Anatech Electronics (RF Filters) - RF Cafe
Berkeley Nucleonics Academy RF Boot Camp - RF Cafe
 

About RF Cafe

Kirt Blattenberger - RF Cafe Webmaster

Copyright: 1996 - 2024
Webmaster:
    Kirt Blattenberger,

    BSEE - KB3UON

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

AirplanesAndRockets.com

Electronics World articles Popular Electronics articles QST articles Radio & TV News articles Radio-Craft articles Radio-Electronics articles Short Wave Craft articles Wireless World articles Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs LinkedIn Crosswords Engineering Humor Kirt's Cogitations RF Engineering Quizzes Notable Quotes App Notes Calculators Education Engineering magazine articles Engineering software Engineering smorgasbord RF Cafe Archives RF Cascade Workbook 2018 RF Symbols for Visio - Word Advertising RF Cafe Homepage Thank you for visiting RF Cafe!
Exodus Advanced Communications Best in Class RF Amplifier SSPAs

Ft, Fmax and transistor operating point - RF Cafe Forums

RF Cafe Forums closed its virtual doors in 2012 mainly due to other social media platforms dominating public commenting venues. RF Cafe Forums began sometime around August of 2003 and was quite well-attended for many years. By 2010, Facebook and Twitter were overwhelmingly dominating online personal interaction, and RF Cafe Forums activity dropped off precipitously. If the folks at phpBB would release a version with integrated sign-in from the major social media platforms, I would resurrect the RF Cafe Forums, but until then it is probably not worth the effort. Regardless, there are still lots of great posts in the archive that ware worth looking at.

Below are the old forum threads, including responses to the original posts.

-- Amateur Radio
-- Anecdotes, Gripes & Humor
-- Antennas
-- CAE, CAD, & Software
-- Circuits & Components
-- Employment & Interviews
-- Miscellany
-- Swap Shop
-- Systems
-- Test & Measurement
-- Webmaster

guest
Post subject: Ft, Fmax and transistor operating point Posted: Wed Nov 09, 2005 5:19 pm
hello guys,

why is there a rule of thumb that one should operate a transistor at 1/5 of its ft (transient frequency)? Let's say I don't buy this rule and try to operate at a frequency that's 1/3 of ft. what's wrong in doing so and how would it affect my circuit?

also, there are plots for ft or fmax vs. Ic or Id....people make a point of where the maximum ft is occuring (at how much collector/drain current). my question is why would i care about it if i'm going to operate at 1/3 of ft and i'm going to bias at a certain point for my design and that bias tells me how much current i want to pass through collector/drain?

similarly, looking at ft vs. Id plot, i see that to operate at 1/3 of ft i need a certain amount of Id but my design calculations tell me that i need to dc bias at a lower current than what i'm reading from ft vs. Id plot. so if i dc bias it at the point that my calculations lead to, would i not be able to operate the transistor at 1/3 of ft? answers are much appreciated...thanx


Top

Guest
Post subject: ftPosted: Wed Nov 09, 2005 7:30 pm
First, a minor point: "ft" means "transition frequency".

It's the frequency where the transistor is useless - you can't build an amplifier of any use with it.

How it affects your circuit: if you run your transistor at ft/3, you'll have a lot less gain than at ft/5 or some larger ratio.

If your parameters Id and Vds are already determined, then so is ft - and ft won't be the same as the data sheet value, unless by coincidence your values were the same as the manufacturer's test conditions.

That might mean you would have even less gain than you thought, even counting the ft/3 bit.

You might want to consider a different transistor.

Good Luck!


Top

IR
Post subject: Posted: Thu Nov 10, 2005 5:13 pm

Site Admin


Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
Rules of thumb are the result of many years of experience, and data sheets are the outcome of the companies which developed the transistors/ devices that you are using. Both of these are meant to protect you from making mistakes! Use them and don't go against them

Good luck!

_________________
Best regards,

- IR



Posted  11/12/2012
KR Electronics (RF Filters) - RF Cafe
Werbel Microwave - RF Cafe
LOTUS Communications Systems Modular RF Component Building Blocks - RF Cafe
 

Please Support RF Cafe by purchasing my  ridiculously low−priced products, all of which I created.

These Are Available for Free