Copyright: 1996 - 2024
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...
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Challenges facing microwave filter engineers - 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
Gripes & Humor
-- CAE, CAD, &
Test & Measurement
Post subject: transient freq. vs. ic and transistor sizing
Posted: Fri Dec 30, 2005 12:51 pm
hello...i am designing a RF amplifier
for a certain gain. the transistor i am using provides the usual plot
for ft vs. Ic for a certain dimension...but i need to operate at a Ic
value which is higher than the Ic that gives max. ft....how do i insure
that i can use the Ic i want and still have that Ic correspond to max.
ft....obviously i increased the transistor size but that would drop
my ft at the Ic i want.
the related question how can i find out
where the max. ft is for my modified transistor size...is there any
way of plotting ft. vs. ic by yourself using cadence/ads, etc or in
other words given a certain transistor size and Ic value how can i figure
out what the ft is for size/Ic combination...thanx much
Post subject: FtPosted: Fri Dec 30, 2005
The issue isn't what the transition frequency ft is - that's
the frequency where your transistor becomes as useless as a lump of
coal. The issue is what gain (hfe or beta) you have at the operating
frequency. People often use the "rule of thumb" that the amplifier should
run at no more than ft/10, but that's not always possible. I encourage
you to look beyond ft.
You're obviously designing an IC - the
ordinary designer, with discrete devices, has no control over the geometry.
Unfortunately, the ft versus ic curve is characteristic of the process
and layout specifics of the transistor - so you don't really have much
This is really an issue you should take up
with your vendor's support staff. If they can't help you, you might
want to re-consider working with them, as they would be either uninterested
in your order (a bad sign), or incapable of intelligently analyzing
their product (also a bad sign).
Post subject: Posted: Fri Dec 30, 2005 2:00 pm
hello...but isn't there a way to estimate what my ft is using some sort
of analysis?? let's say i have fixed a size and Ic...now just want to
know what my ft would be....i remember something about you can do ac
analysis and then something but i don't recall it
Post subject: Posted: Wed Jan 04, 2006 11:14 pm
Joined: Sat Dec 31, 2005 9:19 am
Making the transistor bigger does not necessarily cause the peak
ft versus Ic to drop in frequency.
What ever you do, DO NOT bias
the transistor beyond the peak ft Ic because ft drops rapidly after
the peak and the ft will vary a lot part to part for the same bias current.
You can always test ft in simulation by running parametric simulations
sweeping both the frequency and the base input current. Then you need
a function that finds the peak ft frequency for each value of collector
current. Then you can plot peak ft versus current density or just current
if you like.