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involves two perceptual qualities or concepts: Concrete and Abstract.
Concrete quality deals with real or actual things that a person can register information through his/hers five senses: sight, hearing, taste, smell, and touch.
Abstract quality allows a person to conceive ideas, visualize and understand things that one cannot actually see.
When using abstract quality, a person is using one's intuition and imagination.
Too often abstract
concepts at schools are taught by using methods that do not aid in understanding
of concepts. Science textbooks particularly seem to be written and illustrated
in a manner that frequently lack many fundamental instructional techniques.
Abstract science topics are often explained only with aid of mathematics. The
problem is that mathematics itself is abstract --> there are no rules but
only assumptions. By trying to explain an abstract concept with an abstract
method can create a situation where the level of understanding decreases and
the level of ambiguity increases.
A method that in early years of education is usually beneficial is "doing it". That is applying the knowledge gained in a class room such as conducting experiments and observing demonstrations. However, at high school and college level this method is rarely successful in increasing students' level of understanding of the study subject. In-class demonstrations are helpful but not very effective specifically when dealing with abstract science topics. If the student does not understand the topic, the relevant lab experiment seldom improves the understanding of that particular topic. Too often the main purpose of the lab experiments is to conform mathematically derived results. Typically the only benefit derived from labs hours is the experience of using many test equipment.
One good method
to help a student to understand and to visualize an abstract concept, such as
a transistor operation, is to introduce an object (or device) that he/she can
identify and has similar characteristics as the topic being studied. It should
be noted that this device does not have to be real or operational.
In our transistor demonstration we have used a water valve as a visualization aid. This valve would not function in real life but its operation is still easy to picture.
In a sense what we have done is taken an abstract object, a transistor, and for visualization purposes compared this object to a less abstract object, a water valve, that is much easier to visualize. We chose a transistor for our example demonstration because this object is quite difficult to understand even for many electronics students. Since it is impossible to see the millions of electrons that form the electric current flowing across a transistor and how this current is controlled, for a person trying to comprehend or picture this in his/hers mind, this can be an overwhelming task.
We have utilized many other visualization and learning methods to explain the usage and operation of a transistor. Originally this example was targeted towards first and second year electronics students. This original version was much shorter and to the point. Since we knew the target audience's electronics background, many of the explanations included in this simplified version were not necessary. We have also added more material for this example because of the importance to show readers the "total picture". In this case, it was not enough to explain to a layperson a basic transistor operation without showing some examples and applications so that a person can form more complete picture. We have also tried to minimize the use of technical jargon. However, we have not omitted any electronics nomenclature from the text and illustrations in order to accommodate readers who are truly interested in this subject matter.
Another learning method we have used is repetition and reminders. Throughout the text many important topics are repeated or referenced to an earlier text. Many of the topics are also supplemented with simple straightforward examples.
Transistors below is far from being perfect example how to utilize various visualization
techniques to improve understanding. Nevertheless, it is a vast improvement
to traditional ways textbooks explain science concepts --> too much mathematics
and not enough visualization aids. To get a better idea what we are talking
about, it is advisable to review some typical textbook pages of the same topics.
Before continuing reading this material, click this link sample
textbook pages (205KB). Of course, this is not completely fair comparison
since some concepts displayed in these sample pages have been covered in previous
chapters in more detail.
There are many types of transistors. For this demonstration purposes we have chosen NPN bipolar junction transistor.
These type of transistors are the most common and familiar to any electronics student.
Note: This material
is simplified to accommodate readers with no or limited electronics background.
However, this subject matter is highly technical in nature. To improve the level
of understanding, it is highly recommended
that after reading each paragraph, a reader makes sure that he/she understands
what is being explained before continuing.
It is helpful to read each paragraph at least couple of times.
Operation of the Imaginary Water Valve:
This valve functions
in similar manner (with few exceptions) as any regular water valve. When the
red handle is turned to clockwise, the shaft of the valve turns also clockwise
and the valve moves towards closed position and thus restricting the water flow.
Turning the red handle counterclockwise will move the valve away from the closed
position and allowing more water to flow through.
(Note: You may skip this paragraph if it seems too technical.)
Look at the graph below the water valve. The horizontal axes depicts the water pressure difference between points C and E and the vertical axis the amount of the water flow (current). The red arrows at the end of the axes point towards increasing value. The blue line represents the range of the valve (from fully closed to fully open) and the red dot, Q-point (quiescent point), moving along the blue line is the position of the valve.
As the Q-point moves down, it indicates that the valve is moving towards closed position and the water flow is decreasing. At the same time the pressure between points C & E (PCE) is increasing.
The greatest pressure difference between points C & E takes place when the valve is completely closed.
Note: If you having difficulties to interpret this graph and the relationship between the variables, do not to worry. The graph shows the relationship between current and pressure difference at different valve positions (Q-point).The most important thing to understand is that the highest current flow through the water valve takes place when the valve is completely open (this is common sense).
Now the imaginary
Lets assume that turning the handle of the valve requires virtually no effort at all (friction and other restrictive forces are nonexistent).
Lets also assume that there is a coil (spring) around the shaft of the valve and a locking pin to lock the coil. (See the picture)
As mentioned earlier,
this imaginary valve works almost the same way as any regular valve with some
modifications. You first select the desired water flow by turning the valve
handle. At any time you can push the locking pin in to lock or "activate"
the coil (spring) mechanism. After you have locked the coil in place, you can
still turn the red handle of the valve in either direction away from this "set"
position. The difference would be that if you released the handle, the coil
would automatically return the valve back to this "set" position. This "set"
position of the valve is called the Q-point. (Setting this valve to the Q-point
is called in electronics biasing).
For instance, you could turn the handle until the valve is half open (green stopper half way in). You could push the locking pin in to "set" the valve to that halfway point (Q-point). Now, if you turned the valve further towards open position, the rate of water flow would increase.
If you suddenly let go the handle, the spring would return the valve to its original "set" position (half open).
If you can imagine this, you have now a basic understanding how transistors work. Well, maybe few clarifications would be helpful…
Instead of water
current flowing, it is electric current flowing through a transistor from the
collector to the emitter. The electric current is generated by a voltage source
such as a 9-volt battery. Battery is a source for direct non-alternating current
A transistor is made up three parts or regions: collector, emitter, and base. Think the collector being that part of the transistor where the main current enters and the emitter where the main current exits the transistor (the blue arrow at the emitter). Base is the part that controls the amount of current flowing through a transistor (like the red valve handle). The two transistor illustrations (above) are different representations of the same transistor. The one on top depicts transistor construction showing different parts. The one below is a symbolic representation of that transistor used in schematics etc.
On the graph below
the water valve, the blue line indicates not only the range of the water valve
but also the operating range of the transistor (from max current to no current
flow) and is called dc load line.
The Q-point is the "set" operating point of the transistor (same as the "set" position of the water valve).
So how does the base control the current flow in a transistor?
Instead of a mechanical
handle to move the valve, a base of a transistor requires current to "open a
path" for the main current to flow from the collector to the emitter. This current
is very small typically around 100 times less than the main current flowing
through that transistor. This small amount of base current can come from any
voltage source (battery, etc.). Remember our water valve with a coil and a locking
pin? This small amount of current to the base works the same way. The base current
sets the main transistor current flow to a certain level (Q-point level). The
transistor current remains at that "set" level unless some additional
outside current is also connected to base of the transistor. You could think
this additional current as person's hand turning the handle of the water valve.
In electronics, this outside current is in form of an alternating wave
such as a sine wave.
What is a sine wave and any other kind of alternating wave? Imagine someone turning the valve handle continuously back and forth so that the water flow changes from the level above the set level (Q-point) to below the set level and then back again. Similarly, an alternating wave changes continuously its direction and has either a positive or a negative value. (Your typical household alternating current works this way and it is called AC). Look at the illustration of an alternating wave. See how the wave moves up and down with respect to the Q-point level. Remember, this level is the initial or the"set" position of the water valve and also a "set" operating point of a transistor.
For example, lets pretend that the valve full open position equals to numerical value of ten (10) and fully closed position has value of zero (0).
Now, if we set the Q-point level to a value of five (5), it means that the valve is halfway open. For the alternating wave, lets assign arbitrarily highest positive value of plus two (+2) and minus two (-2) for highest negative value. Take again look at the illustration of the alternating wave and the Q-point level. You should understand that the wave values are relative values. It means that the wave flows value of two (2) upwards and value of two (2) downwards with respect to reference level (Q-point value of five (5) in this case).
The wave starts at the Q-point level of five and flows upwards increasing its value until it reaches the maximum positive value of seven (5 + 2 = 7). To visualize this, imagine your hand moving the handle counterclockwise to open the valve further away from the Q-point value of five (5) to value of seven (7) (highest valve open position possible in this example).
Since we had decided, that the value for valve full open was ten (10), a value of seven (7) indicates that the valve is not going to be completely open. Now, the wave starts flowing downwards. When it crosses the Q-point level, the wave has once again value of five (5).
Finally, at the maximum negative value the wave has value of plus three (5 - 2 = 3). It tells you that the valve will not be fully closed.
In summary, if the
current wave in the illustration is flowing upwards, then "the valve opens further"
and more current can flow through a transistor. On the other hand, if the current
wave is flowing downwards, "the valve is closing" and less current flows through
Should the alternating current to the base be removed, the main transistor current returns to its original "set" position (value of five (5) in our example). Hopefully, it is now more clear how the outside alternating current wave can control the flow of current through a transistor.
Note: In this text
you find many times descriptions how transistor's "valve opens or closes".It
is important to understand that there are no
actual mechanical moving parts (valves etc.) inside a transistor. These mechanical explanations are intended for reader's benefit to help visualize how transistors function.
A transistor is
just a small bar of silicon with wires attached to each region. Pure silicon
does not conduct current well. In order to increase
the conductivity of silicon, controlled amount of impurities is added to silicon. There are two categories of impurities: N-type and P-type.
N-type can be formed by adding antimony, arsenic, or phosphorus atoms to pure silicon.
P-type can be formed by adding a small amount of boron, aluminum, or gallium atoms to pure silicon.
The process of adding impurities to silicon is called doping. NPN - transistor's emitter and collector regions are called N-type and base region is called P-type, thus the letters NPN. The base region is narrower and less doped than the emitter and collector regions.
To fully understand
how a transistor functions you would need to know about atoms, doping, PN junctions,
depletion layers, bias voltages, hole and electron currents, positive and negative
ions, transistor currents, parameters and ratings, and many other topics.
Sounds difficult? Anyway, those topics belong to the classroom.
There are two main usages for transistors. First, a transistor can be used as an amplifier (amplify = to increase or make larger). For instance a transistor can amplify that alternating current connected to the base. This current or signal can be almost anything such as your voice off the microphone. Picture this small weak alternating current (signal) entering the base of a transistor. What happens to the main direct current (DC) flow through a transistor? It either increases or decreases (alternate) depending on the direction (up or down) of the alternating base current at the time. In other words, the main current flowing through a transistor is controlled by the small alternating current to the base. Imagine what happens to the water flow if someone continuously turns the valve handle in both directions? The rate of water flow through a valve will correspond closely to the movement of the valve handle. In a transistor, this means that the shape of the outside alternating current to the base matches perfectly to the main current flowing through a transistor from the collector to the emitter (perfect replica in shape). The only difference is that the main current flowing through a transistor is between 100 and 150 times stronger than the alternating current to the base. (See the illustration below).
is where the amplification comes from.
The main transistor current is an amplified copy of the alternating current to the base and this amplified current can be passed along to continue to flow in other parts of an electronic circuit.
Note: The term amplify can be confusing even to electronics students. Although it appears that amplification takes place inside a transistor, that is not exactly the case. A transistor is not some sort of a "miracle device" that can create energy from nothing. The weak alternating base current is not amplified. This base current just controls the much larger main transistor current flow and this control gives the impression of amplification.
Second, a transistor can be used as a switch.
In digital electronics
there are only two states or two possible conditions. These
two states can be called On-Off, True-False, High-Low, Yes-No, One(1)-Zero(0),
or logic level 1 - logic level 0. They all mean the same thing. In computers
all information and tasks are processed by
using digits 1's and 0's. These are called binary digits.
So, how are these binary digits created? Transistors are the key. They can be set up in such a way that they function as switches.
Like a light switch, you can select two possible states: On or Off.
Transistor switching is straightforward . Forget about the Q-points and "setting the transistor to a certain level." There is no amplification to concern. Think that a switching transistor is like a valve with only two positions: fully open or fully closed. This means that only alternating current is connected to the base. This alternating wave is in a form of rectangular wave. Compared to a sine wave, this type of wave produces much faster and "cleaner" on-off transition.
Unlike in amplification section where the alternating base current can be almost anything, in computers, waveforms are highly controlled and are typically created by the computer's clock generator. This wave is usually around +5 volts at maximum and around zero volts at minimum. These two voltage levels represents the binary digits (or levels) 1 and 0. The +5 volts (level 1) drives a transistor to is maximum current rate (full open position). This maximum main transistor current is called saturation current (Isat). When no current (zero volts) is applied to the base of the transistor, there is no current flow (full closed position). This is called cutoff (at level 0).
In order to form letters, numbers, symbols, etc., there are various codes that represent these characters. Alphanumeric codes, for instance, can represent all characters on computer keyboard. To "code" a letter or a number, one needs 8 transistors or 8 binary digits (bits) of information.
below is an example of a capital letter P and the respective states of each
These 8 bits or binary digits (1101 0111) that form a single character (P) are called one byte.
Logic operations such as calculations are performed by combining transistors to form logic gates. These logic gates can be arranged in such a way as to perform various tasks such as arithmetic operations. There are many different logic gates (AND, OR, NOT, NAND, NOR, EX-OR, EX-NOR) and each has its own symbol. The left side of the illustration below is an example of an AND - Gate.
See how the first
transistor's emitter is connected to the second transistor's collector. Imagine
two water valves connected in similar way.
Take look at the Truth Table and the inputs A and B. Truth Tables are used to show how a logic circuit's output responds to the various combinations of logic levels at the inputs. When both input waves A and B have value of zero (0) or logic level 0, it indicates that there is no input current flowing to the base in either transistor and the resulting output is also zero (no main current flow). It should be clear that there is no main current flow through transistors if just one of the transistors' input wave (A or B) is at voltage level zero (0). The only way the main transistor current can flow through transistors is when both inputs A and B have value of one (1) or logic level 1. If two water valves were connected this way, the only way the water to flow through the valves is when both valves are open. If one valve is closed, there is not water flow through either valve.
In computers the turning a single transistor on or off happens hundreds of millions of time in a second.
By use of truth tables and Boolean algebra, various logic gates can be combined to create logic circuits such as adders which perform additions of binary digits. Boolean algebra is used to analyze and describe a logic circuit and express its operation mathematically.
Small integrated chips can easily accommodate tens of thousands logic circuits and other circuits such as memory, registers, control and timing.
It should be noted that a typical fingernail-size microprocessor can contain tens of millions of transistors.
Transistors are made out of doped silicon and has three main regions: collector,
where the main transistor current enter transistor; base, that controls
the main current flow; and emitter; where the main current exits. When
used as an amplifier, a transistor has to be set to a certain operating point
(Q-point). This is done by applying a small direct current (DC) to the base
of the transistor. When a small alternating wave (current) is also applied to
the base, the main transistor current then fluctuates around the operating point
(level) according to shape and size of the alternating base current.
The main transistor current is typically more than 100 times larger than the alternating base current.
This main transistor current is then considered to be amplified copy of the alternating base current.
are used as switches, there is no direct current (DC) applied to the base. Main
transistor current is controlled by rectangular waves that drive transistor
either full open (saturation) or fully closed (cutoff). In digital electronics
there are two states or logic levels: logic 1 and logic 0.
Logic 1 usually describes voltage level around +5 volts and logic 0 is around zero (0) volts.
These two voltage levels represents binary digits 1 and 0. Various codes exist to represent characters in binary digits.
Logic operations such as calculations are performed by combining transistors in a form of logic gates. Truth Tables are used to show how the logic circuit's output responds to the various combinations of logic levels at the inputs. Logic circuits can be designed by combining various logic gates and by use of Boolean algebra and truth tables.
Congratulation, you have now a very basic understanding of NPN bipolar junction transistors.
Who said that understanding electronics is difficult?
Note to electronics enthusiasts. This "Understanding Transistors" sample was directed to people with no or limited electronics background. Some of the terms and explanations are not completely scientifically accurate. To explain the "correct terms" would have required a considerable amount extra space. Current flow for instance does not exist but it is much easier to visualize than explaining about "flow of charges". Also, it should be clear to any electronics student that battery does not provide current but to explain the purpose and function of a battery would have taken too long. The real reason for this text was not to educate electronics students but rather give lay persons a fundamental picture how transistors function and what many usage transistors have.
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