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Announcements

Announcements. Assignment 2 due now Assignment 3 posted, due Thursday Oct 6 th First mid-term Thursday October 27 th. Lecture 9 Overview. Transistors. Transistors. Semiconductor device First Active circuit element - gain > 1 Discuss the Bipolar Junction Transistor only

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Announcements

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  1. Announcements • Assignment 2 due now • Assignment 3 posted, due Thursday Oct 6th • First mid-term Thursday October 27th

  2. Lecture 9 Overview • Transistors

  3. Transistors • Semiconductor device • First Active circuit element - gain > 1 • Discuss the Bipolar Junction Transistor only • See Simpson Chapter 5 for more detail.

  4. Collector Base Emitter Bipolar Junction Transistors • NPN Bipolar Junction transistor shown (PNP also possible) • 3 terminals: Emitter, Base, Collector • Contains 2 p-n junctions: emitter-base junction, collector-base junction • Can be thought of as two back-to-back diodes, but operating characteristics are very different • Base region (P-type here) must be thin for transistor action to work

  5. Collector Base Emitter Modes of operation “OFF” “ON” • Use 2 voltage supplies to bias the two junctions (forward or reverse) • 3 basic modes: cutoff, active and saturation, correspond to three different bias conditions.

  6. Active Mode • Collector-Base junction is reverse biased • Emitter-Base junction is forward biased • iC=βiB (β=100 - 500)Active circuit element - gain > 1 !! • How does collector current flow when collector-base junction is reverse biased? iC + iB - VCE Base Collector Emitter VBE iE

  7. Active Mode Direction of current flow + - • What's happening? • Emitter-base is forward biased; collector-base reverse biased. • Forward bias of emitter injects electrons into thin base region • Majority shoot through the base into the collector region where they encounter the voltage source on the collector and produce a current. • Electrons combine with holes in the base region and form negative ions which impede the flow • Drawing off negative charge via the base lead reduces this effect (“making the base smaller”) - so the base current controls the flow of electrons into the collector • Nobel Prize 1956; Shockley, Bardeen & Brattain

  8. Bill Shockley Born 1910 London, England Died 1989 Stanford, California

  9. Active Mode • Can relate iB and iC • βis defined mainly by the properties and geometry of the materials • Ideally constant for any particular type of transistor • Typically around 500 • "Common-emitter current gain" • Actually varies with temperature and emitter current • Collector current is controlled by the base current

  10. Emitter current • Emitter current is the sum of iB and iC(KCL) where • αis called the common-base current gain (~1.0)

  11. Circuit Symbols and Conventions • BJTs are not symmetric devices • Doping and physical dimensions are different for collector and emitter • Collector largest, connected to heat sink as most power dissipated there • Emitter region smaller, and more heavily doped to provide an abundance of charge carriers • Base region is very thin (~50nm) to enhance probability that electrons will cross it • PNP devices also exist - diode senses are reversed, so bias voltage polarities must also be reversed

  12. iC IC VBE i-v Characteristics + iB - • Simplest model for low frequencies (DC condition) "Ebers-Moll". • Relates collector current ICto base-emitter voltage VBE: VCE Base Collector Emitter VBE iE • IS=Saturation Current • Similar to Diode Law • Recall IB=IC/β • Collector current is controlled by the base-emitter voltageVBE

  13. BJT Amplifier "Common emitter" configuration Note labelling scheme: iC=IC+ic • To act as an amplifier, first bias the transistor to get it into active mode • Then superimpose a small signal vbeon the base • Under DC conditions: Added resistor RC

  14. BJT Amplifier • DC condition biases the BJT to the point Q on the plot • Adding a small voltage signal vbe translates into a current signal that we can write as: DC component IC • If vbe/VTis small: Small signal component ic So the small signal current is: i.e. a voltage-to-current amplifier; small signal collector current determined by base-to emitter voltage gmis the "transconductance"; corresponds to the slope at point Q

  15. More on small-signals • Base current also varies with vbe IB =DC component of base current iB ib = small signal component of base current iB So, having small signal voltage at the base, and small signal current at the base, consider the small signal equivalent resistance into the base, rπ: Alternatively, rearrange to give small signal resistance between base and emitter, re:

  16. Small signal models Use these relations to represent the small signal model for the transistor by an equivalent circuit. Hybrid-π model: e.g. as a voltage controlled current source: or as a current controlled current source

  17. Small signal models Other small signal models may sometimes be more convenient. T model:

  18. Using small signal models 1) Determine the DC operating conditions (in particular, the collector current, IC) 2) Calculate small signal model parameters: gm, rπ, re 3) Eliminate DC sources: replace voltage sources with shorts and current sources with open circuits 4) Replace BJT with equivalent small-signal models. Choose most convenient depending on surrounding circuitry 5) Analyze

  19. Voltage gain with small signal model ic + RC eliminate DC sources and apply T-model - vc ic + vbe re vbe - • To convert the voltage controlled current source into a circuit providing voltage gain, connect a resistor to the collector output and measure the voltage. • Find the gain using a small signal model:

  20. Summary of useful equations • Basic DC operating conditions: • Add a small signal:

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