Introduction to BJT Amplifier

BJT (Review)

Still remember about BJT?

CBE III

The emitter current (iE) is the sum of the collector current (iC) and the base current (iB)

iB << iE and iC OTHER PRAMETERS &

EQUATIONS?

CBE iii

BJT

Basic structure and schematic symbol

E C

B

E

B

C

n npE C

B

E C

B

E

B

C

p pnE C

B

npn type pnp type

approximate equivalents

transistorsymbols

Refresh..

Common-emitter current gain, β Range: 50 < β < 300

Common-base current gain, α Range: always slightly

less than 1 The current relationship

between these 2 parameters are as follows:

EC

EC

CE

BE

BC

BCE

ii

ii

ii

ii

ii

iii

1

1

1

1

But

)(

)(

Refresh..

BJT as amplifying device B-E junction is forward-biased B-C junction is reverse-biased

BIASING OF BJT

Remember…! for normal operation

emitter-base junction is always forward-biased

AND

collector-base junction is always reverse-biased

FORWARD BIASING E/B JUNCTION

REVERSE BIASING C/B JUNCTION

BIASING NPN TRANSISTOR

Common-Emitter Circuit

(a) with an npn transistor

(b) with a pnp transistor

(c) with a pnp transistor biased with a positive voltage source

DC Analysis - Common-Emitter Circuit

Transistor current-voltage characteristics of the common-emitter circuit

DC Analysis - Common-Emitter Circuit

Common-emitter circuit with an npn transistor

Common-emitter dc equivalent circuit, with piecewise linear parameters

DC Analysis - Common-Emitter Circuit

B

BEBBB R

onVVI

)(

Look for calculation examples in Neamen (Chapter 3), Example 3.3 & 3.4

Usually VBE(on) = 0.7 V

CCCCCE

CECCCC

BC

RIVV

VRIV

II

or

Common-emitter dc equivalent circuit

DC Analysis - Load Line & Modes of Operation

Base-emitter junction characteristics and the input load line

BQB

BEBBB I

R

VVI

Base on Figure A, using KVL around B-E loop:

Figure A

Base on Figure A, 2 end points of the load line are found by setting IC = 0

So, VCE = VCC = 10 VWhen VCE = 0,IC = VCC/RC = 5 mA

IBQ is the value from the previous slide = 15 µASo, ICQ = βIBQ

If β = 200, ICQ = 3000 µA = 3 mA

So, VEQ = 4 V

DC Analysis - Load Line & Modes of Operation

CECCCC VRIV

CCQCCCEQ RIVV

Common- emitter transistor characteristics and the collector-emitter load line

CECCCC VRIV

BJT as an Amplifier

• Amplification of a small ac voltage by placing the ac signal source in the base circuit

• Vin is superimposed on the DC bias voltage VBB by connecting them in series with base resistor RB:

• Small changes in the base current circuit causes large changes in collector current circuit

BDCC II

END