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ANALOG ELECTRONIC CIRCUITS 1. EKT 204 Basic BJT Amplifiers (Part 1). Analog Signals & Linear Amplifiers. Analog signals Natural analog signals: physical sense (hearing, touch, vision)

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analog electronic circuits 1

ANALOG ELECTRONIC CIRCUITS 1

EKT 204

Basic BJT Amplifiers (Part 1)

analog signals linear amplifiers
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

the bipolar linear amplifier
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

the bipolar linear amplifier1
The Bipolar Linear Amplifier
  • Summary of notation
graphical analysis ac equivalent circuit

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

graphical analysis ac equivalent circuit1
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 circuit2
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 circuit3
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 circuit4
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 circuit5

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

small signal hybrid equivalent circuit

gm=ICQ/VT

r=VT/ICQ

Small-signal hybrid- equivalent circuit

vbe = ibrπ

= diffusion resistance /

base-emitter input resistance

1/rπ

= slope of iB – VBE curve

Using transconductance (gm) parameter

small signal hybrid equivalent circuit1
Small-signal hybrid- equivalent circuit

Using common-emitter current gain (β) parameter

how to construct small signal hybrid

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

small signal equivalent circuit

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

small signal hybrid equivalent circuit2

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.

solutions
Solutions

1.

2.

3.

4.

5.

6.

hybrid model and early effect
Hybrid- Model and Early Effect

transconductance

parameter

ro=VA/ICQ

current gain

parameter

ro = small-signal transistor output resistance

VA = early voltage

basic common emitter amplifier circuit

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.

solution

Ri

Ro

RS

Vo

Vs

R1 \\ R2

r

gmV

rO

RC

Solution

Small-signal equivalent circuit

self reading
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
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)