Quote:
You could make a generalised test circuit to determine the transistion frequency.
However, since you already know the circuit you want to use the transistors in, a 10.7MHz IF, one quick way to solve your problem is to set up the transistor as a single 10.7MHz IF amplifier stage with a 10.7MHz IF transformer.
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Yes, I may have misinterpreted the requirement. I assumed that Lee wanted to know the frequency response (good or bad) of all of the transistors. Otherwise it's best to just make a 10.7MHz test circuit. My little test circuit should work (for silicon BJTs at least) from a few MHz up to maybe 400MHz depending on layout and component quality.
I tried building my little test circuit today and it seems to give realistic results. I've only tried a jellybean silicon BJT (BC547B) but I tried testing it at 5Vce across collector current Ic ranging from 0.7mA through to 10mA. The Ft varied from about 85MHz at 0.7mA Ic to about 350MHz when Ic was up at 10mA. This seems about right?
It's also possible to predict Fmax from Ft if the device capacitance and base resistance is known.
To feed and probe my test circuit I used a Marconi RF sig gen and I used a Marconi 2380 1GHz RF probe feeding to an old HP 54540C 500MHz digital scope. However, for lower frequency Ge parts the 10R resistors could be replaced with 47R and a regular x10 scope probe with a Tek 465 scope could probably measure the Ft up to 50MHz or so. It should be able to display the RF waveform OK on the most sensitive setting on the 465 scope?
It's not an ideal test circuit for Ft but the general idea is as per the image below. This is a simple AC representation showing electron flow up from the emitter. When close to Ft the current sharing should be equal in the base path and the collector path and so the RF voltage at the collector and the base will be the same. This RF voltage can be measured with a decent RF probe.