INTRODUCTION TO ELECTRONICS
EHB 222E
Bipolar Junction Transistors
(BJT)
Asst. Prof. Onur Ferhanoğlu
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BJTs
• Transistors are 3-terminal devices.
• Basic principle: Controlling the voltage between 2 terminals to control the current
flowing in the 3rd terminal.
• Transistors are used as amplifiers & switches.
• Invention: 1948: Bell Lab`s
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs
npn transistor
• 3 terminals: emitter, base, collector
• 2 junction: EBJ, CBJ
• 3 modes: active: amplifer
cutoff:
switch
saturation: switch
pnp transistor
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs (npn) in Active mode
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs (npn) in Active mode
• Electrons diffuse into the p region
• Since base is narrow, with some loss, the electrons will reach the collector
• Since collector is more positive (voltage) than base, the electrons reaching the
collector will be swept across to the collector gate
ELECTRONS ARE COLLECTED AT THE COLLECTOR GATE
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs (npn) in Active mode
CURRENT FLOW
• iC = Isexp(VBE/VT)
• iC is independent of vCB -> since collector is positive with respect to the base
electrons will reach the collector side
• iB = iC /β = (Is/ β)exp(VBE/VT) -> small fraction of the emitter current that does not
reach the collector
• iE = ic + ib = Is [(β +1)/ β]exp(VBE/VT) = (β +1)/ β. iC
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs (npn) in Active mode – Equivalent circuit
• iC is independent of vCB , as long as vCB > 0
Therefore iC behaves as a voltage controlled current source
alternatively, iC can be expresses as a current controlled current source
• iC = β iB , β: COMMON EMITTOR CURRENT GAIN
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Circuit Symbols
• Arrow heads point towards the emitter gate, shows the normal direction of current
• npn -> collector collect electrons, current is in the opposite direction
ACTIVE REGION
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Base is 0V
IE = IC (β+1)/ β = 2.02 mA
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Graphical Representation
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•
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•
Asst. Prof. Onur Ferhanoğlu
Same as diode characteristics
No current when vBE < 0.5 V
Typically vBE = 0.7 V
For pnp, replace vBE with vEB
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Saturation mode
• 1 diode between base and emitter
(pn junction)
• 1 more diode between base and
collector
(OFF in active region, ON in saturation)
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Saturation mode
ACTIVE
Add diode currents: KCL at node C
SAT
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Early effect
COMMON EMITTER (emitter grounded)
• vCE increases width of depletion region
• Depletion width increases
• Base width decreases
• iC increases
(more electrons are collected)
Early voltage
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Early effect
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Early effect
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine the value of VBB that results in
a) Active mode with VCE = 5V
b) At the edge of saturation
c) Deep saturation with β = 10
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine the value of VBB that results in
a) Active mode with VCE = 5V
b) At the edge of saturation
c) Deep saturation with βforced = 10
Assume VBE = 0.7 and β = 50
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine the value of VBB that results in
a) Active mode with VCE = 5V
b) At the edge of saturation
c) Deep saturation with βforced = 10
Assume VBE = 0.7 and β = 50
Edge of saturation:
Still use β = 50, as in active mode
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine the value of VBB that results in
a) Active mode with VCE = 5V
b) At the edge of saturation
c) Deep saturation with βforced = 10
Assume VBE = 0.7 and β = 50
Deep saturation:
Use βforced instead
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – determining the mode of operation
• Assume active mode
• Check if VBC < 0.4 V -> YES -> active
• NO -> assume SAT -> check IC/IB < β
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine node voltages & branch currents
Assume β = 100
Transistor is assumed to be active -> VBC = -1.3 V (reverse biased) -> It is active
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine node voltages & branch currents
Assume β = 50
Assume ACTIVE
BC is forward biased XXX
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
Assume SAT
βforced = 1.5
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BJTs – Exercise
Determine node voltages & branch currents
BASE is grounded!
BE & BC are both reverse biased
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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BJTs – Exercise
Determine node voltages & branch currents (assume β = 100)
`pnp` transistor
Assume ACTIVE
Asst. Prof. Onur Ferhanoğlu
BJT/ INTRODUCTION TO ELECTRONICS
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