Bipolar Junction Transistors (BJTs)

Types of BJTs

NPNPNP

NPN-BJT Structure

n-type p-type n-type

NPN BJT Symbol

E

B

C

PNP-BJT Structure

p-type n-type p-type

PNP BJT Symbol

Emitter is reversed

E

B

C

Current Flow Convention

EEmitter (n-type)

Base (p-type)

Collector (n-type)

B

C

IE

IB

IC

WORKING

BJT structure

note: this is a current of electrons (npn case) and so theconventional current flows from collector to emitter.

Transistor Connections

Common Base Connection Common Collector Connection Common Emitter Conncetion

Common Base Connection

Current Amplification Factor α =IC / IE ( constant VCB)

• The value of α is lies b/w 0.9 to 0.999

• Total collector current • IC=αIE+Ileakage

Common Collector Connection

Current Amplification Factor

• γ = IE/IB

• Expression for collector current.

• IC= IB /1-α + ICBO /1-α

Common Emitter Conncetion

Base Current Amplification Factor

• β= IC / IB

• The range of β is 20 to 500.

• Expression for collector current.• IC = βIB + ICEO