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Ft, Fmax and Transistor Operating Point - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Post subject: Ft, Fmax and transistor operating point Posted: Wed Nov 09, 2005 5:19 pm
hello guys,

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 pm
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.

Good Luck!


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

Site Admin

Joined: Mon Jun 27, 2005 2:02 pm
Posts: 373
Location: Germany
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

Good luck!

Best regards,

- IR

Posted  11/12/2012

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