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Unformatted text preview: T qI C 2 sat ( ) C Jason Woo 51 EE121B Spring 2010 Thus, to have good high-frequency AC performance, we need to have 1 2 3 High IC Small B short base and large "built in" field Small C higher doped collector. Note that (3) is in contradiction to small CBC Jason Woo 52 EE121B Spring 2010 Large signal switching performance of a BJT
In addition to the EC mentioned above, the RC delay (due to RB'B) is also important when BJT is used as a switch (such as in ECL circuit). Since switches are typically VB (not IB) controlled, the delay is characterized by
RB B ( C bc + C b e ) That is, to improve BJT performance, we need to eliminate any parasitic CP and R B'B. Jason Woo 53 EE121B Spring 2010 Using the large signal equivlanet model :
Ce + CD E IB B
C1 Rbi B IB Cc CS C S Rc
C S it can be shown that the delay can be expressed as Td k1 R1C c + k2 R1C e + k3 R1C s + k4 R1C1 + k5 + k6 Rbi C c + k7 Rbi C d + k8 Rbx C d
That is, fast switching speed requires small parasitic, and a thin base.
Jason Woo 54 EE121B Spring 2010...
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