C B E

Transistors

•

They are unidirectional current carrying devices like diodes with capability to control the current flowing through them

• Bipolar Junction Transistors ( BJT ) control current by current • Field Effect Transistors ( FET ) control current by voltage • They can be used either as switches or as amplifiers

•

A transistor allows you to control the current, not just block it in one direction.

•

A good analogy for a transistor is a pipe with an adjustable gate.

• • •

A transistor has three terminals. The main path for current is between the collector and emitter. The base controls how much current flows, just like the gate controls the flow of water in the pipe.

BIPOLAR JUNCTION TRANSISTOR

• • • •

Two back to back P-N junctions Emitter

– –

Heavily doped Main function is to supply majority carriers to base Base

– –

Lightly doped as compared to emitter Thickness 10 -6 m Collector

– –

Collect majority carriers from emitter through base Physically larger than the emitter region E N B P N C E P N B P C

The BJT – Bipolar Junction Transistor

The Two Types of BJT Transistors npn pnp E n p n Cross Section C B B Schematic Symbol E C E p n p Cross Section C B B Schematic Symbol E C

NPN Bipolar Junction Transistor

LP4 6

PNP Bipolar Junction Transistor

LP4 7

STRUCTURE

•

The collector surrounds the emitter region, making it almost impossible for the electrons injected into the base region to escape being collected, thus making the resulting value of α very close to unity, and so, giving the transistor a large β

•

Energy Band diagram of an unbiased transistor

–

N-region moves down and P-region moves up due to diffusion of majority carriers across junction.

–

The displacement of band and carrier migration stops when Fermi levels in the three regions are equalized

Biasing of Transistor

–

Base and emitter current when collector is open

•

EB is forward biased- electron diffusion from emitter to base and hole diffusion from base to emitter

•

Hence I B will be large and is equal to I E

•

Collector is open so no current flows into collector

•

Base and Collector current when the Emitter is open (I

CBO

)

•

CB is reverse biased- electron from base flow into collector region and holes from collector flow into base

•

This current is known as reverse saturation current

•

The base current I B I CBO will be small and is equal to

•

Four Ways of Transistor biasing

–

Both EB and CB junctions are fwd biased- Huge current flows through base. The transistor is said to be operating in Saturation region (mode)

–

Both EB and CB junctions are reverse biased- The transistor is said to be operating in cut off region (mode)

–

EB junction is fwd biased and CB junction is reverse biased. The collector current is controlled by emitter current or base current- The transistor is said to operate in Active region (mode)

–

EB junction in reverse biased and CB junction in fwd biased- inverted region (mode)

Transistor Biasing-Active Region When both Emitter and Collector are closed

• • • • •

Emitter-base junction is forward biased Collector-base junction is reverse biased DC emitter supply voltage (V EE )- Negative terminal of V EE is connected to emitter DC collector supply voltage (V CC )- Positive terminal of V CC is connected to collector I B becomes very small and I C will be as large as I E I E N P N I C V EE I B V CC

• • • •

Transistor currents

Forward biasing from base to emitter narrows the BE depletion region Reverse biasing from base to collector widens the depletion CB region Conduction electrons diffuse into p-type base region Base is lightly doped and also very thin- so very few electron combine with available hole and flow out of the base as valence electrons (small base electron current) V EE I E N I B P N I C V CC

• • • •

Sufficient holes are not avail in base – remote possibility of joining of electrons with holes Electron concentration is large on emitter side and nil on collector Electrons swiftly move towards collector At CB junction they are acted upon by strong electric field due to reverse bias and are swept into collector

Transistor currents

• • • • • •

Most of the electrons diffuse into CB depletion region These electrons are pulled across the reverse biased CB junction by the attraction of the collector supply voltage and form the collector electron current. Therefore I E = I C + I B 1-2% of emitter current goes to supply base current and 98-99% goes to supply collector current Moreover, I E of transistor flows into the transistor and I B & I C flow out Current flowing in is taken as positive and currents flowing out are taken as negative The ratio of the number of electrons arriving at collector to the number of electrons emitted by the emitter is called base transportation factor



• • • •

Important Biasing Rule

Both collector and base are positive with respect to emitter But collector is more positive than base Different potentials have been designated by double subscripts as shown in the figure V CB (Collector is more positive than base) and V BE (base is more positive than emitter) ++ C E ++ C V CB B V BE + V BE V CB B - E

• • • •

Transistor circuit configuration

There are of three types

– – –

Common base (CB) OR grounded base Common emitter (CE) OR grounded emitter Common collector (CC) OR grounded collector Common is the term used to denote the electrode that is common to the input and output circuits and it is generally grounded Common-Base Biasing (CB) : input = V BE & I E output = V CB & I C

• •

Common-Emitter Biasing (CE): input = V BE & I B output = V CE & I C

• •

Common-Collector Biasing (CC): input = V BC & I B output = V EC & I E

• • •

Common Base Configuration Input signal applied between emitter & base Output is taken from collector & base Ratio of collector current to emitter current is called dc alpha (



dc ) of a transistor E ++ C V BE V CB B

• • • • • • 

dc



I C I E OR I C

 

dc I E

The subscript dc on



signifies that this ratio is defined from dc values of I C and I E There is also an ac



which refers to the ratio of change in collector current to the change in emitter current For all practical purposes



dc =



ac =



I E is taken as positive (flowing into transistor) and I C taken as negative (flowing out of transistor)



is the measure of quality of a transistor- higher its values, better is the transistor is Value ranges from 0.95 to 0.999

• •

Common Emitter Configuration

The input signal is applied between the base and emitter and the output signal is taken out from the collector and the emitter Ratio of collector current to base current is called dc beta (



dc ) of a transistor

 

I I C B OR I C

 

I B

Relation between



and



C B +

 

I C I E

and

 

I C I B

E

  

I E I B

using

I

 

I E I



C I C

  1     

B



I

or or

E



I C

then

 

I C

becomes

I B

  

I E

/

I I C E

/ 

I E I C

   1    /

or

I E

    1    /  1    • •

Common Collector Configuration The input signal is applied between the base and collector and the output signal is taken out from the emitter-collector circuit Ratio of emitter current to base current is

I I E B



I I E C

.

I I C B

     /  1      1   

From the figure

I E



I B



I C



I B

 

I B

  1   

I B

Output current=(1+



) x Input current B + Relation between transistor currents

I E

:

I B

:

I C

We know

I C

and

I B



because

   1    

I



C



and



I



E

     

I B

  1    

I E

/ 1    

I E



We get

1

I



E

 1     1   

I E

  1  1  

E C

Therefore

I

1 :

E

: ( 1  ( 1    ) : )

I



E

: 

I E

• • •

This shows that emitter current initiated by the forward biased emitter base junction is split into two parts (1-



)I E which becomes base current in the external circuit



I E which becomes collector current in the external circuit

Static Characteristics

•

Common Base Static characteristics

–

Input characteristics.

I E varies with V BE when voltage V CB

•

is held constant V CB is adjusted with the help of R 1

•

V BE is increased and corresponding values of I E are noted

• • •

The plot gives input characteristics Similar to the forward characteristics of P-N diode This characteristics is used to find the input resistance of the transistor. Its value is given by the reciprocal of its slope R in =



V BE /



I E

BJT Input Characteristics

I E E C I C V EE R 2 V BE V CB R 1 V CC B I E 8 mA 6 mA 4 mA 2 mA 0.7 V V BE

Static Characteristics

•

Common Base Static characteristics

–

Output characteristics.

I C constant

• • • •

varies with V CB when I E is held V BE is adjusted with the help of R 2 and I E is held constant V CB is increased and corresponding values of I C are noted The plot gives output characteristics Then I E is increased to a value little higher and whole process is repeated

•

The output resistance of the transistor is given by R out =



V CB /



I C I E E C I C V EE R 2 V BE V CB R 1 V CC B

BJT Output Characteristics



I C flows even when V CB =0 for different values of I E (due to internal junction voltage at CB junction)



I C flows even when I E =0 (Collector leakage current or reverse saturation current I CBO )



The output resistance is very high (500k



) I C Active Region I E V CB Cutoff I E = 0

Static Characteristics

– –

It can be seen that I C flows even when V CB is zero It is due to the fact that electrons are being injected into base due to forward biased E-B junction and are collected by collector due to action of internal junction voltage at C-B junction

• – –

Another important feature is that a small amount of collector current flows even when the emitter current I E is zero called collector leakage current (

I CBO

) When V CB is permitted to increase beyond a certain value, I C increases rapidly due to avalanche breakdown This characteristics may be used to find



ac



ac =



I C /



I E

• • •

Common Emitter Configuration Transistor is biased in active region Called CE because emitter is common to both VBB and VCC VBB forward biases the EB junction and VCC reverse biases the CB V BB R 2 I B B C I C V BE B E V CE R 1 V CC Common Emitter(CE) Connection

LP4 30

Static Characteristics

•

Common Emitter Static characteristics

–

Input characteristics.

I B varies with V BE when voltage V CE

•

is held constant V CE is adjusted with the help of R 1

•

V BE is increased and corresponding values of I B are noted

•

The plot gives input characteristics

• •

Procedure is repeated for different (constant) values of V CE This characteristics is used to find the input resistance of the transistor. Its value is given by the reciprocal of its slope R in =



V BE /



I B

I B 8 mA 6 mA 4 mA 2 mA 0.7 V V BE

Static Characteristics

•

Common Emitter Static characteristics

–

Output characteristics.

I C when I B

•

I B is held constant is held constant

•

varies with V CE V CE is increased and corresponding values of I C are noted

• •

The plot gives output characteristics Then I B is increased to a value little higher and whole process is repeated

•

The output resistance in this case is very less as compared to CB circuit and is given by R out =



V CE /



I C



As V CE value of I B increases from zero, I C rapidly increases to saturation level for a fixed



I C flows even when I B =0 (Collector leakage current or reverse saturation current I CEO ), the transistor is said to be cutoff



When V CB is permitted to increase beyond a certain value, I C due to avalanche breakdown increases rapidly



This characteristics may be used to find



ac I C Region of Operation Description



ac =



I C /



I B Active Small base current controls a large collector current Active Region Saturation V CE(sat) ~ 0.2V, V CE increases with I C I B Cutoff I Achieved by reducing B to 0, Ideally, I also equal 0.

C will Saturation Region V CE Cutoff Region I B = 0

where

Common Base

I E



I C



I B



dc



I C I E



I C

 

dc I E I E

 0 ,

I C



I CBO

(Reverse saturation current) Therefore, in general

I C

 

dc I E



I CBO

Common Emitter

I E



I C



I B



dc



I C I B



I C

 

dc I B

where

I B

 0 ,

I C



I CEO

(Reverse saturation current)

I C

 

dc I B



I CEO

Relationship between



dc

and



dc

   1    

and

    1   

Common Base Formulas

I E E R E C I C R L V EE I B V CB V BE V CC B

V EE



I E R E



V BE

 0 

I E



V EE



R E V BE

and

V CC



I C R L



V CB

 0 

V CB



V CC



I C R L

Where V BE =0.3 V for Ge and 0.7 V for Si Generally V EE >>V BE so I E =V EE /R E

and

Common Emitter Formulas

I C I B E C R B R L V BB I E V CE V CC V BE B

V BB



I B R B



V BE

 0 

I E



V BB



R B V BE V CC



I C R L



V CE

 0 

V CE



V CC



I C R L

DC



and DC

 

= Common-emitter current gain



= Common-base current gain



= I C



= I C I B I E The relationships between the two parameters are:



=

 

=

 

+ 1 1 -



Note:



and



are sometimes referred to as



dc and



dc because the relationships being dealt with in the BJT are DC.

V CB B V BE I B

BJT Example

Using Common-Base NPN Circuit Configuration C I C Given: I B = 50



A , I C = 1 mA Find: I E ,



, and



E I E Solution: I E = I B + I C = 0.05 mA + 1 mA = 1.05 mA



= I C / I B = 1 mA / 0.05 mA = 20



= I C / I E = 1 mA / 1.05 mA = 0.95238

 

could also be calculated using the value of with the formula from the previous slide.



=

 

= 20 = 0.95238

+ 1 21

Transistor as an amplifier

• •

Transistor as an amplifier

An electronic circuit that causes an increase in the voltage or power level of a signal It is defined as the ratio of the output signal voltage to the input signal voltage

G



OutputVolt age InputVolta ge



v o v i

I E I C V EE I B V CC R L

 

The dc voltage V EE is a fixed voltage and causes a dc current I E to flow through EB junction



In the figure we see that an output voltage is developed across R L When the ac voltage V i is super-imposed on V emitter base voltage varies with time EE , the



Say if V EE =10V and the peak voltage of V i EB voltage swings from 9V to 11V is is 1V, the



The causes corresponding variations in I E gives V o and I C which



The emitter variation due to EB voltage variation can be expressed as



I E



v i r i



The collector current I C changes by



I C

 

dc



I E



This current



I C flows through R L drop causing a voltage

v o v o v o

    

I

 

dc dc



C

  

I v i R L

/

E r i



R L



R L



Hence

G



v o

/

v i

 

dc



R L

/

r i



as



dc

 1 

Where r i is very small (100



) and R L is of the order of kilo-ohms. It means V o is larger than V i indicating that the transistor has amplified small V i to a larger V o

Problems

•

In the CE Transistor circuit V

BB

= 5V, R

BB

= 107.5 k



, R

CC

= 1 k



, V

CC

= 10V. Find I

B

, I

C

, V

CE

,



and the transistor power dissipation

In the CE Transistor circuit shown earlier V BB = 5V, R BB = 107.5 k



, R CC = 1 k



, V CC = 10V. Find I B , I C , V CE ,



and the transistor power dissipation using the characteristics as shown below By Applying KVL to the base emitter circuit

I B



V BB



R BB V BE

By using this equation along with the i B / v BE characteristics of the base emitter junction , I B = 40



A By Applying KVL to the collector emitter circuit

I C  V CC  R CC V CE

By using this equation along with the i C of the base collector junction , i C / v CE characteristics = 4 mA, V CE = 6V

 

I C I B

 4

mA

40 

A

 100

Transistor power dissipation = V CE I C = 24 mW We can also solve the problem without using the characteristics if



and V BE values are known i B 100



A i C 10 mA 100



A 80



A 60



A 40



A 20



A 0 0 5V v BE v CE Input Characteristics Output Characteristics