Bipolar Junction
Bipolar Junction
Transistors
Transistors
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An in-depth explanation of the operation of bipolar junction transistors (bjts), focusing on npn transistors. Topics covered include the behavior of the base-emitter and base-collector junctions, forward-reverse biasing, dc biasing, and the current gain (βdc) and alpha (αdc) parameters. The document also includes an example of determining the dc current gain and an explanation of the transistor dc model.
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2018/2019
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Bipolar JunctionBipolar Junction
Transistors Transistors
Bipolar Transistor Operation
• We will consider npn transistors
- (^) pnp devices are similar but with different polarities
of voltage and currents
- (^) when using npn transistors
- (^) collector is normally more positive than the emitter
- (^) VCE might be a few volts
- (^) device resembles two back-to-back diodes
- (^) with the base open-circuit, negligible current flows from the collector to the emitter
Bipolar Transistor Operation
- (^) because the base region is thin, most of the electrons entering the base get swept across the base-collector junction into the collector
- (^) this produces a collector current that is much larger than the base current – this gives current amplification
Bipolar Transistor Operation
• Transistor action
Bipolar Transistor Operation
DC BIASING
- (^) When a transistor is connected to dc bias voltages, VBB forward-biases the base-emitter junction, and VCC reverse-biases the base-collector junction.
DC Beta β DC & DC Alpha α DC
- (^) The dc current gain of a transistor is the ratio of the dc collector current to the dc base current ( IB) and is designated dc beta (β DC ).
- (^) Typical values of β DC range from less than 20 to 200 or higher. In data sheets it is written as
DC Beta β DC & DC Alpha α DC
- (^) The ratio of the dc collector current t o the dc emitter
current is the dc alpha ( α DC ). The alpha is a less-used
parameter than beta in transistor circuits.
Typically, values of α DC range from 0.95 to 0.99 or
greater, but α DC is always less than 1.
- (^) The reason is that IC is always slightly less than IE by the amount of IB. For example, if IE = 100 mA and IB = 1
mA, then IC = 99 mA and α DC = 0.99.
Transistor DC Model
- (^) The unsaturated BJT can be viewed as a device with a current input and a dependent current source in the output circuit for an npn.
- (^) The input circuit is a forward-biased diode through which there is base current.
- (^) The output circuit is a dependent current source with a value that is dependent on the base current, IB, and equal to β DC IB.
Transistor DC Model
BJT CIRCUIT ANALYSIS
BJT CIRCUIT ANALYSIS
- (^) The base-bias voltage source, VBB, forward-biases the base-emitter junction, and the collector-bias voltage source, VCC, reverse-biases the base-collector junction.
- (^) When the base-emitter junction is forward-biased, it is like a forward-biased diode and has a nominal forward voltage drop of VBE = 0.7 V
BJT CIRCUIT ANALYSIS
Example
- (^) Determine IB, IC, IE, VBE, VCE, and VCB in the circuit. The transistor has a β DC = 150.