How does an open base transistor affect collector voltage? Can you tell what's open in a transistor circuit by what's happened to the element voltages?

When you  work with tubes, you know that, if the plate voltage is the same as the supply voltage, the cathode resistor is almost certainly open. Why not learn the same diagnostics for transistors?

Look at Fig. 1. A basic n-p-n transistor circuit. Here, the collector voltage will always be the most positive, while the emitter voltage will be the most negative. The base will be biased so that it is slightly positive with respect to the emitter, and negative with respect to the collector.

The same relationship will hold true for the p-n-p transistor except that the polarity of the voltages will be reversed.

One battery supply

For simplicity, two batteries are shown in Fig. I. Most practical circuits use an arrangement like that in Fig. 2 to obtain the operating voltages from a single battery.

In this arrangement, the collector is connected to one end of the battery and the emitter to the other end. The base is forward-biased with respect to the emitter by tapping off a part of the battery voltage via RI and R2. Polarity depends on whether an n-p-n or p-n-p transistor is being used.

Most technicians find it easiest to measure voltages from ground to the various transistor elements. This is quite acceptable. However. when a power supply like the one in Fig. 2 is used, either end of thc battery may be grounded.

Fig. 3-a shows an n-p-n transistor circuit with the negative side of the battery grounded. Fig. 3-b shows the same circuit with the positive side grounded. The voltages measured at the various elements differ not only in value, but also in polarity, depending on which end of the battery is grounded.

For instance, the collector voltage in Fig. 3-a measures + 5.5 from ground. The collector voltage in Fig. 3-b measures -0.5 from ground. So be sure to note which end of the battery is grounded before you begin measuring.

##### Open base circuit

Fig. 4-a shows the effect an open  base circuit has on the operating voltages of an n-p-n transistor. The uncircled values show the normal operating voltages, while the circled values show the voltages measured from ground with the defect.

The collector voltage has increased. This happens because, with the base circuit open, the base-to-emitter bias disappears and the collector circuit stops conducting. When the collector is not conducting, no current flows through R4 and there is no voltage drop across it. As a result, the collector voltage rises to the battery voltage.

Since the collector has stopped conducting, there is no appreciable current flow through R3 and no voltage drop across it, either. Thus, the emitter voltage falls to zero.

The base voltage becomes zero because it is no longer connected to its operating voltage.

In the p-n-p circuit of Fig. 4-b, an open base resistor has quite a different effect on the operating voltages. The collector voltage has dropped to zero, and the base and emitter voltages have risen to the full battery voltage.

This is because the battery polarity has been reversed, compared to the n-p-n circuit of Fig. 4-a, to supply the proper operating voltages for a p-n-p transistor. As a result, when you measure between collector and ground, you are reading the drop across R4. With thc base circuit open, there is no drop across this resistor because there is no collector current, and you measure zero voltage.

When you put the probes from ground to emitter, the battery voltage, less the drop across R3, is being measured. But since the collector circuit has stopped conducting, there is no drop across R3. As a result, you find the full battery voltage at the emitter.

The base circuit is open and no longer connected to its operating voltage, yet it measures the same voltage as the emitter voltage. This happens because a transistor has a low internal resistance between base and emitter, so the base rises to the emitter voltage.

Open emitter circuit

Fig. 5-a shows tile effect of an open emitter circuit. The collector stops conducting and there is no current in the collector circuit. This results in no voltage drop across R4. With no drop across R4, the full battery voltage appears at the collector.

The open emitter circuit also stops the slight current flow in the base emitter circuit. When this current flow is stopped, the voltage at point A rises slightly, causing the base voltage to rise also (go more positive).

The open emitter, because of the low internal resistance of the transistor, then assumes the base voltage.

In Fig. 5-b a p-n-p transistor with an open emitter circuit is shown. Here, the collector voltage is zero because there is no current flow in the collector circuit, and no voltage drop across R4.

The base voltage in this case has dropped slightly. As with the n-p-n transistor, a slight base-to-emitter current flows through RI. When the emitter--base circuit is open, this current flow stops and the voltage at point A drops slightly (goes less positive). This causes the base voltage to drop. The open emitter then assumes the base voltage.

##### Open collector circuit

Fig. 6 shows the effect an open collector circuit will have on the normal operating voltages of a transistor. In both the n-p-n and the p-n-p circuits, the emitter and collector voltages have become equal. The base voltage has changed very little.

Since the collector circuit is open, no current flows through it and the collector voltage rises or drops to the same voltage as the emitter.

Because of the large change in the collector voltage, and only a small change in emitter and base voltages, the defect is clearly in the collector.

KN

Classic Column

By

Nancy Kott, WZ8C

Inside every Ham is a CW operator just waiting to get out. I know this from personal experience. Let me tell you a bit about my Amateur Radio history. My father, W8ROG, has been a Ham since the 1930s, so I always knew Amateur Radio existed, but learning the code never appealed to me. I took electronics in college and worked in an engineering laboratory as an electronics technician so the technical part of the exams didn’t bother me. I simply didn't want to get a Ham license enough to warrant learning the code.

In retrospect, I'm very grateful that I didn’t have that option. My mother and I soon discovered chatting on 2 Meters wasn't all it was cracked up to be. We began to get intrigued by the low band activity my father would casually mention in conversation. Not wanting to invest much money, we built HW-99s, Heathkit 50-watt CW only rigs. After all, we thought, how much fun could it be to communicate using something as slow and boring as Morse code? I quickly found out that using Morse code on the air opens up a whole new world. There is something magical about sending a CQ into the airwaves and the possibility of someone from anywhere in the world answering your call. The adrenaline you feel when you hear your call sign come back to you in Morse code is just incredible. I was hooked. I never would have believed it if hadn't happened to me.

The first contacts using any mode are memorable landmarks in your Amateur Radio career, but I don't think any come close to the memories created by your first Morse code chats.

However, the adrenaline that makes it so exciting can also make it the most nerve-wracking of your Amateur Radio experiences. I doubt if there is a CW operator in the world who didn't sweat bullets during his or her first CW chat! Some of the most experienced SSB operators get butterflies or freeze up when they think of answering a CQ using Morse, so this phenomenon isn't unique to new Hams. Unfortunately, this fear keeps many Hams from upgrading, because one of the best ways to increase your code speed is to get on the air and use CW.

Luckily there are thousands of experienced operators who get a 'kick out of' helping Hams through their first CW contacts. Many times, when we cruise the Novice subbands you will hear more non-Novice call signs calling CQ than Novices!

Before getting on the air, have a band plan in front of you so you can be sure you are in a subband where you are legally licensed to operate. Another thing you may find helpful is a "a cheat sheet" of a typical QSO format. Basic QSOs consist of the signal report (RST), name, location (QTH), type of rig and antenna and the sign off. Write it out, word for word.

Of course, you wouldn't send all this information during one transmission. Turn the conversation over to the other Ham after the RST, name and QTH and again after telling about your station setup. If it's going well and you want to ad lib, that's great. But it helps to have the cheat sheet to fall back on if you need it. I had such a piece of paper taped to my desk, which may sound silly but it was a lifesaver when I got flustered.

Now that you're ready to get on the air, the easiest way to get your feet wet is to answer a CQ. Slowly scan the frequencies in the subbands of your license class, listening for a station sending CQ at a speed you can comfortably copy. Carefully tune your radio to the calling station's frequency. Watch the “S” meter on your receiver and tune around the signal until the “S" meter is at the highest point. This is where you have the best chance of being heard. When they finish their CQ -- generally it will sound like CQCQCQ de call sign call sign call sign-- immediately send their call sign de your call sign three times. If they can't copy you well, they may send "QRZ?" which means “who is calling me?" Send

your call sign again two or three times. Don't let this throw you, and don't take it as an insult if the other station doesn't copy you the first time.

After you've exchanged your basic information, don't forget to tell the other station that it is one of your first CW  QSOs. Trust me, it will make their day!

Don't make the mistake I did of hating CW just because it's a requirement. It's been around for decades because it’s functional and fun. If you give it half a

chance, you just might discover that CW operator who is hiding inside you too!

HF DIGITAL COMMUNICATION COURSE

By

Bob Wexelbaum, W2ILP

I recently completed a distance learning course in HF Digital Communication, via the Internet. The course was offered by  the ARRL Certification and Continuing Education Program. This was the  first time that this course was offered and I was one of the first 50  students to take it. All were hams and the course turned out to be very  hands on.

Students had to actually operate most of the modes. They had  to set up their ham stations to operate the digital modes and make real  QSOs in each mode if possible. An instructor/mentor received the  assignments, which called for on the air digital communication and helped  us if we ran into problems.

I plan to give an overview of this topic at  the August GARC meeting and give handouts telling how to build a simple  PC computer to HF SSB transceiver interface, as well as how to download  free software for operating RTTY, PSK-31, MFSK, Hellschreiber and SSTV.  All can be operated using the same interface hardware. Each requires  different free software. Audio processing and conversions are  accomplished by the computer's sound card.

This article will be too late to announce my August talk but I  will be continuing in the following months to go into detail as to how each mode works. Hopefully I will cover RTTY in September and will  continue with the other modes in the following months, unless there is a  conflict with other presentations.

In September I will get somewhat  technical; explaining the advantages and disadvantages of RTTY from an  information theory point of view; its history and how it can be accomplished with a modern HF SSB transceiver by using audio tones and  not actually shifting the RF frequency. You need not have heard my August talk to start in September, because I will make the basic interface and web addresses for downloading software and information  available to all who are interested. There are now many hams, worldwide,  using these digital modes.

These modes work strictly by radio propagation and do not depend on linking via the internet and so they are true ham  radio. It is possible to work DX using digital modes with QRP power or  a poor antenna, when SSB phone signals are not readable. These modes  have many of the advantages of the first digital mode known to all of us on HF...CW!

KN

MINUTES OF GENERAL MEETING – 8/21/02

By Pete, N2PYV

TREASURER'S REPORT

Finances continue to be in good shape.

REPEATER REPORT –

Gordon, KB2UB

Repeaters appear to be working OK. Gordon got a brochure for refurbished GE mobile units that are converted to amateur 2-meter repeaters. A unit would cost about \$660.00. These units will be evaluated against other potential units.

Dave, AB2EF, has obtained the material necessary to mount the HF beam on the roof of Plant 14. He needs some dimensions of the antenna to complete the bracket.

NET REPORT –

Zak, WB2PUE

Today’s 20-Meter Net had four checkins. The Sunday morning 40-Meter Net was good. Zak reported that Emmet, W4GOL, a regular on the net had a heart problem, but seemed to be recovering OK.

VE REPORT –

Bob W2ILP

There were six applicants and six VE’s present. Three made new Techs, one made General and two failed. Gordon and Bob discussed the cases of some VE’s in California who had been selling ham licenses without passing an exam.

WAG REPORT - Bob, W2FPF

No Activity

HOUSE REPORT

Gordon reported that Northrop/Grumman is expanding on Long Island. They are now fighting over office space and will have to lease additional facilities.

PROGRAM

Bob, W2ILP, presented a talk giving an overview of various digital modes used by amateurs. He will explain these modes in greater detail at future meetings