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

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 β >>1 and C C ~0 we get A I ~ with h ie � Let’s call ω T the ω where A I =1 ω T

� We find ( f T =2 πω T is the transition frequency ) and g m

� Similarly, for a FET in CS configuration, we get � For a MOSFET, it can be found [Fonstad] that � so,

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