Electronics II Lecture 2(a): Bipolar Junction …Analysis of this transistor circuit to predict the dc voltages and currents requires use of Ohm’s law, Kirchhoff’s voltage law
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Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transistor !• Transistor=Transfer+Resistor. When
Transistor operates in active region its inputresistance is high and output resistance islow. Transistor as a device which transfersits resistance from high to low and by thisproperty transistor amplifies/control inputcurrent signal.
• Transistor is a three terminal semiconductordevice used as amplifier, switch oroscillator.
• bipolar junction transistor (BJT ) is a typeof transistor that relies on the contact of twotypes of semiconductor junctions for itsoperation
2 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Basic Transistor Operation
3 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
Look at this one circuit as two separatecircuits, the base-emitter(left side) circuitand the collector-emitter(right side)circuit.
Note that the emitter leg serves as aconductor for both circuits. The amountof current flow in the base-emitter circuitcontrols the amount of current that flowsin the collector circuit. Small changes inbase-emitter current yields a large changein collector-current.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transistor Structure
4 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
The BJT (bipolar junction transistor) is constructed with three doped semiconductorregions separated by two pn junctions, as shown in Figure (a). The three regions arecalled emitter, base, and collector. Physical representations of the two types of BJTsare shown in Figure (b) and (c).
One type consists of two n regions separated by a p regions (npn), and other typeconsists of two p regions separated by an n region (pnp).
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transistor Currents
5 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
The directions of the currents in both npn and pnp transistors and their schematicsymbol are shown in Figure (a) and (b). Notice that the arrow on the emitter of thetransistor symbols points in the direction of conventional current. These diagramsshow that the emitter current (IE) is the sum of the collector current (IC) and thebase current (IB), expressed as follows:
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transistor Characteristicsand Parameters
6 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
Figure shows the proper biasarrangement for npn transistorfor active operation as anamplifier. Notice that the base-emitter (BE) junction is forward-biased and the base-collector(BC) junction is reverse-biased.As previously discussed, base-emitter current changes yieldslarge changes in collector-emittercurrent. The factor of this changeis called beta().
= IC/IB
The ratio of the dc collector current (IC)to the dc emitter current (IE) is the alpha.
Electronic Devices and Circuit Theory, 10/eRobert L. Boylestad and Louis Nashelsky
Transistor Characteristicsand Parameters
8 Copyright @2013 by Dept. of Electrical Engineering,College of Engineering, Maysan University
Copyright @2013 by Dept. of Electrical Engineering, Electronics IICollege of Engineering, Maysan University
The collector current is determinedby multiplying the base current bybeta.
Analysis of this transistor circuit to predictthe dc voltages and currents requires useof Ohm’s law, Kirchhoff’s voltage law andthe beta for the transistor.
Application of these laws begins with thebase circuit to determine the amount ofbase current.
Using Kichhoff’s voltage law, subtract the.7 VBE and the remaining voltage isdropped across RB.
Determining the current for the base withthis information is a matter of applying ofOhm’s law. VRB/RB = IB