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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 1:35 pm

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 flexibility here.

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).

Good Luck!



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

Posts: 4

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.



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