Drivers for GaAs FET Switches and Digital Attenuators

Rev. V5

Application Note

S2079

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2

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M/A-COM Technology Solutions Inc. and its affiliates reserve the right to make

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

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.

The SW-277, SW-349, SW-394,

and SW-399 include level shifting components for

compatibility with positive CMOS or TTL control voltages,

but these switches still require complementary control

logic.

The SW-335, SW65-0xxx series and related

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

fabrication

techniques

for

low

power

consumption.

The devices consist of input buffers, inverters to

generate

complementary

logic

values,

voltage

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.

Although GaAs FET

switches and attenuators will operate well with nominal 0

V and -5 V for control, careful selection of the control

With proper selection of positive and negative supply

voltage, the SWD-109 and SWD-119 can both provide

output control voltages in these ranges.

Another consideration in design with switches and

attenuators is the elimination of crosstalk that can arise

from RF leakage onto control lines.

Most board

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.

Figure 3: GaAs SPDT Switch with CMOS Driver

Control A Control B

RF Common

to RF 1

RF Common

to RF 2

TTL Low

TTL High

ON

OFF

TTL High

TTL Low

OFF

ON

RF Common

R1

+5 VDC

C1

Q2

Q3

RF1

C5

Q1

Q4

C4

RF2

R3

+5V

C3

Control

"B"

Control

"A"

R2

+5V

C2

TTL Control

+5 VDC

C6

VCC

GND

CD54HCT04