Current Gain: - Current Gain:
- α is the fraction of electrons that diffuse across the narrow Base region
- 1- α is the fraction of electrons that recombine with holes in the Base region to create base current
- The current Gain is expressed in terms of the β (beta) of the transistor (often called hfe by manufacturers).
- β (beta) is Temperature and Voltage dependent.
- It can vary a lot among transistors (common values for signal BJT: 20 - 200).
BJT characteristics
Emitter is grounded. - Emitter is grounded.
- Base-Emitter starts to conduct with VBE=0.6V,IC flows and it’s IC=b*IB.
- Increasing IB, VBE slowly increases to 0.7V but IC rises exponentially.
- As IC rises ,voltage drop across RC increases and VCE drops toward ground. (transistor in saturation, no more linear relation between IC and IB)
NPN Common Emitter circuit
Common Emitter characteristics
No current flows
Collector current controlled by the collector circuit. (Switch behavior)
In full saturation VCE=0.2V.
Collector current proportional to Base current
The avalanche multiplication of current through collector junction occurs: to be avoided
Operation Region
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IB or VCE Char.
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BC and BE Junctions
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Mode
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Cutoff
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IB = Very small
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Reverse & Reverse
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Open Switch
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Saturation
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VCE = Small
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Forward & Forward
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Closed Switch
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Active Linear
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VCE = Moderate
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Reverse &
Forward
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Linear Amplifier
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Break-down
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VCE = Large
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Beyond Limits
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Overload
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Operation region summary
BJT as Switch
- Vin(Low ) < 0.7 V
- BE junction not forward biased
- Cutoff region
- No current flows
- Vout = VCE = Vcc
- Vout = High
- Vin(High)
- BE junction forward biased (VBE=0.7V)
- Saturation region
- VCE small (~0.2 V for saturated BJT)
- Vout = small
- IB = (Vin-VB)/RB
- Vout = Low
- Basis of digital logic circuits
- Input to transistor gate can be analog or digital
- Building blocks for TTL – Transistor Transistor Logic
- Guidelines for designing a transistor switch:
- VC>VB>VE
- VBE= 0.7 V
- IC independent from IB (in saturation).
- Min. IB estimated from by (IBmin» IC/b).
- Input resistance such that IB > 5-10 times IBmin because b varies among components, with temperature and voltage and RB may change when current flows.
- Calculate the max IC and IB not to overcome device specifications.
BJT as Switch 2
- Common emitter mode
- Linear Active Region
- Significant current Gain
Example:
- Let Gain, b = 100
- Assume to be in active region -> VBE=0.7V
- Find if it’s in active region
BJT as Amplifier
BJT as Amplifier
VCB>0 so the BJT is in active region
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