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Distributed Amplifiers

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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.

Distributed Amplifiers

Here's a cascode distributed amp, made by Bookham:

Distributed Amplifiers

Vintage distributed amplifier!

Bonus for November 2018!  Bruno from Italy sent us these pics of a pretty old distributed amplifiers... but we'll let him explain!

Please find attached the front and back photos of a quite old DA that was designed and built here at RAI (Italian public radio/TV broadcasting company) R&D, where I have worked since 1988.

I had picked up this DA  from the scrap heap many years ago, but I ignored the exact diagram and its features. It is marked “power amplifier for 20MHz link”. Probably it was a kind of video distribution amplifier. Why distributed? Probably to obtain “high”, linear, power at “wide” bandwidth was not easy in that era (end of the sixties – beginning of the seventies?).

A couple of parts of the DA had been salvaged in the output circuit. Maybe one day I will manage to find some spare time to fix it and to test it with a VNA just out of curiosity to see how it performs!

 

 

Editor's note: considering that capacitors in this unit are fifty years old, you might want to replace some of them before you fire it up.  Especially if it was stored in a place with high humidity or was hotter than room temperature (outdoors, basement, attic). Any electrolytic or or paper caps should be replaced before turn-on, but the ceramic caps (the discs) in the RF paths should be OK.  Here's a website on this subject...Good luck and send us some data!  UE 

 

Author : Unknown Editor

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