Transistor Lecture Text
Transistors:
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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