ANALOG ELECTRONIC CIRCUITS 1EKT 204

Basic BJT Amplifiers (Part 1) *

Analog Signals & Linear AmplifiersAnalog signalsNatural 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 & voltagesMagnitude: any value which vary continuously with timeAnalog circuitsElectronic circuits which produce analog signalsE.g. linear amplifierLinear amplifierMagnifies input signal & produce output signal that is larger & directly proportional to input signal

DC voltage sourceAmplifier LoadSignal sourceDC powerLow signal powerHigh signal power(a)(b)Block diagram of a compact disc player system*

The Bipolar Linear AmplifierTo 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*

The Bipolar Linear AmplifierSummary of notation*

VariableMeaningiB, vBETotal instantaneous valuesIB, VBEDC valuesib, vbeInstantaneous ac valuesIb, VbePhasor values

Graphical Analysis & AC Equivalent Circuit(D) Common-emitter transistor characteristics, dc load line, and sinusoidal variation in base current, collector current, and collector-emitter voltage(C) Common-emitter circuit with time varying signal source in series with base dc sourceiBiCvBEvCEFig. CFig. D*

Graphical Analysis & AC Equivalent CircuitBase on Fig. C & D (time-varying signals linearly related & superimposed on dc values)

If signal source, vs = 0:*

Graphical Analysis & AC Equivalent CircuitFor 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 signalSmall 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.

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Graphical Analysis & AC Equivalent CircuitRules for ac analysisReplacing all capacitors by short circuitsReplacing all inductors by open circuitsReplacing dc voltage sources by ground connectionsReplacing dc current sources by open circuits*

AC equivalent circuit of C-E with npn transistorGraphical Analysis & AC Equivalent CircuitEquationsInput loop

Output loop0.026 V*

Small-signal hybrid- equivalent circuitUsing transconductance (gm) parametervbe = ibr

r = diffusion resistance /base-emitter input resistance

1/r = slope of iB VBE curve*

Using common-emitter current gain () parameterSmall-signal hybrid- equivalent circuit*

How to construct Small-signal hybrid- Place a terminal for the transistor Common Terminal as groundWe know that i across B ib i across C ib i across E (+1)ib r between B -Er*

Small-signal equivalent circuitSmall-signal hybrid- equivalent circuitOutput signal voltageInput signal voltage*

Example

Given : = 100, VCC = 12VVBE = 0.7V, RC = 6k, RB = 50k, and VBB = 1.2VCalculate the small-signal voltage gain.Small-signal hybrid- equivalent circuit*

Solutions1.

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Hybrid- Model and Early Effecttransconductance parametercurrent gain parameterro=VA/ICQro = small-signal transistor output resistanceVA = early voltage*

Early Voltage (VA)Hybrid- Model and Early EffectEarly Voltage (pg 299)*

Basic Common-Emitter Amplifier CircuitExample

Given : = 100, VCC = 12VVBE(on) = 0.7V, RS = 0.5k, RC = 6k, R1 = 93.7k, R2 = 6.3k and VA = 100V.Calculate the small-signal voltage gain.*

SolutionSmall-signal equivalent circuit*

Self-ReadingTextbook: Donald A. Neamen, MICROELECTRONICS Circuit Analysis & Design,3rd Edition, McGraw Hill International Edition, 2007Chapter 5:The Bipolar Junction TransistorPage: 334-339Chapter 6: Basic BJT AmplifiersPage: 370-388.

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ExerciseThe circuit parameters in Figure are changed to VCC = 5V, R1=35.2k, R2=5.83k, RC=10k and RS =0, =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)

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