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EE130/230M Review Session

EE130/230M Review Session. Small Signal Models for MOSFET/BJT MOS Electrostatics. BJT Small Signal Model. A small change in I B or V BE will result in a small change in I C and V CE. output current =. input voltage =. the cutoff frequency ( w T = 2 p f T ) is defined at.

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EE130/230M Review Session

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  1. EE130/230M Review Session Small Signal Models for MOSFET/BJT MOS Electrostatics

  2. BJT Small Signal Model • A small change in IBor VBEwill result in a small change in IC and VCE output current = • input voltage = the cutoff frequency (wT= 2pfT) is defined at • input current = output voltage = =

  3. Example A BJT is biased at IC = 1 mA and VCE = 3V. bdc= 90, tF= 5ps, T = 300K. Find (a) gm , (b) rp , (c) Cp .Solution: (a)(b) rp = bdc / gm= 90/0.039 = 2.3 kW (c) EE130/230M Spring 2013 Lecture 27, Slide 3

  4. MOSFET Small Signal Model • A small change in VGwill result in a small change in IDand VDS output current = • input voltage = • input current = the cutoff frequency (wT= 2pfT) is defined at output voltage = =

  5. Summary of Small Signal Models • Inputs/Outputs • Linearized equivalent circuits • Key components • Performances • Gain • Cut-off frequency

  6. MOS Capacitor Energy Band Diagram Problem: Solution:

  7. Flat-band Condition

  8. Equilibrium Condition

  9. Accumulation Condition

  10. Strong Inversion Condition

  11. Charge distribution Charge amount Capacitance Equivalent circuits

  12. MOS Capacitor Charge vs. Gate Voltage Accumulation Depletion Inversion VT Flat-band voltage Maximum depletion charge

  13. MOS Capacitance vs. Gate Voltage Accumulation Inversion Depletion Minimum capacitance VT Flat-band voltage

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