Re: Gaasfet switch driver needed

On Thu, 26 Oct 2006 22:37:13 GMT, no_email@xxxxxxxxxxxxx (Charles
Luke) wrote:

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.


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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