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GaAs FETs as control devices

Posted: 24 Oct 2002     Print Version  Bookmark and Share

Keywords:power 

/ARTICLES/2002OCT/A/2002OCT24_ICD_RFD_POW_AN02.PDF

GaAs FETs as Control Devices

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 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 5A @ -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 k 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.

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).

Alpha Industries, Inc. [781] 935-5150 7 Fax [617] 824-4579 7 Email sales@alphaind.com 7 www.alphaind.com 1

Specifications subject to change without notice. 3/98A

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 3.5 V). Figure 5 shows a TEE VVA

that uses two series FETs and one shunt FET. The

attenuator requires two independent control voltages for

operation. Alpha also offers a single positive voltage

control VVA (AT002S3-12). See "Single Control VVA"

application note for more information.

Power handling of VVAs is significantly less than

switches since the FET is not normally fully biased `on' or

fully biased `off'. See "Dual Voltage Controlled VVA"

application note for more information.

Source

5 k

Gate

Drain

-5 V

N

SI

Figure 1A. MESFET Control Device in

High Impedance State ("Off" State)

Source

5 k

Gate

Drain

0 V

N

SI

Figure 1B. MESFET Control Device in

Low Impedance State ("On" State)

GaAs FETs as Control Devices

2 Alpha Industries, Inc. [781] 935-5150 7 Fax [617] 824-4579 7 Email sales@alphaind.com 7 www.alphaind.com

Specifications subject to change without notice. 3/98A

Figure 2. Shunt/Series FET

RF Path

5K

DC "On" 0 V

"Off" -5 V

Isolation

Insertion Loss

Shunt FET Series FET

RF Path

5K

DC

"On" 0 V

"Off" -5 V

Insertion Loss

Isolation

Figure 3. Single Bit Configuration for Digital Attenuator

V2

RF1 RF2

V1

Fixed Pad

Fixed Pad

Figure 4. FET as a Variable Resistor

Resistive Path

DC

1 mm FET

"0" V ~ 3.0

-1 V ~ 10

-2 V ~ 30

-3 V ~ 300

-3.5 V ~ 1000

-5 V ~ 10 k

Figure 5. Voltage Variable Attenuator (VVA)

in Tee Configuration





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