November 21, 2004
to go to our main page on amplifiers.
Fellow microwave Dudes, we sure
could use some pictures to go along with this text! Send us a picture
and you'll receive a cool Microwaves101
pocketknife! Don't make us draw stuff with Microsoft Word, that
gets ugly for everyone.
A distributed amp is a clever
way to provide enormous bandwidths, as much as 100 GHz. Some distributed
amplifiers can operate down to DC as well, so they are used as opto-electronic
amps. The theory behind the distributed amplifier is that a number
of FETs (at least two but more typically four, five or six) are
fed by a periodic structure at the input that resembles a terminated
transmission line. The combination of FET capacitance with the high-impedance
connection lines resembles a lumped-element version of a fifty-ohm
line. This trick is used to impedance match the input and output.
One limitation of distributed
amplifiers is they don't make efficient power amplifiers. The load
that each FET sees is not even close to optimum, and the signal
distribution is such that some FETs get far greater voltage stress
than others. They make mediocre LNAs for the same reasons.
Here are some pictures of distributed
amplifiers. The first one is from Agilent.
Here's a cascode
distributed amp, made by Bookham: