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 7:30
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
Post subject: Posted: Thu Nov 10, 2005 5:13 pm
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