specifications subject to change without notice. 9/03A
Similar pages