MNQRP NEWS

April 1998 Volume 1 Number 4

Presidential Address

April Meeting Highlights from the Speaker Himself

Claton Cadmus

KA0GKC

Yours truly is going to be the fool of this month's program and in keeping with the season, I have entitled the program "April Tools". I plan on talking a bit about the following:

Basic tools for electronic building fools.

Easy to make but foolish looking building helpers.

Ideas for apartment homebrewing fools.

A fool's guide to electronic bench design.

Taking care of the fool's tools.

So with that said I would also like to ask those planning to attend April's meeting to bring your foolish toolish ideas to share too!

73 de Cla KA0GKC

Meeting Reminder

We are returning to our normal meeting location for the April meeting as well as for the remainder of 1998. See everyone at the Minnetonka Community Center, 1:30PM! Talk in on the 145.05 repeater. For more information on the meeting location as well as the MNQRP club in general, visit the website at

http://www.qsl.net/mnqrp/

The Reluctant Editor - A Look to the Future

Chris Wilken

KB0DAL

Once again, it appears as though I managed to figure out the rudiments of newsletter editing and get this issue out the door to one and all. While I still find myself "stretching" the size of the fonts a bit to fill pages, I think we are well on our way to having a decent newsletter that will hopefully serve as a useful supplement to our regular membership meetings.

Looking at the latest club roster, we are inching ever closer to the 60 member mark. This is quite an accomplishment for a club of our longevity, and through such things as operating events, educational meeting presentations, and interesting newsletter articles, this number should only grow.

Although not everyone can make the meetings all the time, I look forward to using the newsletter to better keep our non-meeting members informed, and perhaps entice them to attend a Saturday session now and again. With summer coming, the demands over and above our radio hobby will make it harder for even more to come - but that should be no excuse not to keep in touch!

Starting with the next issue, I hope to publish a set of the minutes from the previous meeting as well as a summary of any presentations given at that meeting. For those of you not able to attend last month, David Donaldson gave a very interesting presentation on basic radio propagation and shared a number of computer and internet based sources for additional information.

Those of you that have expressed a desire to help with the newsletter, but are reluctant to contribute a full article, are more than welcome to help take notes and forward whatever "interesting" things heard or seen at the meeting that others may find interesting as well.

I try to pay attention to everything, but as our meetings become larger and involve more "post meeting discussion sessions," I can't cover it all. Everyone's help in this matter would be appreciated, not only by me, but the growing numbers of our fellow QRPers unable to attend the regular meetings.

We also need more pictures. If anyone has pictures of their rigs, projects, hamshacks, etc. I would like to include them in the newsletter. If you are willing to share printed photos, I can have them scanned. For those of you that take a lot of pictures, please consider having a "photo disk" made. This is a nice feature a number of the metro area drug and convenience store photo counters are offering. At the time you drop your film off, you can choose to have your pictures copied to 3.5" disk in JPEG format. The cost is an additional $4 or so over and above the regular processing fee. This is a fantastic idea, in that you now have a disk full of "computer" ready pictures that you can email to friends, family, and so on. And if you send me a picture of your latest homebrew project, so much the better! You can be guaranteed that it will make it in the newsletter!

That's it for now. Keep your ideas coming. I appreciate those that have helped and look forward to making things bigger and better as we go.

'73's Chris - KB0DAL

A BIPOLAR TRANSISTOR PRIMER

Bob Liesenfeld

WBØPOQ

PART II

Last time, we looked at the basic operation of a bipolar junction transistor or BJT. In this part we will begin to investigate the methods used to apply these devices in practical small signal audio amplifiers. Most importantly, we will learn the rules to design and construct stable, predictable circuits. This is an important distinction. I recall as a youth stumbling my way through some audio power amps that while they worked, the devices got VERY hot, and my allowance was rapidly depleted buying 9V batteries for it.

One of the first steps is to decide upon some DC values. Some of these values are simply selected from a range of possible values. I have had many students ask questions such as "what equation did you use to arrive at that value of current?" The answer is frequently "you need to pick a starting point and build up the other values from there". This will become clear in the examples to follow. The first example will demonstrate 'simple bias'. "Bias" is the term used to describe the DC resting values for the various currents in the circuit, or quiescent point. I think you will see the reason for 'simple' shortly.

SIMPLE BIAS DESIGN EXAMPLE

First, as mentioned above, we need some starting values. Lets assume we are using a 9V battery as the power supply. This supply is called 'V_CC ' Next, we need to pick a value for collector current. Since this is to be a small signal amp using a battery supply, a few mA would be an appropriate choice. I'm going to use 2mA. Something up to 10mA might also be appropriate, but at decreased battery life. From a data book we can find that the H_FE of a 2N2222 is about 180-200. I'll use 200. Ok, we are ready to start our design!

Recall from last time the following equation:

IC=IB*H_FE

that is collector current = base current times beta. Since we desire a collector current of 2mA, and H_FE is 200, IB must = 10uA. Another way to look at this is, if 10uA of base current is made to flow, collector current will = 2mA. But how do we get the desired value of base current to flow? With a 'bias' network. Please refer to figure 1.

Note the collector battery is in the same place as one of the power supplies from our last discussion. But now observe that I have added a resistor from the base to the positive side of the battery. This will be the path for base current. We will call this resistor "RB". But how do we arrive at the correct value of this resistor? We use Ohms Law. Much BJT design and troubleshooting work is simply Ohms Law.

To do this, we need one more bit of information. That is that in a functioning BJT amp, the voltage measured from the base to the emitter will be .6V, nearly independent of the value of base current. This voltage is surprisingly constant with widely different values of RB and is tied up with the physics of the PN junction where the base joins the emitter.

Ok, Now we have the information needed to calculate RB. Figure 2 depicts the same circuit with all known values labeled. Notice that since from point A to point B is 9V (the battery), and from point C to B is .6V, the voltage across RB must be the difference between these two values or 8.4V. Recall that we desire 10uA of base current to flow. The path for this current will be from the negative pole of the battery, into the emitter, out the base, and through RB back to the positive side of the battery. Each and every electron that leaves the base (base current) must also flow through RB. Conversely, any electron that flows through RB came from the base. Base current and the current through RB are one and the same. We now know the voltage across RB, (8.4V) and the desired current through it (10uA). Ohms law states R=E/I, or in this case 8.4V/10uA, or 840K Ohm. This technique will be the basis for all the biasing schemes we will investigate.

Now it is time to add a resistor in the collector circuit to act as a load, which together with the transistor will generate our output signal. Recall that our device has a Beta of 200, so with 10uA of base current, 2mA of collector current will flow. Please see figure 3. Here is the same circuit, but with a resistor (RC) in series with the collector to the positive pole of the battery. As before, any electron that leaves the collector must flow through RC, and any electron that flows though RC came from the collector. Again physics rears it's head in the fact that in a BJT, the collector current is determined almost entirely by the value of the base current and is nearly independent of the value of RC. This suggests we have wide latitude in selecting the value of RC. However, for reasons that will become clear when we do an AC analysis of this circuit, it is highly desirable to have 1/2 Vcc across RC. Note I have indicated this value in figure 3. As before, if there is 1/2 Vcc across RC, the other half (4.5V) must be from point D to B. Now we can calculate RC's value. RC=4.5V/2mA or 2250 Ohms.

At this point we have a usable BJT amplifier, albeit, not a particularly stable or repeatable one. This design while simple, has several pitfalls. First, the computed resistor values are heavily dependent on the value of Beta, which can vary a fair amount between samples of the same device. Secondly, the computed values will rarely be standard resistor values, and the circuit is quite sensitive to small changes in these resistor values. However, it does have it's uses, one of which will be to model the AC behavior of the circuit which we will do next time.

In the meantime, why not breadboard this circuit, and take some readings: There is nothing like the sweet smell of the magic smoke from a 2N2222!

As always, E-mail me with questions/comments.

72

Bob 'POQ

wb0poq@visi.com

Newsletter contributions are always welcome. Please forward your materials to:

Chris Wilken

5000 147^th Street West

Savage, MN 55378

Fax 712-262-1001

cwilken@greatlakesav.com

See you at the meeting!

Calendar of Events

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