Lecture Text Transistor

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Transistor Lecture Text


A transistor is an active component that can be used as an amplifier or a switch.  In a Class A amplifier a small signal is applied to the base and an amplified undistorted output is produced on the collector and the emitter.  The output can exhibit current and voltage gain, but what is most significant is that a transistor is capable of generating considerable power gain.  The transistor circuit does not actually create power, but gets its power from a battery or DC power supply.  A transformer can increase voltage at the expense of current, or it can increase current at the expense of voltage .  Put the power output of a transformer in watts will always be less than input wattage.  A transistor generally has three or more leads.  Bipolar transistors have a Base, Emitter, and Collector.  The base emitter and the base  collector junctions behave like back to back diodes.  This actually is a simple way to  test a transistor, since transistor failures frequently consist of open or shorted PN-junctions. 

Transistor Physics:

When the base emitter junction is forward biased the depletion zone is decreased and base-emitter current will flow.  This is what one would expect from diode theory.  Something new also occurs.  Minority carrier electrons become more abundant in the depletion zone, these electrons enable current conduction between the collector and the emitter.  When the transistor is correctly biased, than typically a hundred times more current will flow through the collector than flows through the base.  The ratio of collector to base current is called Beta.  I will include links to more extensive and more correct presentations of transistor physics in this section.  The truth is that Quantum Mechanics is required to fully explain semiconductor operation; however, only a few simple rules need to be understood in order to troubleshoot transistor circuits or even design simple transistor circuits.  My way of teaching involves explaining operation of circuits that I have animated or simulated.

Biasing Bipolar Transistors: 

A transistor must be biased so that both base current and collector current flow in the direction of arrow on the transistor symbol.  The arrow on the transistor schematic symbol points in the direction of conventional current flow.  Conventional current flow is in the opposite direction of electron flow.  I use NPN transistors in most of my animations.  Silicon require above .6 volts to drive transistor towards saturation and Germanium require above .3 volts to drive transistor toward saturation. 

 Memory Tool: NPN = Not Pointing In.

Other Types of Transistors:  I have only animated Bipolar transistors.  I will still talk a little about other types.

Field Effect Transistors:

Field Effect transistors are controlled by the Gate voltage, the Gate draws very little gate current.  I will only discuss  N-type channel Field Effect Transistors.  In Junction type Field Effect transistor, a P type gate is bonded to N-type channel.  This results in an actual PN-junction being formed at the Gate to channel bond.  Junction Field Effect Transistors are reverse biased, and therefore no current flows through the gate.  The gate voltage determines the E field in the junction area.  That E field controls or effects the conductivity of the N channel.  One end of the channel is called the Source and the other end of channel is called the Drain.   The input impedance of the Junction Field Effect Transistor is in the Meg-ohms.  The Insulated Gate Field Effect transistor has an insulating layer of metal oxide separating the gate from the channel.  Thus, the Insulated Gate Field Effect transistor does not have to be reverse biased.  The metal oxide layer provides billions of ohms of insulation.   This makes the Insulated Gate Field Effect Transistor excellent for amplifying the high impedance output of a transducer without effecting the transducer DC output. 

Field Effect Transistors are often used in the input stage of operational amplifiers in order to give the input a very high impedance.   Operational amplifiers are usually integrated circuits, and they will be discussed in the next lecture.

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Last modified: Monday July 07, 2014.