BIPOLAR JUNCTION
TRANSISTORS (BJTs)
Dr Derek Molloy, DCU
What are BJTs?
• Two PN junctions joined together is a BJT
– Simply known as a transistor!
• Bipolar? Current carried by electrons and
holes
• Will see FETs (Field Effect Transistors)
– BJTs have a higher gain (amplification).
– BJTs can supply more current.
– FETs are less complex and require less
power.
Production
Example of a bipolar transistor production
Transistor types
• So, they are like diodes:
– Semiconductor material
– Doped p and n regions
• Unlike a diode:
– 3 alternated doped regions p-n-p or n-p-n
– Narrow channel between 2 terminals is
controlled by a voltage on a 3rd terminal.
Transistor controlled to operate as a switch or a variable resistor
Key element in the design of amplifier
BJT CONFIGURATIONS
BJT PRINCIPLE OF OPERATION
Common Emitter Configuration
I
C
Collector
I
n
b
+
V
BE
Base
+
p
V
CE
n
Emitter
A Current Amplifier!
A small current flowing in the base-emitter circuit can control the amount of
a much larger current flowing in the collector-emitter circuit
BJT PRINCIPLE OF OPERATION
-
BE diode is forward biased.
Closed circuit between
the collector and emitter.
Current flowing between C-E,
BE diode is reverse biased.
Open circuit, high resistance between
the collector and emitter.
Small current flowing between C-E, ICEO
SOME CHARACTERISTICS
I C (mA)
I
C
β=hFE
I ( A )
B
Current transfer characteristic
(slightly non-linear)
IF VCE is large IC α IB
0.7V
VBE
Current-voltage transfer characteristic
(highly non-linear)
IC depends on VBE
BASICS BJT
IE=IC+IB
Numbers for illustration
of measurements only
Not a worked example!
CURRENT GAIN
• DC or large signal gain: hFE or β
hFE
IC

IB
For most practical purposes,
hfe and hFE are considered equal
• Small signal gain: hfe
h fe 
dI C I C

dI B I B
Voltage Gain is the ratio of output voltage, to input voltage.
Current Gain is the ratio of output current, to input current.
Transconductance is the ratio of output current, to input voltage.
Transimpedance is the ratio of output voltage, to input current.
COMMON EMITTER BIPOLAR AMPLIFIER
Coupling Capacitor acts as a high-pass filter, allowing AC signal
voltage on to the transistor, while blocking all DC voltage from being
shorted through the AC signal source.
Input signal between base and emitter.
Output signal between collector andh emitter.
= 100
FE
Base needs to be biased.
R
B
V
CC
RB
RC
= 910k
Base-Emitter is forward biased due to RB
R = 4.7k
C
RB sets the quiescent (steady-state with
no
input signal applied) base IBQ, ICQ, VVOQ
,=V10V
CEQ
CC
VBE=0.7V
C
B
E
V
i
V
O
VBE
0V
I BQ
ICQ

VCC  VBE
RB
 hFE I BQ

10V  0.7 V
910k
 10.2 A
 100  10.2 A  1.02 mA
VOQ VCC  I CQ RC 100.00102  4700  5.206V
Vcc=10V
RB=910k
RC=4.7k 
hFE=100
INPUT CHARACTERISTICS
I
C
(mA)
I
I
B
(A )
Current transfer characteristic
(slightly non-linear)
C
IB
0.7V
V
BE
0.7V
Current-voltage transfer characteristic
VBE
(highly non-linear)
• DC input characteristics
  eVkTBE  
I B I BS  e    1I BS e 40VBE




• AC input impedance
hie 
dVBE
1

()
dI B
40 I B
at room temperature
IBS is a constant determined
by the base characteristics
OUTPUT CHARACTERISTICS
OUTPUT CHARACTERISTICS
Saturation region
Active region
Ic(mA)
Saturation Region:
VCE is not large enough, IC is
Independent of IB and depends on VCE.
60
Active Region:
VCE is large enough, IC is
independent of VCE and depends on IB
(and the gain).
600
50
500
40
IB(µA) 400
30
300
20
200
10
100
0
0
VCE(V)
For amplification, BJT operates in active region
For switch or digital applications, BJT swings
between saturation (switch on ) and cut-off (switch off)
OPERATING REGION
B: Maximum of Ic
C: Maximum Power
Dissipation VCE and Ic
Damage Transistor
Ic
P = VCE x Ic
B
C
A
A: Saturation region
Highly non-linear
IB
D
D: Maximum VCE
Result in Avalanche Breakdown
of the transistor
VCE
Operating point placed in the white area
A suitable operating point, Icmax/2 to avoid any red zones, close to centre
Maximum swing possible with AC input.
VARIATION AROUND THE
QUIESCENT POINT
Ic
IB
iB
ic
Operating Point
VCE
vCE
VCC
VB
IB
VRC
VCE Vout
Avoid: clamping the wave
DC DESIGN PARAMETERS OF A
COMMON EMITTER AMPLIFIER
Saturation region
The load line graphical method
Ic(mA)
B
h
FE
R
Active region
B
= 100
V
CC
RB
= 910k
60
600
R = 4.7k
C
50
V = 10V
CC
40
IB(µA) 400
30
300
20
200
C
500
10
A
0
1
Rc
100
0
VCE(V)
RC
B
E
V
i
Ic 
Vcc  Vce
V
V
  ce  cc  mx  b
Rc
Rc Rc
A: cut-off B: saturation
Vcc
1) Chose an operating point (IC , VCE) and a power rail VCC .
Then draw the load line and calculate the slope. RC is given by -1/slope.
2) Chose RC and VCC. Draw a load line through VCE = VCC with a slope of -1/RC.
Then select an operating point somewhere along this load-line.

V
V
0CE