Lecture6sankary

Lecture6sankary - Introduction to Electronics ECED3201-...

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Page 1 ECED3201- Introduction to Electronics Prof. Kamal El-Sankary, Ph.D Kamal.El-Sankary@Dal.ca Introduction to Electronics
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Page 2 Part II: Bipolar Junction Transistors (BJTs) (cnt’d) ECED3201- Introduction to Electronics
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Page 3 Lecture Outline ECED3201- Introduction to Electronics IV. Analysis of BJT circuits V. Biasing in BJT Amplifier Circuits VI. Single-Stage BJT Amplifiers
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Page 4 Lecture Outline ECED3201- Introduction to Electronics IV. Analysis of BJT circuits V. Biasing in BJT Amplifier Circuits VI. Single-Stage BJT Amplifiers
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Page 5 IV.1. DC Analysis vs. Small-Signal Analysis Transistors must be biased because: – They must operate in the active region, and – Their small-signal model parameters are set by the bias conditions. Steps for using small-signal models 1. Determine the DC operating point of the BJT – in particular, the collector current I C 2. Calculate small-signal model parameters: g m , r e ,r 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 one depending on surrounding circuitry 5. Analyze
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Page 6 IV.1. DC Analysis vs. Small-Signal Analysis (cnt’d) DC analysis AC analysis
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Page 7 IV.2. Graphical Analysis Can be useful to understand the operation of BJT circuits First, establish DC conditions by finding I B (or V BE ) Second, figure out the DC operating point for I C
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Page 8 IV.2. Graphical Analysis (cnt’d) Apply a small signal input voltage and see how i b or v be translate into v ce Can get a feel for whether the BJT will stay in active region of operation The location of the dc bias point in the i C - v CE plane significantly affects the maximum allowable signal swing at the collector – The positive peaks of v ce cannot go beyond V CC , otherwise the transistor enters cut-off region – The negative peaks of v ce cannot go below 0.2 or 0.3V otherwise the transistor enters saturation region
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Page 9 What happens if R C is larger or smaller? –much larger R C Load-line A results in bias point Q A with a corresponding V CE which is too close to V CC and thus limits the positive swing of v CE (there isn’t sufficient “head room”) –much smaller R C load-line B results in an operating point too close to the saturation region, thus limiting the negative swing of v CE (there isn’t sufficient “leg room”) IV.2. Graphical Analysis (cnt’d)
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This note was uploaded on 02/12/2011 for the course ECED 3201 taught by Professor Al-sankary during the Spring '11 term at Dalhousie.

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Lecture6sankary - Introduction to Electronics ECED3201-...

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