GROUP:- GROUP MEMBERS Roll no1. Muhammad Fahim 42 2. Abdur Rehman 413. Athar Gul 544. Adeel Abbasi 565. Nasir Rafique 13

BJT’S (BIPOLAR JUNCTION TRANSISTOR)

The BJT (Bipolar junction transistor) is connected with three doped semiconductor region separated with two p-n junction. The three region are called emitter, base, and collector. The physically representation of two types of BJT’s are

N

P

N

collector

base

emitter

Base collector junction

Base emitterjunction

B

E

C

One type consist of two n-region separated by p-region called N-P-N transistor. Similarly the other type consist of two p-region which is separated by n-region are called P-N-P region. The p-n junction joining the base and emitter region is called base-emitter junction. A wire lead is connected to each region and is indicated by B, E, C. the base region is lightly doped and very thin while the emitter is heavily doped and the collector is moderately doped.

Symbolic representation of n-p-n and p-n-p BJT’s are shown as

B

C

E

n-p-n transistor

C

B

E

P-n-p transistor

BASIC TRANSISTOR OPERATION

In order to make transistor as an amplifier the two p-n junction must be correctly biased with external D.C voltage. The proper bias arrangement for both n-p-n and p-n-p transistor for operation as an amplifier in both cases base-emitter(BE) is forward bias and the base-collector(BC) junction is reversed biase

Diagram of Operation of Transistor:

The forward bias from base to emitter (BE) depletion is narrow and the reverse bias from base to collector (BC) depletion is wider. Since the emitter region is heavily doped there are large number of electron in conduction band. These electrons easily diffused through the forward biased base emitter junction. In the p-type (in case of n-p-n) transistor. Since base region is thin and lightly doped thus maximum electrons goes to base collector (BC) region.

Which are pulled by heavily battery connected in reverse bias this forms the collector current (Ic) while small number of free electron flow out the base lead wire and that current is called base current (IB)

N

P

N

base

emitter

Base collector junction

Base emitterjunction

B

E

N

P

N

base

emitter

Base collector junction

Base emitterjunction

B

E

Transistor Diagram

In above slide notice that the arrow on the emitter of the transistor symbol point in the direction of conventional current the both diagram shows that the emitter current (IE) is the sum of collector current IC and base current IB mathematically

IE=IC+IB

Base current is very small compare to emitter and collector current

Dc Beta(βDc)and Dc alpha (αDc) The ratio of the dc collector current Ic to the base current IB is called Dc beta.β= Ic ÷ Ib

From 20 to 200 typical value of b dc.The ratio of Dc collector current Ic to the dc emitter current IE.αDc= Ic / IE

Transistor current and voltage analysis

The ratio of alpha dc is 0.095 to 0.99 a Dc is always less then 1.

The reason is that Ic is always slightly less than IE

VBc=DC voltage at Base w.r.t collector. VCB=DC voltage at collector w.r.t base. VCE= DC voltage at collector w.r.t

emitter. VBB forward biase the base emitter

junction and Vcc reverse biase the base collector junction.

When base emitter junction is forward biase it act like a forward biased voltage.

VBE=0.7 V Since the emitter is ground (0v). Apply (kvl) across RB VRB=VBB-VBE BY ohm law V=IR VRB=IB .RB Put in IB.RB=VBB-VBE IB=(VBB-VBE)/RB

1

2

3 2

3

4

Similarly the voltage at the collector w.r.t emitter.

VCE=VCC-VRC

By ohm law V=IR VRC=IC.RC 6 in 5 VCE=VCC-IRC

5

6

7

βDC=IC/Ib IB=IC/βDC The voltage across reverse biased base

collector junction. VCB=VEB-VEB

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