What is the meaning of transition frequency?

transistor transition frequency

Can someone tell me the definition of transition frequency of a BJT? Is it same as the cutoff frequency? Thank a lot! 8O

transition frequency ft

fT: Frequency Transition - the frequency (MHz) where gain falls to unity.

transition frequency

The exact definition of transistion frequency is frequency at which the current gain is equal to 1. So you put a current source between base and emittor, and a shortcut between collector and emittor.
1. ft cannot be measured (frequencies are too high to be able to define well controlled current source). It is defined through interpolation.
2. most of the time the transistor operating frequency is a fraction of the ft. (like 1/5)
3. this is just a characteristic of the die. Bond wires introduce parasitics that lower the actual ft.
So ft is just a so called "figure of merit" to compare different transistors. It does not have any direct use, because at the frequency ft, parasitics dominate transistor performance! When designing chips, ft has a clearer meaning and can be simulated!

transition frequency transistor

I really appreciate your help, your answer is very important for me,,,Thanks a lot

ft transition frequency

What about freq transition in MOS?
How do we measured the ft? 8O

meaning of frequency

One more question. For the application switching circuit, is it a transistor with higher transition freq. will have faster switching speed?

Thanks a lot
Best rgd

mos transition frequency

hi,
the answer is yes,one problem in switching transistor are parasitic capacitance,higher ft => lower parasitic capacitance.I think you know what happen with pulse when go through RC circuit.

transition frequency mos

The transition frequency in MOS is defined completely the same way. It is correct that de DC input resistance is very high. But at higher frequencies, the input capacitance becomes dominant. So, the input voltage get smaller as frequency rises with a constant current source on the input. So yes, the same definition goes for MOSFETS!

Hello
Radiohead:

The exact definition of transistion frequency is frequency at which the current gain is equal to 1.

Click to expand...

Why current gain? Why voltage gain isn't used?

april1975 said:

Hello
Radiohead:

Why current gain? Why voltage gain isn't used?

Click to expand...

Short circuited current gain...

The transition frequency in MOS is defined completely the same way.

Click to expand...

I'm not aware of a transit frequency with RF MOSFET. Where do you see it?