Ft, Fmax and transistor operating point - RF Cafe Forums
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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
Post subject: ftPosted: Wed Nov 09, 2005 7:30 pm
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 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