OUTLINE BJT small signal model BJT cutoff frequency BJT transient (switching) response Reading : Finish Chapter 12. Lecture #27. Small-Signal Model. Common-emitter configuration, forward-active mode:. “hybrid-pi” BJT small signal model:. Transconductance:. Small-Signal Model (cont.).
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BJT small signal model:
EE130 Lecture 27, Slide 2
where QF is the magnitude of minority-carrier charge stored in the base and emitter regions
forward transit time
EE130 Lecture 27, Slide 3
A BJT is biased at IC = 1 mA and VCE = 3 V. bdc=90, tF=5 ps, and T = 300 K. Find (a) gm , (b) rp , (c) Cp .Solution: (a)(b) rp = bdc / gm= 90/0.039 = 2.3 kW
EE130 Lecture 27, Slide 4
The cutoff frequency is defined to be the frequency (f = w/2p) at which the short-circuit a.c. current gain equals 1:
EE130 Lecture 27, Slide 5
fT is commonly used
as a metric for the
speed of a BJT.
SiGe HBT by IBM
To maximize fT:
EE130 Lecture 27, Slide 6
Consider an npn BJT:
At high current levels, the density of electrons (n IC/qAvsat) in the collector depletion region is significant, resulting in widening of the quasi-neutral base region.
As W increases, the depletion width in the collector also increases, since the charge density decreases:
At very high current densities, the excess hole concentration in the collector is so high that it effectively extends the p-type base.
Top to bottom :
VCE = 0.5V, 0.8V,
EE130 Lecture 27, Slide 7
Hybrid-pi model for the common-emitter configuration, forward-active mode:
EE130 Lecture 27, Slide 8
EE130 Lecture 27, Slide 9
EE130 Lecture 27, Slide 10
EE130 Lecture 27, Slide 11
Example: Add recombination centers (Au atoms) in the base
EE130 Lecture 27, Slide 12
reduced stored charge in quasi-neutral base
EE130 Lecture 27, Slide 13