EEE 1

(Essentials of Electrical and

Electronics Engineering)

Lecture 14
Transistors

1
EEE 1 (Essentials of Electrical and Electronics Engineering)
The Semiconductor “Family Tree”
Semiconductor

Diode Transistor Others

Rectifier Schottky BJT FET Diac Quadrac

Triac
Zener

These devices have 2 main functions:

• amplification à output signal is a larger signal
proportional to the input signal
• switching à larger signal is controlled by a
weaker signal
2
EEE 1 (Essentials of Electrical and Electronics Engineering)
Bipolar Junction Transistors (BJT)
• A three-layer (emitter, base, collector)
semiconductor device

• Consist of P or N semiconductor between

opposite types
– close enough that minority carriers interact
– far apart enough that depletion regions
don’t interact

• 2 Types: PNP and NPN

3
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Bipolar Junction Transistors (BJT)
• Emitter (E)
– Heavily doped
– Job is to emit or inject free electrons to the
base
• Base (B)
– Lightly doped and very thin
– It passes most of the emitter-injected
electrons on the collector
• Collector (C)
– Doping level is between the heavy doping of
emitter and light doping of the base
– It collects or gather electrons from the base

4
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Perspective View
Example Silicon p-n-p Bipolar Junction Transistor

5
EEE 1 (Essentials of Electrical and Electronics Engineering)
 p-n-p Transistor

Looks sort
of like two
diodes back
to back

6
EEE 1 (Essentials of Electrical and Electronics Engineering)
 p-n-p Transistor

Emitter “emits” holes Collector “collects” holes

emitted by the emitter
Narrow Base controls
number of holes emitted

7
EEE 1 (Essentials of Electrical and Electronics Engineering)
 n-p-n Transistor

Looks sort
of like two
diodes back
to back

8
EEE 1 (Essentials of Electrical and Electronics Engineering)
 n-p-n Transistor

Emitter “emits” electrons Collector “collects” electrons

emitted by the emitter
Narrow Base controls
number of electrons emitted

9
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Emitter-Base Junction
• has forward V applied to the PN or NP junction
in order to allow free charges of the E to move
into B

• Forward bias is necessary for transistor to be

operated: Ge => 0.2V; Si => 0.6V

• No current can come out of the collector

unless VF allows the E to inject free charges
into the B.

" \ normally off device and needs forward V

applied to start conducting.

10
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Collector-Base Junction
• removes charges into B

• C must have reverse voltage wrt B

• Reverse voltage across C-B junction means

that no majority charges can flow from C to B.

• But in the opposite direction, from B to C, C

voltage attracts the charges in the B supplied
by the E.

11
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Transistor Action

1. E has heavy doping to supply free

charges.
2. B has only light doping and is very
thin.
3. C voltage is relatively high.

12
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Regions of Operation
1. Active region
* E-B junction forward biased; C-B junction
reverse biased
* largest signal gain and smallest distortion
* IC = bIB
- b(current gain) typical values : 50 to >
400
2. Saturation region
* E-B and C-B junctions forward biased
* device acts like an “ON” switch
* VCE = VCE,sat, IC = IC,sat
* further increase in IB will not increase IC
13
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Regions of Operation
3. Cut-off region
* E-B and C-B junctions reverse biased
* device acts like an “OFF” switch
4. Reverse-Active region
* E-B junction reversed biased; C-B junction
forward biased
* Basically the forward active region with
roles of emitter and collector reversed
* typically avoided region

14
EEE 1 (Essentials of Electrical and Electronics Engineering)
 p-n-p n-p-n
Ø Two of the currents and two of the voltages are independent.
If two of the currents or voltages are known, third terminal current or voltage can determined.

Current flowing into a device

= current flowing out of device

(VCE = - VEC )

15
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Formulas
true for all operating regions:

VEB + VBC + VCE

IE = I C + I B

For active region only:

IC = bIB IE = (b+1)IB
IC = aIE a = b/(b+1)

16
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Notation
• VBE,FA/VBE,ON – VBE when the transistor is
forward active
• VBE,SAT – VBE when the transistor is
saturated
• VCE,SAT – VCE when the transistor is
saturated
• IC,SAT – IC when the transistor is saturated

17
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Basic Steps in Analyzing Transistor
Circuits
1. Assume transistor is forward active
IC = bIB and IE = (b+1)IB are applicable.
2. Take KVL at the base-emitter loop
Equation will be in terms of IB, and thus, IB can be
solved.
3. Take KVL at the collector-emitter loop
Solve for VCE and/or VO.
4. Check validity of assumption
a. If VCE > VCE,sat then assumption is valid.
b. If VCE < VCE,sat then transistor is saturated.
i. Replace VCE with VCE,sat and set B-E to on.
ii. Redo all calculations.

18
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Transistor Switch
Vcc = 5V

VCE,SAT = 0.2 V Find Vo if

VBE,FA = 0.7 V IC RC = 10k a. Vi = 0
b. Vi = 5V
IB
Vi
What is IC,SAT?
RB= 1k
Vo

19
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Fixed Bias Circuit
Vcc KVL at B-E loop:
VCC = IBRB + VBE
IB = (VCC – VBE) / RB
RB IC RC
KVL at C-E loop:
IB VCC = ICRC + VCE
VCE = VCC - ICRC
Vo VO = VCE
Find Vo. IC = bIB
VO = VCC – bIBRC

20
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Emitter-Stabilized Bias Circuit
Vcc KVL at B-E loop:
VCC = IBRB + VBE + IERE
IE = (b+1)IB
RB IC RC IB = (VCC– VBE) / [RB+(b+1)RE]

IB KVL at C-E loop:

VCC = ICRC + VCE + IERE
Vo VCE = VCC - ICRC - IERE
Find Vo. RE VO = VCE + IERE
IC = bIB
IE = (b+1)IB
VO = VCC – bIBRC
21
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Voltage Divider Bias Circuit
Find Vo. Vcc
take thevenin
equivalent at base:
R1 IC RC
R1 IB
VCC
IB

R2
Vo
R2 RE
VTH = VCCR2 / (R1 + R2 )
RTH = R1R2 / (R1 + R2 )

22
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Voltage Divider Bias Circuit
Vcc
KVL at B-E loop:
IC
RC
VTH = IBRTH + VBE + IERE
IB = (VTH–VBE)/[RTH+(b+1)RE]

RTH IB KVL at C-E loop:

VTH VCC = ICRC + VCE + IERE
VCE = VCC - ICRC - IERE
Vo
VO = VCE + IERE
RE
IC = bIB
IE = (b+1)IB
VO = VCC – bIBRC

23
EEE 1 (Essentials of Electrical and Electronics Engineering)
DC Bias with Voltage Feedback
Vcc KVL at B-E loop:
I1 VCC = I1R1 + IBRB + VBE + IERE
R1 I1 = IB + IC = (b+1)IB = IE
RB
IB = (VCC–VBE)/[RB+(b+1)(RE+R1)]
IC
IB
KVL at C-E loop:
VCC = I1R1 + VCE + IERE
Vo VCE = VCC - I1R1 - IERE
RE VO = VCE + IERE
IC = bIB
I1 = IE = (b+1)IB
Find Vo. VO = VCC – (b+1)IBR1
24
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Exercise 1

Vcc = 16V

470k IC 2.7k
Find IB, IC, VCE, VC,
VC
IB VB, VE
β = 90
VE
VCE,sat = 0.2V

25
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Exercise 2
Vcc = 12V

Ic = 2mA
RB IC RC

IB
7.6V Find RB, RC, VCE,
β = 80 VB, RE
VCE,sat = 0.2V 2.4V
RE

26
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Exercise 3
Vcc = 16V
1. Find IB, IC, VCE,
62k
VC, VE, VB.
IC 3.9k

IB VC 2. Determine IC,sat?
β = 80
VE
9.1k 0.68k

VCE,sat = 0.2V

27
EEE 1 (Essentials of Electrical and Electronics Engineering)
 Exercise 4
Vcc = 12V

Determine the range

RB IC 2.7k
of RB that will operate
IB the transistor in
β = 100
saturation.

VCE,sat = 0.2V
680

28
EEE 1 (Essentials of Electrical and Electronics Engineering)