What is fT of a transistor?

[matbob]

ft transistor

Can anyone please say what is meant by transition frequency fT seen on transistor datasheets.

For BC547 fT=300MHz and for BF494 fT=120MHz.

Why it is low for BF494 compared to BC547 even though BF494 is a medium frequency transistor and BC547 is a low frequecy one?

 

[FvM]

transistor ft

fT is defined by the small-signal current gain dropping to 1. BF494 has been typically used for IF amplifiers or LF/MF receiver front ends. A BC547 wouldn't be used for such applications because of higher capacitances, although it has sufficient gain at this frequencies.

 

[matbob]

ft of a transistor

Thanks for your reply. But its still unclear for me.

Is it because the DC current gain hFE, is higher for BC547(110 to 800) compared to BF494(60 to 200)?

By capacitance, I think , you mean the input and output capacitance. I checked the datasheets again and found it to be 1pF for BF494 and 6pF for BC547 (max).
Input capacitance for BC547 was found to be 9pF and I was not able to find the same for BF494 as it is not specified in the datasheet I referred.
Could you please tell me why we should take this capacitance into account even though we require only higher gain.

Also please tell me which transistor is good for an FM transmitter circuit(80 to 150 MHz).

 

[FvM]

ft of transistor

To my opinion it's not the higher hFE respectively β, that makes the difference. For an RF amplifier, particularly the feedback capacity Ccb is a critical parameter. Because of the higher capacitance, BC547 is less suited as an RF amplifier, that's the whole story.

 

[arman_arian2005]

bf494

The frequency at which the current gain of the transistor from base to collector is unity. You shouldn't operate the transistor in frequencies near ft.