Definition of ft in terms of S21

Hi all, I don't have access to my books, since I have them packed, because I am moving to live in another city.

Could somebody therefore remind my of the definition of ft in terms of S21 ?

Kindest regards,

StoppTidigare

They are completely different items measured under different conditions.

Ft is the frequency for the common emitter configuration where the small signal AC collector current into a shorted load equals the base current. This measurement is done at low frequencies and the slope of the beta-frequency curve graph is projected to the crossover point.

S21 is the square root of power out/power in with a 50 ohm (or other finite value) load.

These two measurements are only slightly related.

Hi, flatulent.

I was reading a datasheet of the Philips BFG425W NPN transistor, and it is said that it has a ft of 25 GHz.

Simulating it, I found that S21 has 0 dB of gain at around 8.9 GHZ.

I had the idea that ft was defined as the frequency where S21 = 0dB....

Still I am must be way wrong. I guess I've implemented the transistor erroneously..

Kindest regards,

StoppTidigare

One thing that is vastly different between these two measurements is the load. The Miller effect is not active in Ft because the output is shorted and there is no voltage gain.

what is Ft for a fet tarnsistor?