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

S2079

Manufacturer Part Number

S2079

Description

Drivers for GaAs FET Switches and Digital Attenuators

Manufacturer

MA-COM [M/A-COM Technology Solutions, Inc.]

Datasheet

1.S2079.pdf (4 pages)

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changes to the product(s) or information contained herein without notice.

Drivers for GaAs FET Switches and Digital Attenuators

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.

GaAs FET’s

GaAs MMIC control devices such as switches and

digital

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.

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.

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)

opposite (or complementary) gate biases.

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

corresponding to digital attenuation bits, switched in or

out of the transmission path, either individually or in

combination.

voltages for each state, while digital attenuators require

complementary bias voltage to activate each bit.

Application Note

S2079

attenuators

that

Switches require complementary bias

conduction

form

typically

The series FET's provide a

fixed

states

employ

attenuator

and

Field

The gate

therefore

Effect

pads,

For

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.

Dual Control Switch Truth Table

Typical complementary logic control voltages:

Logic low

Logic high

Figure 1: Typical Dual Control Switch (SW-239, etc)

Figure 2: Digital Attenuator Based on Switched Pads

Control A Control B

• North America Tel: 800.366.2266 • Europe Tel: +353.21.244.6400

• India Tel: +91.80.4155721

RF 1

RF1

-5 V

0 V

VC1

16 dB

(AT-230)

Pad

VC1

-5 V

0 V

Control

"A"

0 V to –2 V @ 20 μA max.

-5 V to 40 μA typ. To –8 V @ 200 μ

A max.

VC2

RF Common

8 dB

Pad

to RF1

VC2

RF Common

Off

• China Tel: +86.21.2407.1588

Control

"B"

VC3

RF Common

4 dB

Pad

to RF2

VC3

Off

Rev. V5

RF2

RF 2

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S2079 Summary of contents

Page 1

... Application Note S2079 Drivers for GaAs FET Switches and Digital Attenuators 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 ...

Page 2

... Application Note S2079 Drivers for GaAs FET Switches and Digital Attenuators Built-in Drivers Some of M/A-COM's newer switches and attenuators feature simplified control using CMOS ( TTL ( logic, with no need for negative control voltages. The Appendix to this application note lists some popular M/A-COM switches ...

Page 3

... Application Note S2079 Drivers for GaAs FET Switches and Digital Attenuators M/A-COM's dual control, negative bias switches and attenuators have intrinsic switching speeds as low ns. A disadvantage of level shifting by floating the FET's is that the time constants of the bypass and blocking capacitors charging through the internal FET gate resistors will introduce some switching delays ...

Page 4

... Application Note S2079 Drivers for GaAs FET Switches and Digital Attenuators Additional Notes: Application Note M537, GaAs MMIC Based Control Components with Integral Drivers defines performance parameters for switches and attenuators. 1. See Application Note M517, MASW6010 GaAs SPDT Switch Performance and Driver Circuit Techniques for additional information on designing with the SWD-109 and SWD-119 drivers ...

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