Hi there, I am trying to understand more about High Frequency RF measurements of Bipolar Transistors. I have a question about measuring Ft.
Ft is defined as the frequency for which the magnitude of the AC current gain (h21) drops to unity for a transistor in the common-emitter configuration with an AC-shorted collector. Now most of the readings I have done discuss calculating Ft through a method of extrapolation. They discuss that there is a -8dB/decade slope in the curve of (h21) vs Frequency, and that a measurement should be taken in the region displaying this behaviour, then a linear extrapolation (in the semi-log plot) should be done to determine the cut-off frequency.
Now my question is why do we even bother with the extrapolation? Why not attempt to measure Ft at the cut-off frequency? I understand that Ft is measured using S-parameter data which is converted to h-parameter then the calculation takes place, but why not devise a system to systematically calculate h21 at each frequency and increment the frequency till h21 is equal to unity (i.e. stop when h21 = 1).
Is there a stability reason, or is it something else?
Thanks in advance for the help!!!
:D