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Lecture #27

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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Lecture #27

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  1. OUTLINE BJT small signal model BJT cutoff frequency BJT transient (switching) response Reading: Finish Chapter 12 Lecture #27 EE130 Lecture 27, Slide 1

  2. Small-Signal Model Common-emitter configuration, forward-active mode: “hybrid-pi” BJT small signal model: Transconductance: EE130 Lecture 27, Slide 2

  3. Small-Signal Model (cont.) where QF is the magnitude of minority-carrier charge stored in the base and emitter regions forward transit time EE130 Lecture 27, Slide 3

  4. Example: Small-Signal Model Parameters 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 c) EE130 Lecture 27, Slide 4

  5. Cutoff Frequency, fT 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

  6. For the full BJT equivalent circuit: fT is commonly used as a metric for the speed of a BJT. SiGe HBT by IBM To maximize fT: • increase IC • minimize CJ,BE, CJ,BC • minimize re, rc • minimize tF EE130 Lecture 27, Slide 6

  7. Base Widening at High IC: the Kirk Effect • At very high current densities (>0.5mA/mm2), base widening occurs, so QB increases. • tF increases, fT decreases. 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, 1.5V, 3V. EE130 Lecture 27, Slide 7

  8. Summary: BJT Small Signal Model Hybrid-pi model for the common-emitter configuration, forward-active mode: EE130 Lecture 27, Slide 8

  9. BJT Switching - Qualitative EE130 Lecture 27, Slide 9

  10. Turn-on transient where IBB=VS/RS • We know: • The general solution is: • Initial condition: QB(0)=0. since transistor is in cutoff EE130 Lecture 27, Slide 10

  11. Turn-off transient • We know: • The general solution is: • Initial condition: QB(0)=IBBtB EE130 Lecture 27, Slide 11

  12. Reducing tB for Faster Turn-Off • The speed at which a BJT is turned off is dependent on the amount of excess minority-carrier charge stored in the base, and also the recombination lifetime tB • By reducing tB, the carrier removal rate is increased Example: Add recombination centers (Au atoms) in the base EE130 Lecture 27, Slide 12

  13. Schottky-Clamped BJT • When the BJT enters the saturation mode, the Schottky diode begins to conduct and “clamps” the C-B junction voltage at a relatively low positive value.  reduced stored charge in quasi-neutral base EE130 Lecture 27, Slide 13

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