Drivers for GaAs FET Switches and Digital Attenuators

Rev. V5

Application Note

S2079

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3

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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.

M/A-COM's dual control, negative bias switches and

attenuators have intrinsic switching speeds as low as ~ 5

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. As shown

in the appendix, switches that incorporate level shifting

on-chip typically have switching speeds ranging from

several tens of nanoseconds to microseconds.

Figure 3 shows a dual control GaAs FET switch driven

by the Texas Instruments CD54HCT04 high speed

CMOS logic hex inverter, a CERDIP packaged device

that can operate with CMOS logic input levels (0 V, 2.7

V) and drive TTL loads.

Driving a dual control switch

stage requires using two gates of the CD54HCT04, one

to generate a buffered output, one to generate its

complement. Each gate in the CD54HCT04 introduces a

switching propagation delay of 20 ns. The DC current

consumption of the entire hex device is less than 1 mA at

+ 5 VDC.

When designing with the CD54HCT04 as a driver,

choose the DC blocking capacitors C1, C4, and C5, to

give minimum insertion loss at the lowest desired

operating frequency. Choose the bypass capacitors C2

and C3 to give maximum isolation at the highest desired

operating frequency.

Bypass capacitor C6, which has

the same value as C2 and C3, shunts any RF signal

leakage on the DC bias line at the hex inverter to ground.

Use low series resistance, high Q capacitors, such as the

American Technical Ceramic ATC100A series, for the

lowest possible insertion loss.

The resistors R1 and R3 that connect the DC bias to the

switch should have a value in the range of 10 to 50

kilohms to keep RF crosstalk as low as possible. Place

the resistors, capacitors and ground vias as close to the

body of the switch as possible to reduce inductance for

the best RF performance.

Other popular logic IC's work well as drivers, depending

upon your requirements for switching speed, DC power

consumption, and RF linearity.

Other hex inverters

related to the CD54HCT04 that work well include the

SOIC

or

plastic

DIP

packaged

CD74HC04

and

CD74HCT04, the slower CD54HC04, and the Fairchild

DM74LS04.

With a 5 VDC supply voltage, the DM74LS04 provides

output logic voltages of 0.25 V (logic low) and 3.4 V

(logic high). To substitute the pin-compatible DM74SL04

for the CD54HCT04, you will have to add additional pull-

up circuitry connected between the driver and the switch

to raise the logic high to 5 VDC.

This will result in slower switching speed and higher

current consumption compared to the CD54HCT04.

The Texas Instruments SN54HC139 two to 4 line

decoder also works well, as does the CD4041UB quad /

true complement buffer. The CD4041UB provides four

pairs of complementary outputs, and can provide a range

of logic output voltages depending upon the supply

voltage that you choose. With the CD4041UB supplying

a bias of 8 VDC for the logic high, many GaAs FET

switches will operate with a higher P1dB power level,

higher by perhaps 5 or 6 dB.

For driver switching speeds less than 10 ns at the

expense of higher current consumption, consider using

an ECL driver such as the Motorola MC10H350 ECL to

TTL translator, as shown in Figure 4. This circuit can

drive the switch directly without the need for level shifting

capacitors and resistors.

Figure 4: MC10H350 for Driving Dual Control

Switches

Conclusion

This application note has explained how to control

M/A-COM's GaAs FET switches and digital attenuators

using

drivers

provided

by

M/A-COM,

or

using

commercially available digital logic IC's. The appendix

summarizes M/A-COM's most popular switches and

classifies them by drive requirements. Careful choice of

the switch or digital attenuator and the driver can provide

optimum RF linearity, fast switching speed, low power

consumption and small board footprint.