Gaasfet switch driver needed

Oct 26, 2006 4 Replies
C

I need a switch driver that can switch in 1 to 3 nsec. Complementary drive outputs, 0 to -5 or -8 volts. Driving a SPDT Gaasfet switch.



I've tried differential A-to-D drivers but they are not fast enough. Input to the driver is from a cmos asic. Also looked at comparators with ECL output but they don't have the voltage swing.



Impellimax makes a driver but they want mucho$.



Other constraints. Battery powered(low powered). Small size.



I'm looking at a discrete driver or biasing the Gaasfet switch.



Can bias the Gaasfet switch for a 5V ground and use 0 to 5V switch outputs for the driver. This will be at 4.5GHz. Anyone ever done this? What are the ramifications? Lost isolation? Bad RL? If I go this way, is CPWaveguide the way to go to minimize via inductance?



Charlie Luke Luke Engineering, Inc.


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J

Shifting the supplies would sure be easiest. I can't see why that would affect the RF, since you're probably AC coupling that in/out anyhow. Now you could use fast CMOS gates as drivers, and have zero static power dissipation. Maybe a TinyLogic flipflop, or a tiny dual XOR, as a diff driver?

If you wanted to stay with the negative supply, an LVDS-to-TTL receiver chip would be the basis for a decent driver. Say a DS90C402 powered by ground and -5, with a little level shifting on the input.

500 ps edges!

Or just shift a 5-volt CMOS swing down 5 volts, to 0 and -5, with a bypassed zener or bandgap reference and a pulldown resistor... the GaAs gate input probably needs very little current.

I've used the Hittite GaAs switches at speeds like this, but the ones I use are TTL compatible when run from +5 and ground. No surprises, except that whan they say "DC to xx GHz" they don't really mean DC... check the S-params for hints of low frequency funnies.

CPW is fine for stuff like this. Avoid signal vias if possible, but

4.5 GHz isn't *really* fast.

Whose switch are you using? Why do you need to switch so fast?

John

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R

What is your control signal ? TTL 50 Ohms ? PECL, LVPECL, NECL, LVDS ? And you need to provide a current down into -5V .. -8V ?

Rene

Ing.Buero R.Tschaggelar - http://www.ibrtses.com & commercial newsgroups - http://www.talkto.net
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R

The last question was to verify an open collector. For a pin diode it very like is.

Rene

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C

How about using a pair of gaasfet or phemt as a driver ? There was also an ecl driver mentioned here with larger voltage swing although I dont think it was that much.

Colin =^.^=

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