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ANALOGUE ELECTRONICS I

ANALOGUE ELECTRONICS I. EMT 112/4 Basic BJT Amplifiers (Part 1). Analog Signals & Linear Amplifiers. Analog signals Natural analog signals: physical sense (hearing, touch, vision)

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ANALOGUE ELECTRONICS I

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  1. ANALOGUE ELECTRONICS I EMT 112/4 Basic BJT Amplifiers (Part 1)

  2. Analog Signals & Linear Amplifiers • Analog signals • Natural analog signals: physical sense (hearing, touch, vision) • Electrical analog signals: e.g. output from microphone, output signal from compact disc – form of time-varying currents & voltages • Magnitude: any value which vary continuously with time • Analog circuits • Electronic circuits which produce analog signals • E.g. linear amplifier • Linear amplifier • Magnifies input signal & produce output signal that is larger & directly proportional to input signal DC voltage source • Low signal power • High signal power DC power Block diagram of a compact disc player system (a) (b) Signal source Amplifier Load

  3. The Bipolar Linear Amplifier • To use circuit as an amplifier, transistor needs to be biased with DC voltage at quiescent point (Q-point) transistor is biased in forward active region • Time-varying output voltage is directly proportional to & larger than time-varying input voltage  linear amplifier (a) Bipolar transistor inverter circuit; (b) inverter transfer characteristics

  4. The Bipolar Linear Amplifier • Summary of notation

  5. VCC RC vO RB vs VBB Graphical Analysis & AC Equivalent Circuit Fig. D Fig. C iC vCE vBE iB (C) Common-emitter circuit with time varying signal source in series with base dc source (D) Common-emitter transistor characteristics, dc load line, and sinusoidal variation in base current, collector current, and collector-emitter voltage

  6. Graphical Analysis & AC Equivalent Circuit • Base on Fig. C & D (time-varying signals linearly related & superimposed on dc values) • If signal source, vs = 0:

  7. Graphical Analysis & AC Equivalent Circuit • For B-E loop, considering time varying signals: Rearrange: Base on (5), left side of (7) is 0. So: • For C-E loop, considering time varying signals: • Base on (6), left side of (11) is 0. So:

  8. Graphical Analysis & AC Equivalent Circuit • Definition of small signal • Small signal : ac input signal voltages and currents are in the order of ±10 percent of Q-point voltages and currents. e.g. If dc current is 10 mA, the ac current (peak-to-peak) < 0.1 mA.

  9. Graphical Analysis & AC Equivalent Circuit • Rules for ac analysis • Replacing all capacitors by short circuits • Replacing all inductors by open circuits • Replacing dc voltage sources by ground connections • Replacing dc current sources by open circuits

  10. RC ic vO RB + vce + ib - vbe vs - Graphical Analysis & AC Equivalent Circuit • Equations • Input loop • Output loop 0.026 V AC equivalent circuit of C-E with npn transistor

  11. gm=ICQ/VT r=VT/ICQ Small-signal hybrid- equivalent circuit vbe = ibrπ rπ = diffusion resistance / base-emitter input resistance 1/rπ = slope of iB – VBE curve Using transconductance (gm) parameter

  12. Small-signal hybrid- equivalent circuit Using common-emitter current gain (β) parameter

  13. VCC RC vO RB vs VBB B C B C βib E E How to construct Small-signal hybrid- • We know that • i across B  ib • i across C βib • i across E  (β+1)ib • rπ between B -E • Place a terminal for the transistor • Common Terminal as ground rπ

  14. Small-signal hybrid- equivalent circuit Ic RB Vo + + Ib Vbe r gmVbe Vce Vs RC - - Small-signal equivalent circuit Output signal voltage Input signal voltage

  15. VCC RC vO RB vs VBB Small-signal hybrid- equivalent circuit Example Given :  = 100, VCC = 12V VBE = 0.7V, RC = 6k, RB = 50k, and VBB = 1.2V Calculate the small-signal voltage gain.

  16. Solutions 1. 2. 3. 4. 5. 6.

  17. Hybrid- Model and Early Effect transconductance parameter ro=VA/ICQ current gain parameter ro = small-signal transistor output resistance VA = early voltage

  18. Hybrid- Model and Early Effect Early Voltage (pg 299) Early Voltage (VA)

  19. VCC R1 RC vO CC RS vs R2 Basic Common-Emitter Amplifier Circuit Example Given :  = 100, VCC = 12V VBE(on) = 0.7V, RS = 0.5k, RC = 6k, R1 = 93.7k, R2 = 6.3k and VA = 100V. Calculate the small-signal voltage gain.

  20. Ri Ro RS Vo Vs R1 \\ R2 r gmV rO RC Solution Small-signal equivalent circuit

  21. Self-Reading Textbook: Donald A. Neamen, ‘MICROELECTRONICS Circuit Analysis & Design’,3rd Edition’, McGraw Hill International Edition, 2007 Chapter 5:The Bipolar Junction Transistor Page: 334-339 Chapter 6: Basic BJT Amplifiers Page: 370-388.

  22. Exercise The circuit parameters in Figure are changed to VCC = 5V, R1=35.2kΩ, R2=5.83kΩ, RC=10kΩ and RS =0, β =100, VBE(on) =0.7V and VA =100V. Determine thequiescent collector current andcollector-emitter voltageand find thesmall-signal voltage gain. Ans: ICQ = 0.21mA, VCEQ =2.9V, Av =-79.1)

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