S2079 MA-COM [M/A-COM Technology Solutions, Inc.], S2079 Datasheet - Page 3

• Manufacturer Part Number: S2079
• Description: Drivers for GaAs FET Switches and Digital Attenuators
• Manufacturer: MA-COM [M/A-COM Technology Solutions, Inc.]

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Drivers for GaAs FET Switches and Digital Attenuators
Application Note
S2079
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.
related to the CD54HCT04 that work well include the
SOIC
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.
or
plastic
DIP
packaged
Other hex inverters
CD74HC04
and
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
Conclusion
This application note has explained how to control
M/A-COM's GaAs FET switches and digital attenuators
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.
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