BJT e MOS at HF Let us compute the A I in CE configuration: With R - - PowerPoint PPT Presentation

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Mar 16, 2023 158 likes •216 views

BJT e MOS at HF Let us compute the A I in CE configuration: With R L =0 we find h ie h ie With >>1 and C C ~0 we get A I ~ with h ie Lets call T the where A I =1 T We find ( f T =2 T is the transition

SLIDE 1

BJT e MOS at HF

SLIDE 2

Let us compute the AI in CE

configuration:

With RL=0 we find With β >>1 and CC ~0 we get

with

Let’s call ωT the ω where AI=1

AI ~ hie hie hie ωT

SLIDE 3

We find

(fT=2πωT is the transition frequency) and

SLIDE 4

Similarly, for a FET in CS

configuration, we get

For a MOSFET, it can be

found [Fonstad] that

so,

SLIDE 5

[Fonstad p. 492] In a saturated FET the voltage across the channel is VGS -VT so, neglecting the channel length modulation, = (VGS-VT)/l The average speed of the electrons is µ, so the transit time τT is so that, apart from the 3/2 factor, we see that τT=1/ωT