lecture 10 bipolar junction transistor (bjt)eng.staff.alexu.edu.eg/.../lecture_10_bjt.pdf ·...
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Lecture 10
Bipolar Junction Transistor (BJT)
BJT 1-1
Outline Bipolar Junction Transistor (BJT) – Cont’d
BJT configurations DC biasing circuits and analysis
BJT 1-2
Test Yourself
Choose the more appropriate answer
1) The β of a transistor is its (a) current gain (b) voltage gain (c) power gain (d) internal resistance
BJT 1-3
BJT Configurations
BJT 1-4
The base is common to both input (emitter–base) and output (collector–base) of the transistor
Common-Base configuration
BJT Configurations
BJT 1-5
This curve shows the relationship between of input current (IE) to input voltage (VBE) for three output voltage (VCB) levels
Common-Base configuration
Emitter (or input)
Characteristics
BJT Configurations
BJT 1-6
This graph demonstrates the output current (IC) to an output voltage (VCB) for various levels of input current (IE)
Common-Base configuration Collector (or output)
Characteristics
BJT Configurations
BJT 1-7
• 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 1-8
Common-Emitter configuration
Collector (or output) Characteristics Base (or input) Characteristics
BJT Configurations
BJT 1-9
• 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 1-10
Load line
I-V characteristics of BJT (cont’d)
BJT 1-11
Operating Limits for Each BJT Configuration
BJT 1-12
• 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 1-13
CCBCmax IVP
CCECmax IVP
ECECmax IVP
Example (1)
BJT 1-14
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
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 1-17 No Bias
Good Bias
BJT Circuits at DC
The BJT operation mode depends on the voltages at EBJ and BCJ
The I-V characteristics are strongly nonlinear
Simplified models and classifications are needed to speed up the hand-calculation analysis
BJT 1-18
DC analysis of BJT circuits
Step 1: assume the operation 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 1-19
DC Biasing Circuits
Fixed-bias circuit
Emitter-stabilized bias circuit
Voltage divider bias circuit
DC bias with voltage feedback circuit
BJT 1-20
Fixed-bias circuit
BJT 1-21
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 1-22
Design the following circuit so that Ic = 2 mA and Vc= 5 V. For this particular transistor, β =100 and VBE=0.7 V
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, 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 1-23
BJT 1-24
Lecture Summary
Covered material Bipolar Junction Transistor (BJT) – Cont’d
BJT configurations DC biasing and analysis
Material to be covered next lecture
Continue BJT DC biasing and analysis
• Examples
BJT switching time