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Question about Ft (cut-off frequency)

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

Reply to
Joe
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It's 20+ years since I did this but..... I think it's because in practice the Ft you measure won't be at the same frequency as that predicted by extrapolation - at such high frequencies secondary and parasitic effects take over. So which Ft is more useful? Let's say you are trying to predict the gain at a particular frequency. If you use the Ft extrapolated from the data and -8dB/decade you the right answer (eg a point somewhere near the actual measured data). If you use the measured Ft and -8dB/decade then you are you will be out.

... and I've long since forgotten that stuff!

Colin

Reply to
CWatters

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