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.
|
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.
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
1985, I moved about 60 miles from the Detroit area where I had grown up. The
phone bill from talking to my parents was enormous. My father pointed out that
if my mother and I got Ham tickets, we could save a fortune on phone calls by
talking on 2 Meters. This made sense to us and we set about getting our
licenses. My mother, WO8E, had to learn all the theory AND the code from
scratch -- at least I had an advantage on the theory from taking it in school.
She got her ticket before I did! I simply hated Morse code and resented having
to learn it. After all, I wasn't ever going to use it. I just wanted to chat on
2 Meters using a repeater and a handitalkie! Why should I have to learn the
code? I fought it every step of the way and consequently it took two years to
pass my 5-wpm exam. If there had been a no-code license in 1988, I would have
been first in line to sign up.
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!
Reprinted from WORLDRADIO ISSUE
February 2002.
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
GRUMMAN AMATEUR RADIO CLUB
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