Sampler diodes with more barrier height?

John Larkin wrote:
> On Thu, 27 Feb 2014 17:56:05 -0800, Joerg <invalid@invalid.invalid>
> wrote:
> 
>> John Larkin wrote:
>>> On Thu, 27 Feb 2014 17:59:16 +0100, Gerhard Hoffmann <dk4xp@arcor.de> wrote:
>>>
>>>> Am 27.02.2014 16:02, schrieb John Larkin:
>>>>> Several people still make SRDs. M-pulse, Metelics, MAcom (all SRD makers start
>>>>> with "M")
>>>>>
>>>>> MA44767 and MA44769 are SOT23 distributor items, 50-75 cents range.
>>>>>
>>>> Ah, found them under varactors @Mouser, at least the MA144769-287T
>>>> But 600 ps transition time is not that wonderful anymore, got
>>>> half a ns from Fairchild NC7sz04p5x :-)
>>>> Nobody escapes the CMOS steamroller!
>>>>
>>>> thanks, Gerhard
>>> The 44769 is rated 150 ps transition time, but they will go a bit faster if you
>>> drive them hard.
>>>
>>> The fastest catalog items are around 25-30 ps. 
>>>
>>> http://www.aeroflex.com/AMS/Metelics/micro-metelics-prods-SRDs-beamlead.cfm
>>>
>>> Somebody makes some really fast 0402 sized parts, can't remember who.
>>>
>>> Fast SRDs don't store much charge, so they are hard to drive.
>>>
>> <100psec should be doable with a BFP740 and similar ones with the base
>> driven sledgehammer-style. Of course, only very few volts allowed at the
>> collector.
> 
> Have you tried that? My experiments along those lines have been
> disappointing.
> 

Not with the BFP yet but with a BFR92 and it performed quite nicely.
What happened in your case? How did you drive them?


> PHEMTS switch screamingly fast, and the gates are relatively easy to
> drive. Good SRD drivers. In the old tek samplers, they usually used an
> avalanche transistor to drive the SRD. HP tended to use a transistor
> driving a slow srd driving a fast srd.
> 

I want to keep things simple. PHEMTs are nice but need a lot of fast
gate swing unless followed by an SRD. BJTs almost snap from zero to
saturation withing about 100mV on the drive ramp. I don't really care
about saturation and recovery because the PRF is low.

BJTs are often used for cheap transmitters at UHF or above. Example:

http://www.nxp.com/documents/data_sheet/BFG480W.pdf

Compression is at +20dBm at 2GHz which is pretty much full swing. Some
of the MMIC should also be able to do this but unfortunately have a
different pinout. Northrop-Grumman makes massive (and probably very
expensive) BJT if you need tons of pulse energy for pulsed radar:

http://www.northropgrumman.com/Capabilities/BipolarRFTransistors/Documents/WPTB48F2729Cx.pdf?pdf=Datasheet

-- 
Regards, Joerg

http://www.analogconsultants.com/