Lecture 9 Bipolar Junction Transistor (BJT)eng.staff.alexu.edu.eg/.../fall_2017/Lecture_9_BJT.pdf · Bipolar Junction Transistor (BJT) BJT 1-1 Sunday 22/10/2017 . Outline ... I-V
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Lecture 9
Bipolar Junction Transistor (BJT)
BJT 1-1 Sunday 22/10/2017
Outline
Continue BJT Configurations BJT Biasing DC analysis
• Fixed-bias circuit
BJT 1-2
BJT Configurations
BJT 1-3
• The emitter is common to both input (base-emitter) and output (collector-emitter)
• The input is on the base and the output is on the collector
Common-Emitter configuration
BJT Configurations
BJT 1-4
Common-Emitter configuration
Collector (output) Characteristics Base (input) Characteristics
BJT Configurations
BJT 1-5
• The input is on the base and the output is on the emitter • The characteristics are similar to those of the common-
emitter configuration, except the vertical axis is IE
Common-Collector configuration
I-V characteristics of BJT
BJT 1-6
Load line
I-V characteristics of BJT (cont’d)
BJT 1-7
Operating Limits for Each BJT Configuration
BJT 1-8
• VCE is at maximum and IC is at minimum (IC_min= ICEO) in the cutoff region • IC is at maximum and VCE is at minimum (VCE_min = VCEsat = VCEO) in the
saturation region • The transistor operates in the active region between saturation and
cutoff
Power Dissipation in BJT
Common-base
Common-emitter
Common-collector
BJT 1-9
CCBCmax IVP
CCECmax IVP
ECECmax IVP
Example (1)
BJT 1-10
Analyze the BJT circuit, shown in the figure, to get the values of: IB, IC, IE,α,VCE, VCB, VBE and max. power dissipated in the BJT
Solution We assume an operating mode
for the BJT (active mode VBE=0.7V)
The base-emitter KVL equation is: 5.7 − 10IB −VBE − 2IE = 0
Also, IE =(β+1)IB=100IB
Then, 5.0 – 210 IB =0
IB = 23.8 μA
IC =βIB= 2.356 mA
IE = 2.38 mA
α = IC/IE= 0.9899 BJT 1-11
Solution (cont’d) VBE=0.7V from assumption
VC = 10.7 – IC . 1 kΩ = 8.34 V
VE = 0 + IE . 2 kΩ = 4.76 V
So, VCE=VC - VE = 3.58 V
By applying KVL to the BJT transistor: VCE = VCB + VBE
Then, VCB = VCE – VBE = 2.88 V
Max. power = VCE IC = 8.43 mWatt
Last thing to check assumption: VCE = 3.58 V > 0.3 V ok (no saturation)
IB = 23.8 μΑ > 0.0 ok (no cutoff) BJT 1-12
BJT Biasing
Definition: The DC voltages applied to a transistor in
order to turn it ON so that it can amplify the AC signal
BJT 1-13 No Bias
Good Bias
BJT Circuits at DC
The BJT operating mode depends on the
voltages at EBJ and BCJ
Simplified models and classifications are
needed to speed up the hand-calculation
analysis
BJT 1-14
DC analysis of BJT circuits
Step 1: assume the operating mode
Step 2: use the conditions or model for circuit analysis
Step 3: verify the solution
Step 4: repeat the above steps with another assumption if necessary
BJT 1-15
DC Biasing Circuits
Fixed-bias circuit
Emitter-stabilized bias circuit
Voltage divider bias circuit
DC bias with voltage feedback circuit
BJT 1-16
Fixed-bias circuit
BJT 1-17
Base-Emitter Loop
From Kirchhoff’s voltage law:
+VCC – IBRB – VBE = 0
Solving for base current:
B
BECCB
R
VVI
Collector-Emitter Loop
/R
VVII
B
BECCBC
Collector current:
From Kirchhoff’s voltage law: CCCCCE RIVV
Example (2)
BJT 1-18
Design the following circuit so that Ic = 2 mA and Vc= 5 V. For this particular transistor, β =100 and VBE=0.7 V
Vc
Solution To design the circuit, we need to determine
values of RC and RE
We assume BJT works in active mode
Ic = 2 mA = (15- Vc)/ RC then RC = 5 kΩ
VB = 0 V, from assumption VBE = 0.7 V
VE = VB – VBE = 0 – 0.7 = - 0.7 V
RE = (VE -(-15))/ IE
IE = (β +1)/ β Ic = 2.02 mA
RE = 14.3/2.02 = 7.07 kΩ
BJT 1-19
Load Line Analysis The end points of the
load line are:
BJT 1-20
ICsat
IC = VCC / RC
VCE = 0 V
VCEcutoff
VCE = VCC
IC = 0 mA
The Q-point is the operating
point that sets the values
of VCE,Q and IC,Q
VCE,Q
IC,Q
Circuit Values Affect the Q-Point
BJT 1-21
Circuit Values Affect the Q-Point
BJT 1-22
Circuit Values Affect the Q-Point
BJT 1-23
Example (3)
BJT 1-24
Given the load line and the defined Q-point, as shown in figure 2-a, determine the required values of VCC, RC, and RB for a fixed-bias configuration as depicted at figure 2-b.
Figure 2-a
Figure 2-b
Solution
BJT 1-25
sat
sat
BJT 1-26
Lecture Summary
Covered material Continue BJT
Biasing DC analysis
• Fixed-bias circuit
Material to be covered next lecture
Continue BJT Continue DC analysis
• More examples
Introduction to ac signal analysis
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