1. BIJUNCTION TRANSISTORE

2. Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals.
A transistor is made up of three layers – an ‘n’ layer sandwiched between two ‘p’ layers or a ‘p’ layer between two ‘n’ layers.  
Doping of each layer is different and that is what is responsible for the operation(amplification).

3. The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors:
N P N n p E B C
Cross Section
Schematic Symbol
P N P p n E B C
Cross Section
Schematic Symbol
Collector is moderately doped
Base is lightly doped
Emitter is heavily doped
Click to view NPN Transistor
Click to view PNP Transistor

4. BJT Relationships - EquationsC E IE IC IB - +
VEB
VCB
VEC B C E IE IC IB - +
VBE
VBC
VCE
npn
IE = IB + IC
VCE = -VBC + VBE
pnp
IE = IB + IC
VEC = VEB - VCB

5. DC  and DC   = Common-base current gain  = IC  = IC IB IE
 = Common-emitter current gain
 = Common-base current gain
 = IC  = IC
IB IE
The relationships between the two parameters are:
 =   = 
 
Note:  and  are sometimes referred to as dc and dc because the relationships being dealt with in the BJT are DC.

6. Modes of Operation Active: Most important mode of operation
Central to amplifier operation
Emitter –Base junction Forward biased and Collector –base Reverse Biased
Saturation:
Barrier potential of the junctions cancel each other out causing a virtual short
Ideal transistor behaves like an closed switch
Both junction are Forward biased
Cutoff:
Current reduced to zero
Ideal transistor behaves like an open switch
Both junction are Reverse biased

7. Three Types of BJT Configurations
Biasing the transistor refers to applying voltage to get the transistor to achieve certain operating conditions.
Common-Base Biasing (CB)
input = VEB & IE
output = VCB & IC
Common-Emitter Biasing (CE)
input = VBE & IB
output = VCE & IC
Common-Collector Biasing (CC)
input = VBC & IB
output = VEC & IE

8. BJT Transconductance Curve
Typical NPN Transistor
Collector Current:
Transconductance:
(slope of the curve)
gm = IC /  VBE
IES = The reverse saturation current of the B-E Junction.
VT = kT/q = 26 mV
T=300K)
 = the emission coefficient
and is usually IC
8 mA
6 mA
4 mA
2 mA
VBE
0.7 V

9. Common-Base Circuit Diagram: NPN Transistor
The Table Below lists assumptions that can be made for the attributes of the common-base biased circuit in the different regions of operation. Given for a Silicon NPN transistor.
Region of Operation IC
VCE
VBE
VCB
C-B Bias
E-B Bias
Active
βIB
=VBE+VCE
0.7V 0V
Rev.
Fwd.
Saturation
Max
-0.7V<VCE<0
Cutoff
None/Rev.

10. Common-Base input characteristics
Input characteristics for the CB configuration gives relation between the input quantities, input voltage VEB and input current IE for fixed VCB values
The input circuit in CB configuration involves the emitter-base diode, which is forward biased in active region. Therefore, the relationship between VEB­ and IE is nothing but the forward characteristics of a diode

11. Common-Base input characteristics
In the above characteristics, VCB = Open represents the characteristics of the forward biased emitter
With increase of VCB, the curves shift downwards i.e., we get the same IE with less VEB. This is because, from the early effect increases the IE increases with VEB held constant

12. Common-Base out put characteristics
Although the Common-Base configuration is not the most common biasing type, it is often helpful in the understanding of how the BJT works.
Emitter-Current Curves

13. Common-Emitter Circuit Diagram Collector-Current Curves VCE IC IC+ _
Active Region
VCC IB IB
Region of Operation
Description
Active
Small base current controls a large collector current
Saturation
VCE(sat) V, VCE increases with IC
Cutoff
Achieved by reducing IB to 0, Ideally, IC will also equal 0.
VCE
Saturation Region
Cutoff Region
IB = 0

14. Common Emitter Input Characteristics
The input quantities for C.E. configuration are base current IB and base emitter voltage VBE
The input characteristics curves are in between IB and VBE for various values of collector to emitter voltage VCE
If VCE = 0 and if the base-emitter junction is forward biased, the input characteristics is the same as the characteristics of forward biased diode
If VCE is increased then VCB increases
By applying KVL around the transistor

15. Common Emitter Input Characteristics
Increase in VCB leads to decrease in effective base width WB| due to early effect, resulting in decrease of recombination and consequently, decrease in base current due to recombination.

16. Common Emitter output Characteristics
The output quantities in C.E. configuration are IC and VCE the o/p characteristics gives a relationship between IC and VCE with base current IB as a parameter.
This family of curves may be divided into three regions those are active region, saturation region and cutoff region.

17. Common-Collector
It is often called an emitter follower since its output is taken from the emitter resistor.
Is useful as an impedance matching device since its input impedance is much higher than its output impedance.
It is also termed a "buffer" for this reason and is used in digital circuits with basic gates.

18. Common-Collector
The Common-collector biasing circuit is basically equivalent to the common-emitter biased circuit except instead of looking at IC as a function of VCE and IB we are looking at IE.
Also, since  1, and
 = IC/IE that means IC IE
Emitter-Current Curves
VCE IE
Active Region IB
Saturation Region
Cutoff Region
IB = 0

19. Common collector input Characteristics
As VCB increases according to early effect base width decreases and IB decreases.

20. Common collector output Characteristics
The common-collector circuit is basically same as the common-emitter, with the exception that the load resistor is in the emitter circuit, the output characteristics are similar to that of CE configuration. It is because

21. Transistor as amplifier
Transistor amplifies current as well as voltage and is a current operated device.
The CE configuration is widely used as it amplifies current and voltage unlike the other configurations.
Click to view Image

22. COMPARISION of CB, CE AND CC PARAMETERS
Input impedance
(Ri)
Output Impedance
(Ro)
D.C current gain
Voltage gain
Very Large
Moderate
Applications
For High Frequency
For Audio frequency
For impedance matching
Phase relationship between i/p and o/p
In- phase
Out-of phase

23. Transistor summary