Slide 1: BIPOLAR JUNCTION TRANSISTORS
Slide 2: Bipolar Transistor
Bipolar transistors are three-terminal devices that act as electrically controlled switches or as amplifier controls. These devices come in either npn or pnp configurations An npn bipolar transistor uses a small input current and positive voltage at its base (relative to its emitter) to control a much larger collector- toemitter current. Conversely, a pnp transistor uses a small output base current and negative base voltage (relative its emitter) to control a larger emitter-to collector current.
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Slide 3: Bipolar Transistor
An npn bipolar transistor is made by sandwiching a thin slice of p semiconductor between two n-type semiconductors. When no voltage is applied at the transistor’s base, electrons in the emitter are prevented from passing to the collector side because of the pn junction. (Remember that for electrons to flow across a pn junction, a biasing voltage is needed to give the electrons enough energy to “escape” the atomic forces holding them to the n side.) Notice that if a negative voltage is applied to the base, things get even worse—the pn junction between the base and emitter becomes reverse-biased. As a result, a depletion region forms and prevents current flow.
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Slide 4: CONSRUCTION AND SYMBOL
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Slide 5: Bipolar Transistor
If a positive voltage (of at least 0.6 V) is applied to the base of an npn transistor, the pn junction between the base and emitter is forwardbiased. During forward bias, escaping electrons are drawn to the positive base. Some electrons exit through the base, but—this is the trick—because the p-type base is so thin, the onslaught of electrons that leave the emitter get close enough to the collector side that they begin jumping into the collector. Increasing the base voltage increases this jumping effect and hence increases the emitter-to collector electron flow. Remember that conventional currents are moving in the opposite direction to the electron flow. Thus, in terms of conventional currents, a positive voltage and input current applied at the base cause a “positive” current I to flow from the collector to the emitter.
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Slide 6: OPERATION
Emisor N e h Ie Vbe -+ Je Ib Jc Base P Colector N Nd >> Na Na == Nd
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Slide 7: OPERATION
e
h Je Ib Jc Vcb -+ Ic
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Slide 8: OPERATION
Nd >> Na e heh Vbe -+ IE
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Na == Nd
e Ic
Ib
Vcb -+
Slide 9: OPERATION
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Slide 10: Bipolar Transistor
Saturation region refers to a region of operation where maximum collector current flows and the transistor acts much like a closed switch from collector to emitter. Cutoff region refers to the region of operation near the voltage axis of the collector characteristics graph, where the transistor acts like an open switch—only a very small leakage current flows in this mode of operation. Active mode/region describes transistor operation in the region to the right of saturation and above cutoff, where a near-linear relationship exists between terminal currents (IB, IC, IE)
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Slide 11: BJT BIAS CONDITION AND REGION OF OPERATION
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Slide 12: EBER’S MOLL MODEL
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Slide 13: ACTIVE REGION
EBER’S MOLL MODEL
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Slide 14: CUTOFF REGION
EBER’S MOLL MODEL
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Slide 15: SATURATION
EBER’S MOLL MODEL
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Slide 16: Bipolar Transistor Types
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Slide 17: TRANSISTOR LEAD IDENTIFICATION
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Slide 18: TRANSISTOR GAIN TEST
A TRANSISTOR GAIN TEST can be made using an ohmmeter and a simple test circuit. The principle behind this test lies in the fact that little or no current will flow in a transistor between emitter and collector until the emitter-base junction is forward biased. A 10to-1 resistance ratio in the test between meter readings indicates normal gain.
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Slide 19: TRANSISTOR JUNCTION RESISTANCE TEST
TRANSISTOR JUNCTION RESISTANCE TEST can also be made using an ohmmeter by measuring the baseemitter, base-collector, and collector-emitter forward and reverse resistances.
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Slide 20: OPERATION
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Slide 21: CHARACTERISTIC CURVE
Ic Ic= α Ie P Ie Iex Vcb P Vbex Icx
Vbe
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Slide 22: EFFECTS OF TEMPERATURE
Ie Ie
T1>T2>T3
0,5
1
Vbe
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Slide 23: TRANSISTOR FORMULAS
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Slide 24: BJT AS AN OPEN SWITCH
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Slide 25: BJT AS A CLOSED SWITCH
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Slide 26: TRANSISTOR AS A SWITCH
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Slide 27: PROBLEM
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Slide 28: TRANSISTOR AS AN AMPLIFIER
Slide 29: TRANSISTOR AS AN AMPLIFIER
The need for amplification arises because transducers provide signals that are said to be weak and possessing little energy, such signals are too small for reliable processing and processing is much easier if the signal magnitude is made larger. The functional block that accomplishes this task is the signal amplifier An amplifier that preserves the details of the signal waveform is characterized be the relationship
Slide 30: TRANSISTOR AS AN AMPLIFIER
Slide 31: TRANSISTOR CONFIGURATIONS
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Slide 32: COMMON-EMITTER CONFIGURATION
The COMMON-EMITTER CONFIGURATION (CE) is the most frequently used configuration in practical amplifier circuits, since it provides good voltage, current, and power gain. The input to the CE is applied to the base-emitter circuit and the output is taken from the collector-emitter circuit, making the emitter the element "common" to both input and output. The CE is set apart from the other configurations, because it is the only configuration that provides a phase reversal between input and output signals.
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Slide 33: COMMON-BASE CONFIGURATION
The COMMON-BASE CONFIGURATION (CB) is mainly used for impedance matching, since it has a low input resistance and a high output resistance. It also has a current gain of less than 1. In the CB, the input is applied to the emitter, the output is taken from the collector, and the base is the element common to both input and output.
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Slide 34: COMMON-COLLECTOR CONFIGURATION
The COMMON-COLLECTOR CONFIGURATION (CC) is used as a current driver for impedance matching and is particularly useful in switching circuits. The CC is also referred to as an emitter-follower and is equivalent to the electron-tube cathode follower. Both have high input impedance and low output impedance. In the CC, the input is applied to the base, the output is taken from the emitter, and the collector is the element common to both input and output. GAIN is a term used to describe the amplification capabilities of an amplifier. It is basically a ratio of output to input. The current gain for the three transistor configurations (CB, CE, and CC) are ALPHA (a), BETA (b), and GAMMA (g), respectively.
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Slide 35: TRANSISTOR CONFIGURATION COMPARISON CHART
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Slide 36: TRANSISTOR AS AN AMPLIFIER
Rc Vcb Ic Vce Vbe Ie Rb Ib Vbb Vcc
Slide 37: DC LOADLINE-GRAPHICAL ANALYSIS
Slide 38: TRANSISTOR AS AN AMPLIFIER
