1. Transistors and transistor circuits
Lecture (9, 10)
Transistors
and transistor circuits

2. Introduction
Transistors are three terminal devices that replaced vacuum tubes.
They are solid state devices that are used for…
Amplification
Switching
Detecting Light
The three terminals are the…
Emitter, Base, Collector (BJT)
Source, Gate, Drain (FET)

3. The BJT
Bipolar Junction Transistors are made with n-type and p-type semiconductors.
There are two types: npn and pnp.
Circuit Symbols
Transistor Analogy

4. The three layers of the sandwich are conventionally called the Collector, Base, and Emitter.
A Bipolar Transistor essentially consists of a pair of PN Junction Diodes that are joined back-to-back.
This forms a sort of a sandwich where one kind of semiconductor is placed in between two others.
There are therefore two kinds of Bipolar sandwich, the NPN and PNP varieties.

5. The BJT – Bipolar Junction Transistor
The Two Types of BJT Transistors:
npn
pnp n p n p n p E C E C C C
Cross Section
Cross Section B B B B
Schematic Symbol
Schematic Symbol E E
Collector doping is usually ~ 106
Base doping is slightly higher ~ 107 – 108
Emitter doping is much higher ~ 1015

6. BJT Relationships - EquationsIE IC IE IC -
VCE + +
VEC - E C E C - - + +
VBE
VBC IB
VEB
VCB IB + + - - B B
npn
IE = IB + IC
VCE = -VBC + VBE
pnp
IE = IB + IC
VEC = VEB - VCB
Note: The equations seen above are for the transistor, not the circuit.

7. Some of the basic properties exhibited by a Bipolar Transistor are immediately recognizable as being diode-like. However, when the 'filling' of the sandwich is fairly thin some interesting effects become possible that allow us to use the Transistor as an amplifier or a switch. To see how the Bipolar Transistor works we can concentrate on the NPN variety.

11. What are the primary uses of transistors?
Switching (Logic Gates)
Amplification (Op-Amps)

12. Architecture of BJTs
The bipolar junction transistor (BJT) is constructed with three doped semiconductor regions separated by two pn junctions
Regions are called emitter, base and collector

13. Architecture of BJTs There are two types of BJTs, the npn and pnp
The two junctions are termed the base-emitter junction and the base-collector junction
The term bipolar refers to the use of both holes and electrons as charge carriers in the transistor structure
In order for the transistor to operate properly, the two junctions must have the correct dc bias voltages
the base-emitter (BE) junction is forward biased(>=0.7V for Si, >=0.3V for Ge)
the base-collector (BC) junction is reverse biased

14. FIGURE Transistor symbols.

15. Basic circuits of BJT

16. Operation of BJTs BJT will operates in one of following four region
Cutoff region (for digital circuit)
Saturation region (for digital circuit)
Linear (active) region (to be an amplifier)
Breakdown region (always be a disaster)

17. Operation of BJTs

18. DC Analysis of BJTs Transistor Currents: IE = IC + IB alpha (DC)
IC = DCIE
beta (DC)
IC = DCIB
DC typically has a value between 20 and 200

19. Transistor Equations Alpha Factor Current Gain
Kirchhoff’s Current Rule

20. DC Analysis of BJTs DC voltages for the biased transistor:
Collector voltage
VC = VCC - ICRC
Base voltage
VB = VE + VBE
for silicon transistors, VBE = 0.7 V
for germanium transistors, VBE = 0.3 V

21. DC  and DC   = Common-emitter current gain
 = Common-base current gain
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.

22. BJT Example Using Common-Base NPN Circuit Configuration C
Given: IB = 50  A , IC = 1 mA
Find: IE ,  , and 
Solution:
IE = IB + IC = 0.05 mA + 1 mA = 1.05 mA
= IC / IB = 1 mA / 0.05 mA = 20
 = IC / IE = 1 mA / 1.05 mA =
 could also be calculated using the value of  with the formula from the previous slide. IC
VCB + _ IB B
VBE IE + _ E

23. BJT Transconductance Curve Typical NPN Transistor 1
Collector Current:
IC =  IES eVBE/VT
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 ~1 IC
8 mA
6 mA
4 mA
2 mA
VBE
0.7 V

24. Modes of Operation Active: Saturation: Cutoff:
Most important mode of operation
Central to amplifier operation
The region where current curves are practically flat
Saturation:
Barrier potential of the junctions cancel each other out causing a virtual short
Current reduced to zero
Ideal transistor behaves like an open switch
Cutoff:
* Note: There is also a mode of operation called inverse active, but it is rarely used.

25. Three Types of BJT Biasing
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

26. Common-Base
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 IC
Active Region IE
Saturation Region
Cutoff
IE = 0
VCB

27. Orange = Actual IC (IC’)
The Early Effect (Early Voltage) IC
Note: Common-Emitter Configuration IB
-VA
VCE
Green = Ideal IC
Orange = Actual IC (IC’)

28. Early Effect Example
Given: The common-emitter circuit below with IB = 25A, VCC = 15V,  = 100 and VA = 80.
Find: a) The ideal collector current
b) The actual collector current
Circuit Diagram a) IC
VCE + _
VCC IB b)

29. Breakdown Voltage The maximum voltage that the BJT can withstand.
BVCEO = The breakdown voltage for a common-emitter biased circuit. This breakdown voltage usually ranges from ~ Volts.
BVCBO = The breakdown voltage for a common-base biased circuit. This breakdown voltage is usually much higher than BVCEO and has a minimum value of ~60 Volts.
Breakdown Voltage is Determined By:
The Base Width
Material Being Used
Doping Levels
Biasing Voltage

30. BJT as an amplifier
Class A Amplifiers
Class B Amplifiers

31. BJT Class A Amplifiers
In a class A amplifier, the transistor conducts for the full cycle of the input signal (360°)
used in low-power applications
The transistor is operated in the active region, between saturation and cutoff
saturation is when both junctions are forward biased
the transistor is in cutoff when IB = 0
The load line is drawn on the collector curves between saturation and cutoff

32. BJT Class A Amplifiers

33. BJT Class A Amplifiers Three biasing mode for class A amplifiers
common-emitter (CE) amplifier
common-collector (CC) amplifier
common-base (CB) amplifier

34. BJT Class A Amplifiers A common-emitter (CE) amplifier
capacitors are used for coupling ac without disturbing dc levels

35. BJT Class A Amplifiers A common-collector (CC) amplifier
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 voltage gain  ~ 1, but current gain is greater than 1 and  = IC/IE that means IC~IE
Emitter-Current Curves

36. BJT Class A Amplifiers The third configuration is the common-base (CB)
the base is the grounded (common) terminal
the input signal is applied to the emitter
output signal is taken off the collector
output is in-phase with the input
voltage gain is greater than 1
current gain is always less than 1

37. BJT Class B Amplifiers
When an amplifier is biased such that it operates in the linear region for 180° of the input cycle and is in cutoff for 180°, it is a class B amplifier
A class B amplifier is more efficient than a class A
In order to get a linear reproduction of the input waveform, the class B amplifier is configured in a push-pull arrangement
The transistors in a class B amplifier must be biased above cutoff to eliminate crossover distortion

38. BJT Class B Amplifiers

39. The BJT as a Switch
When used as an electronic switch, a transistor normally is operated alternately in cutoff and saturation
A transistor is in cutoff when the base-emitter junction is not forward-biased. VCE is approximately equal to VCC
When the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated

40. The BJT as a Switch