Post subject: Ft, Fmax and transistor operating point Posted:
Wed Nov 09, 2005 5:19 pm
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
Post subject: ftPosted: Wed Nov 09, 2005
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.
Post subject: Posted: Thu Nov 10, 2005 5:13
Joined: Mon Jun 27, 2005 2:02 pm
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