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EKT104 ANALOG ELECTRONIC CIRCUITS [LITAR ELEKTRONIK ANALOG] BASIC BJT AMPLIFIER (PART I) DR NIK ADILAH HANIN BINTI ZAHRI [email protected] 1 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 Block diagram of a compact disc player system DC power (a) Signal source a) Low signal power b) High signal power DC voltage source (b) Amplifier Load 2 The Bipolar Linear Amplifier (a) Bipolar transistor inverter circuit; (b) inverter transfer characteristics • 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 operating mode Time-varying output voltage is directly proportional to & larger 3 than time-varying input voltage linear amplifier The Bipolar Linear Amplifier Variable iB, vBE • Meaning Total instantaneous values DC values IB, VBE Instantaneous ac values ib, vbe Summary of notation Phasor values Ib, Vbe 4 Graphical Analysis & AC Equivalent Circuit VC C iC vO RB vs iB RC vCE vBE VBB Figure (c) (c) Common-emitter circuit with time varying signal source in series with base dc source Figure (d) (d) Common-emitter transistor characteristics, dc load line, and sinusoidal variation in base current, collector current, and collector-emitter voltage 5 Graphical Analysis & AC Equivalent Circuit • Base on Figure (c) & (d) (time-varying signals linearly related & superimposed on dc values) • iB I BQ ib (1) iC I CQ ic (2) vCE VCEQ vce (3) vBE VBEQ vbe (4) If signal source, vs = 0: VBB I BQ RB VBEQ (B - E loop) (5) VCC I CQ RC VCEQ (C - E loop) (6) 6 Graphical Analysis & AC Equivalent Circuit • For B-E loop, considering time varying signals: VBB vs iB RB vBE ( I BQ ib ) RB (VBEQ vbe ) • Rearrange: VBB I BQ RB VBEQ ib RB vbe vs • (7) (8) Base on (5), left side of (7) is 0. So: vs ib RB vbe (9) 7 Graphical Analysis & AC Equivalent Circuit For C-E loop, considering time varying signals: VCC iC RC vCE ( I CQ ic ) RC (VCEQ vce ) (10) VCC I CQ RC VCEQ ic Rc vce (11) Base on (6), left side of (11) is 0. So: ic Rc vce 0 (12) 8 Graphical Analysis & AC Equivalent Circuit • Definition of small signal • Small signal : ac input signal voltages and currents which 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 Graphical Analysis & AC Equivalent Circuit • Equations RC • ic vs ib RB vbe vO RB vs ib + vbe - + vce - I BQ vbe ib VT • AC equivalent circuit of C-E with npn transistor (ac equivalent circuit of Figure (c)) Base-emitter loop (Input loop) Thermal voltage, 0.026 Collector emitter loop (Output loop) ic RC vce 0 ic ib 11 Small-signal Hybrid- Equivalent Circuit gm=ICQ/VT vbe = ibrπ rπ = diffusion resistance / base-emitter input resistance 1/rπ = slope of iB – VBE curve r=VT/ICQ vbe V V r T F T , ib I BQ I CQ gm Small signal hybrid-π equivalent circuit for npn transistor using transconductance (gm) parameter I CQ VT ic g m vbe 12 Alternative Form of Small-signal Hybrid- Equivalent Circuit ib ( I b ) Using common-emitter current gain (β) parameter ic ib 13 Constructing Small-signal hybrid- VCC RC vO RB We know that i across B ib vs i across C βib VBB i across E (β+1)ib rπ between B -E Place a terminal for the transistor Common Terminal as ground B C B C βib rπ E E 14 Small-signal Voltage Gain RB Ic B C Vo + Vs Ib Vbe - r Vs Vbe r RB + r gmVbe E RC Vce - Vo Vce g mVbe RC Output signal voltage r Vo Small signal voltage gain, Av g m RC Vs r RB Input signal voltage 15 Voltage Gain Measurement VC C Example RC RB Given : = 100, VCC = 12V VBE = 0.7V, RC = 6k, RB = 50k, and VBB = 1.2V vO vs VBB Calculate the collector-emitter voltage at q-point and small-signal voltage gain. 16 SOLUTIONS 1. I BQ 2. 3. 4. VBB VBE ( on) RB 1.2 0.7 10 A 50 I CQ I BQ 100(10A) 1 mA VCEQ VCC I CQ RC 12 (1)(6) 6V r VT I CQ I CQ (100)(0.026) 2.6 k 1 5. 6. r Vo 11.4 Av g m RC Vs r RB VT 1 38.5 mA / V 0.026 gm 17 Hybrid- Model and Early Effect Early Effect • Collector voltage has some effect on collector current • Collector current increases slightly with increases in voltage Early Effect • Modeled as a linear increase in total current with increases in VCE 18 Hybrid- Model and Early Effect Early Voltage (pg 296) Early Voltage (VA) 19 Hybrid- Model and Early Effect transconductance parameter ro=VA/ICQ current gain parameter ro = small-signal transistor output resistance 20 VA = early voltage Basic Common-Emitter Amplifier Circuit VCC Example Given : = 100, VCC = 12V R1 VBE(on) = 0.7V, RS = 0.5k, RS CC RC vO RC = 6k, R1 = 93.7k, R2 = 6.3k and VA = 100V. vs R2 Calculate the small-signal voltage gain. 21 SOLUTION Small-signal equivalent circuit Ri RS Vs R1 \\ R2 Ro B C r gmV rO Vo RC E Ri R1 R2 r R1 R2 r V V R1 R2 r RS s Ro ro RC Vo g mV ro RC R1 R2 r Vo Av g m R1 R2 r RS Vs r R o C Ans: ICQ = 0.95mA, VCEQ =6.3V, Av =-163) 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 the quiescent collector current and collector-emitter voltage and find the small-signal voltage gain. Ans: ICQ = 0.21mA, VCEQ =2.9V, Av =-79.1) 23 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. 24