What is transition frequency of a transistor?

 Yesterday, when I read about NPN transistor BC547B and the corresponding PNP transistor BC557B, I noted that the former had a typical transition frequency (fT) of 300 MHz and the latter had a typical transition frequency of 150 MHz. Now what is this transition frequency? In simple terms, it can be considered as the frequency at which the amplifier gain becomes unity. That is the highest frequency below which the transistor can function as an amplifier. It indirectly reflects the total time current carriers (electrons or holes) take to travel from the emitter to the collector. In actual circuits, the effective higher end operating frequency will be only a fraction of the transition frequency as parasitic capacitances lower the maximum frequency at which it can operate. 


Radio amateurs planning to design radio frequency circuits will be interested in transistors with higher transition frequency which we often call as RF transistors, in comparison to AF transistors for audio frequency applications which need only lower transition frequency. While designing RF circuits, methods of implementing frequency stability is also important, especially as you go from HF to VHF/UHF/SHF ranges. That is where a GPS Disciplined Oscillator (GPSDO) is needed for good frequency stability. GPSDO also known as GPS Clock, can be considered as a combination of a Global Positioning System receiver and a high quality stable oscillator whose output is controlled by the signals broadcast by GPS or other Global Navigation Satellite System satellites.



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