Ft - Current-Gain - Bandwidth - why f = 200 MHz??

[ahmed osama]

Dear all

in a bjt datasheet i find something strange

Current-Gain - Bandwidth Product
(IC = 50 mAdc, VCE = 15 Vdc, f = 200 MHz) Ft = 1200 MHz

i understand why IC & VCE but why f= 200 Mhz

thanks
bye

 

[FvM]

Ft is a fictional parameter that doesn't exist as such in a device. It has to be calculated from a gain measurement at a particular frequency, in this case 200 MHz. You'll notice, that the result will be different for different measurement frequencies, because the real device is more complex than a simple model can represent.

 

[ahmed osama]

Ft is a fictional parameter that doesn't exist as such in a device. It has to be calculated from a gain measurement at a particular frequency, in this case 200 MHz. You'll notice, that the result will be different for different measurement frequencies, because the real device is more complex than a simple model can represent.

we get ft by shorting the output in the AC simulation and we begin to sweep the frequeny at a certain DC op pt (IC 50ma Vce 15vdc) and see when the current gain drops to 1 so for example in our case the gain will drop to 1 at 1.2Ghz

so we sweep the freq. so why 200 !!!
thanks
bye

 

[FvM]

so why 200 !!!

For the practical reasons I already mentioned, the manufacturer apparently preferred to measure it at 200 MHz.

It's a different thing to measure a real transistor, or to "measure" ft in a simulation of an ideal device. The latter will retrieve the said fictional ft parameter that has been put into the simplified model before.

Or simply accept, that the manufacturer wanted to tell the exact conditions how he determined ft. He don't need to explain why, you don't need to understand.

 

[ahmed osama]

For the practical reasons I already mentioned, the manufacturer apparently preferred to measure it at 200 MHz.

It's a different thing to measure a real transistor, or to "measure" ft in a simulation of an ideal device. The latter will retrieve the said fictional ft parameter that has been put into the simplified model before.

Or simply accept, that the manufacturer wanted to tell the exact conditions how he determined ft. He don't need to explain why, you don't need to understand.

well umm i guess i got u now , but now if i am at lab and i get the gain at 200 Mhz say 100 so how can i calc. the Ft. ?? thanks alot
bye

 

[FvM]

The assumption for calculating ft is a single pole roll-off of the current gain. Then the (small signal) current gain β is decreasing above a cut-off frequency fg, in the simplified model the product f*β equals ft. A transistor with β = 100 at 200 MHz would have a ft of 20 GHz, which is unrealistic at least for a silicon BJT.