March 1998 Volume 1
Number 3
That's about the only way to describe it! We did very well at our first FYBO! 218,448 points. It was great fun, if you didn't come and see you really missed out. The weather was so good, we were wishing last Field Day had been this nice! Thanks go to all those that had fun with us and we hope to see more of you at upcoming club events. Don't be afraid to get in there and help, it's always welcome and good for laughs. Besides, where else can you hang out with people that understand what you're talking about!
This month's meeting is Saturday the 7th and I'm going for a new business meeting record! Leaving plenty of time for our program on Propagation, presented by Dave Donaldson WB7DRU. This is very timely now that we're creeping out of the bottom of the cycle. I've noticed 10 meters has been opening up a bit lately. What about it Dave?
I'd like to introduce our new editor Chris Wilken, KB0DAL. Chris was kind enough to take this job and we all need to help him. Write an article, post an classified ad, ask a question, even send in a joke! Chris will use the stuff. So here's the email address to send this stuff to:
cwilken@greatlakesav.com
See you at the meeting!
73 de Cla KA0GKC
We have our normal meeting room reserved for the remainder of 1998 however, as you will recall, someone else had previously reserved it for them on Sat. March 7th. Therefore, we will be meeting in the St. Alban's room, just down the hall from the Club station room. We won't have access via that door to the right just inside the main entry; instead, come all the way in and turn to the right. There is another access door just across from the Information Counter. We will still be there for Talk-In on the 145.05 repeater.
73 and hope to see a good turnout.
Wayne KB0PCI kb0pci@n0bsn.ampr.org
WBØPOQ
This series of articles, expressly written for the MNQRP newsletter, will provide some basic information on the operation and use of common bipolar junction transistors (BJTs). It is not meant to be an exhaustive treatment of the subject, but more an introduction with emphasis on the practical implementation of these devices.
Assuming I do my job, at the conclusion of this series you shall be able to design and build stable, predictable, audio and low power tuned rf amplifiers.
PART ONE: THE DEVICE
For much of this series we will be using a 2N2222 as the device of choice. These devices (and the multitude of similar types) are readily available, and have characteristics that make them useful for many applications. They work from DC up through the audio range, well into the HF region. A bin with a few dozen 2N2222s will stand the homebrewer in good stead.
A BPJ transistor is a relatively simple device, at least on the surface. See fig 1.
The device is composed of 3 sections. The emitter, base, and collector. For us old tube types, these correspond roughly to the cathode, grid, and plate, respectively. However, there the similarity ends. The transistor is composed of pieces of silicon that have been "grown" or diffused together to form one integral unit. The 3 sections have somewhat differing chemical and electrical properties. Each section has been "doped" with what is known as an "impurity" element. Normally, a piece of pure silicon forms a very stable, electrically neutral (equal number of protons and electrons) material, where electrons are not easily moved. After doping, the silicon material retains it's electrical neutrality, but it's crystalline structure has been modified to change it's chemical nature. This can be done in one of two ways.
One way is to dope the silicon with a material that causes a number of atoms in the structure to be moderately receptive of any free electron that may find it's way into the material. This is called "P" type material, and is used for the "base" in our 2N2222. This affinity for electrons is sometimes called a "hole", suggestive of the place an electron could plop into.
The other option is to use a doping agent that causes the silicon structure to contain a number of loosely bound electrons that can be motivated to move, especially if a hole is nearby. This point will be of great import later. This is called "N" type. Our emitter and collector are composed of "N" type material. Therefore, our device is called an "NPN" transistor.
It is important to note that the doping process does not lend a net positive or negative charge to the silicon materials. Pure silicon,
N type, or P type silicon are all electrically neutral (same total number of protons and electrons). It is the "affinity" to accept, or the "willingness" to relinquish an electron that is being manipulated.
One of my old professors used to use the analogy of the "chemistry" between males and females to illustrate this concept, but that is another story....
It is also important to understand two other factors that are manipulated to control the operation of our device. First, the base is made physically quite thin in comparison to the emitter and collector. Generally, the thinner the base, the higher the achievable gain, albeit at the cost of many other characteristics that can reek havoc at higher frequencies.
Secondly, the collector material is heavily doped, which in N type material, means it has a great number of electrons that can encouraged to move. This is a second vital point.
Consider fig 2. In this example we have connected a variable power supply between the emitter and base. Note the polarity. If this supply is over approximately .6VDC, electrons will leave the negative side of the battery, stimulating the loosely bound electrons in the emitter to move towards to base region (Remember, like charges repel).
Meanwhile, the base region (with it's P type affinity to accept electrons) is faced with a like minded positive battery terminal. Again like charges repel. The net effect is that emitter electrons are "herded" towards the base, whilst base "holes" are motivated towards the emitter.
The region where the base joins the emitter is the stuff of kilo page physics textbooks, but suffice it to say that there is an "exchange". "Holes" appear in the emitter material, where they eagerly combine with an electron from the power supply while electrons appearing in the base are drawn into the positive terminal of the supply. For every electron that leaves the negative terminal of the supply, one will enter the positive terminal. The net number of electrons in our device has not changed, just moved.
Increasing the applied voltage slightly will increase the amount of current significantly. If carried too far, the base-emitter junction will be destroyed, so a resistor is placed in series with the battery to allow some control over the current.
Now for the good stuff.....!
Please refer to fig 3. Note a second power supply has been added. Also, note the polarity. The negative side of this supply also motivates emitter electrons towards the base. The positive side of this supply pulls on the heavily doped collector electrons, and moves a great many of them to the top of the material, away from the collector-base region. Note that these electrons do not leave the collector material, they simply move away from the vicinity of the base. This leaves a great number of holes (remember the collector is heavily doped) in the collector material abutting the base, many more per unit area than to be found in the thin base.
Also, understand that these holes are quite special, in that they represent a place where a electron has been taken out of an electrically neutral atom, and moved away. This means that these atoms are now electrically charged and desperately want electrons.
Consider the following....The power supply between base and emitter causes electrons to enter the base from the emitter as described above, where they encounter chemically induced, (by doping) holes. These holes have a moderate desire for electrons. But now, just on the other side of the thin base material are many, many, desperately electron-hungry holes. Most (not all) of these electrons will be "sucked in" by the collector material, while a few make their way to the supply attached to the base. For every electron sucked in by the collector, one will leave the top of the collector material and enter the positive terminal of the supply. For every 1 electron that travels to the base battery, 200 may go to the collector. Note that all electrons, whether they end up flowing into the base supply, or getting sucked up by the collector, originated in the emitter material.
In addition, if the base power supply voltage is raised slightly, the current leaving the base will increase, and so will the current leaving the collector by the same ratio as in the example above! That is to say if the base current is made to increase by say 1uA, the collector current will increase by 200uA. The long and short of this is that a small change in base current will result in a much larger change in collector current. This is how we get gain, or amplification from a BJT. It is interesting to note that it is changes in, or amplification of current that is occurring here. BJTs are called current controlled devices for this reason. Another interesting point is that as long as the collector-emitter supply is over 2 or 3 volts, changing it's value will have little effect on collector current. The base current is the predominately controlling factor of collector current. Opening the path to the base power supply will cause the base current to fall to zero, which in turn halts the flow of collector current. (This ignores a factor called leakage current, which is very small can be disregarded for this discussion).
This ratio of current leaving the collector vs current leaving the base, is called HFE, or DC Beta. It is on the order of 180-200 for our 2N2222. HFE ranges from around 10 to about 250 for common BJTs.
To put this discussion in codified terms, we can use the following formulas:
IC=IB*HFE (Collector current = base current*Beta
IE=IC+IB (Emitter current = collector current + base current)
These two equations are at the heart of designing BJT amplifiers.
In the next installment of this series, we will examine using this knowledge to design a rudimentary audio amplifier, and look at some improvements that will produce a stable, practical circuit. Until then 72!
Dit dit
Bob 'POQ
The Reluctant Editor
Chris Wilken
KB0DAL
I apologize to everyone for the delay in getting this issue out. In the last 24 hours, I've had to re-learn everything I thought I knew about MS Word to get this issue to somewhat resemble the fine job Claton did on the premiere edition of our new club newsletter. I would further like to thank Claton for his excellent help in setting up the relatively "idiot proof" templates to build the newsletter from.
I'll quickly figure out how to glue this thing together each month. All I ask of the rest of you is a little help in filling it up.
Next month's MNQRP News is guaranteed to be much better. Hopefully, with your help, contributions such as Bob's, WB0POQ, will be common place.
Calendar of Events
Please send in events to
the editor.
We can't show them if we don't know them!
| JANUARY 99 2 MNQRP Meeting |
FEBUARY 99 6 MNQRP Meeting |
MARCH 98 7 MNQRP Meeting |
| APRIL 98 1 Watch for QST Spoof |
MAY 98 2 MNQRP Meeting |
JUNE 98 6 MNQRP Meeting |
| JULY 98 4 Holiday |
AUGUST 98 1 MNQRP Meeting |
SEPTEMBER 98 5 MNQRP Meeting |
| OCTOBER 98 3 MNQRP Meeting |
NOVEMBER 98 7 MNQRP Meeting |
DECEMBER 98 5 MNQRP Meeting |