ANALOG ELECTRONIC CIRCUITS 1 EKT 204 Basic BJT Amplifiers (Part 1)

Analog Signals & Linear Amplifiers 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

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 region Time-varying output voltage is directly proportional to & larger than time-varying input voltage  linear amplifier

The Bipolar Linear Amplifier Summary of notation Variable Meaning iB, vBE Total instantaneous values IB, VBE DC values ib, vbe Instantaneous ac values Ib, Vbe Phasor values

Graphical Analysis & AC Equivalent Circuit Fig. D RC RB vs vO VBB VCC 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

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

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:

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.

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

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

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

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

How to construct Small-signal hybrid- RC RB vs vO VBB VCC 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 B E C βib B E C rπ

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

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. RC RB vs vO VBB VCC

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

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

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

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. vs RS R1 R2 RC CC vO VCC

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

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.

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)