Because of the high maintenance needed to monitor and filter spammers from the RF Cafe Forums, I decided that it would
be best to just archive the pages to make all the good information posted in the past available for review. It is unfortunate
that the scumbags of the world ruin an otherwise useful venue for people wanting to exchanged useful ideas and views.
It seems that the more formal social media like Facebook pretty much dominate this kind of venue anymore anyway, so if
you would like to post something on RF Cafe's
page, please do.
Below are all of the forum threads, including all
the responses to the original posts.
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.