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ION FROM HEWLETT-PACKARD
V
NOVEMBER-DECEMBER 1974
conditions found in digital Expanding on the above idea leads
circuits. to our second troubleshooting tip.
TRANSISTORIZED
In troubleshooting transistor cir- TIP #2: Modify the control sig-
CIRCUITS FASTER cuits, the most important area to nals present and see if the circuit
examine is the base-emitter junction responds accordingly.
by George Stanley
as this is the control point of the
transistor. Remember that conven-
tional PNP and NPN transistors are
Many books and articles have been basically "off" devices and must
written on transistors and tran- be biased "on" to their operating
sistorized circuits but very few point. This is done by forward bias-
have been written about trouble- ing the base-emitterjunction. There-
shooting transistorized circuits. fore, the status of the base-emitter
This article is aimed at providing diode tells exactly what the tran-
practical troubleshooting tips for sistor should be doing. This diode is
those of you repairing transis- made out of either silicon or germa-
torized equipment. nium. If the transistor is silicon and
has approximately 0.6V forward
Before describing specific tips let's bias between base and emitter, the
take a moment and review several transistor should be "on". The
important transistor characteris- amount it should be on depends
I
~ tics: upon the current gain ( B ) of the For example, Figure 1 shows a
A conventional PNP or NPN tran- transistor, the resistors in series normally biased NPN silicon tran-
sistor has three operating states: with the collector and emitter, and sistor with the bias resistors ad-
the supply voltage. If the transis- justed to have the transistor
A. Off, that isan open switch. tor were germanium and had turned on half way. Now remove
approximately 0.2 volts forward the forward bias on the base-
B. Part way on, bias voltage are bias between base and emitter, emitter diode junction by adding
set so the transistor can am- it would behave in the same general the short as is shown in Figure 2.
plify, i.e. it can be turned fur- fashion. When the short is added, the col-
ther on or further off. This is lector voltage should rise to within
the normal bias condition for If the transistor has zero bias or a few tenths of a volt of the supply
amplifiers. reverse bias on its base-emitter voltage. If it doesn't, we've identi-
junction, it should be turned off. fied a bad transistor. This tech-
C. Saturated, behaves like a If it is not off under these condi- nique is perfectly safe in A C
closed switch. Saturation is ions, it is either shorted or leaky.
defined as where the IR drop
This review .leads us to our first
across the emitter and collec-
troubleshooting tip. IN THIS ISSUE
tor resistors equals the supply
voltage. The interesting thing TIP # I : Measure the base-emitter
about saturation is that both voltage. From this decide how the
the base-emitter and base- transistor should be behaving.
collector diodes are forward Then look at the collector voltage
biased. A saturated germa- and see if the transistor is behav-
nium transistor may have as ing as it should be.
low as 0.05 volts between its
emitter and collector while a For example, if the base-emitter
satured silicon transistor voltage is 0.6V forward biased and
might have about 0.5 volts the collector voltage is the same as
between these leads. the supply voltage, something is
wrong. Probably the collector-
Saturated or off are the usual base junction is open.
@Hewten-PackardCompany 1974
coupled circuits. In some DC coup- TIP #3: When a transistor with ex- Since our transistor checker puts
led circuits we could cause damage cessive leakage is sprayed with out a sine wave that has alterna-
if base-emitter shorts are applied coolant, it often starts behaving tively positive and negative half
around high power levels such as properly. Conversely, heating a cycles we would expect a perfect
the output stage of a power leaky transistor will make the prob- diode to behave as shown in
amplifier. lem much worse. Figure 6.
I I In an amplifier stage excessive
leakage current will cause clip-
ping distortion because of the shift
in the quiescent operating point.
TIP #4: In an amplifier with clip-
ping distortion try cooling each
transistor. Quite likely you will
find that when one transistor is
cooled the clipping distortion dis- In actual practice the waveforms
appears. That transistor probably shown in Figure 7 are obtained
has excessive leakage. because we do not care which lead
is on the base and which lead is on
Even though all the above tips are the collector (or emitter).
good ones there is a transistor
When we use the above technique, tester that will speed up trouble-
the collector voltage would rise to shooting even more. This tester
exactly the supply voltage if there works on the known fact that PNP
was no collector-base leakage cur- and NPN transistors are made up
rent. Since all PNP and NPN tran- of two diodes and examines each
sistors have some leakage let's diode independently. The display
review this area. is shown on an oscilloscope.
b Figure 4 shows a simplified sche-
matic of the transistor checker. The above waveforms are typical
This tester was described in the of out of circuit transistor checks.
September issue of BENCH BRIEFS. Note in Figure 8, which shows
Key characteristics are repeated
here for the sake of completeness.
Figure 3 shows a properly biased
transistor. Note the collector vol-
tage is more positive than the base
voltage- thus in normal operation
the base-collector diode junction is
reverse biased. This reverse-biased
diode should be off but because we
have never been able to make a With the tester connected as
perfect diode there is a very small shown we would expect the follow-
current leaking across it. This ing waveforms:
a complete schematic, there is a
leakage current flows across the switch for "ln-Circuit" and "Out-
collector-base junction and part of Of-Circuit" operation. When per-
it goes thru the base-emitter forming In-Circuit tests there are
(control point) junction. usually resistors and capacitors
Since leakage current is extremely associated with the transistor
temperature sensitive we can use under test. The result is often a
this to our advantage in trouble- waveform such as is shown in
shooting: Figure 9.
WWW.HPARCHIVE.COM
e position as you need the extra
current.
ium transistor at room tempera-
ture but a little high for a silicon
surface-passivated transistor.
Another way to test transistors is
to perform a forward and reverse TIP #8: Measure by short-
ohmmeter check on the two tran- ing the emitter-base junction and
sistor diodes. It's much slower than monitoring the voltage across the
with the transistor checker. Also collector load resistor. /CBO =
you have to be careful about the
short-circuit current and open-
circuit voltage of your ohmmeter.
-(see text).
v~~
RL
On R x l and RxlO scales VOM's One of the most common mistakes
The loop in Figure 9 shows there is often have a very high short cir- in analyzing transistor circuits is to
associated capacitance (probably cuit. This current may be as high as miscalculate a stage gain in a
a coupling capacitor) and the fact several hundred mA and can multistage amplifier. For example,
that the waveform is not a perfect damage small delicate transistors. an excellent approximation of
"right" angle is because of the On the other hand VOM's often
associated resistance (probably have high open circuit voltages stage gain is AeX-- RL where hib
bias or load resistors). (22.5V) on their high resistance h'b
This transistor tester leads to our scales. These voltages also can is 3052 at 1MA of DC emitter cur-
next troubleshooting tip. damage delicate emitter-base rent, 15S2 at 2MA, etc. The prob-
junctions. Usually the R x l K scales lem comes in plugging in the cor-
TIP #5: Use the transistor checker are safe for most meters but it is rect value for RL. Figure 11
for rapid testing. Make sure to test best to measure your own. shows a two-stage amplifier. The
both the base-emitter and base-
correct value for RL is not the
collector diodes. TIP #6: Measure the short-circuit 1
A little experimenting with a current and open-circuit voltage actual value of this resistor but
printed circuit board containing for each resistance scale on your rather the parallel combination of
many transistors will rapidly VOM's and VTVM's. Keep this R L ~ , Ra, Rb and Rin of Q2.
show you the various waveforms information along with the polar-
ity of the leads on a chart on the Usually the Rin of Q2 is the most
you will encounter for good tran- dominant factor in this combina-
sistors. The important thing to look back of the ohmmeter.
t ion.
for is whether or not the wave- TIP #7: If you are using a VTVM
form has a "break" in it (F't A in make sure the range you are using
Figure 9). If it does, the transistor has enough open-circuit voltage
diode is good. Remember, the to overcome the 0.2V for germa-
lower the bias resistors, the less nium and 0.6V for silicon. Other-
defined the "break" (Pt A Figure 9) wise you will get an unsatisfactory
and the more the waveform reading.
appears like a "short". Of course,
when testing out-of-circuit the Since leakage does not show up
"break" will be very sharp - just well on the transistor checker of
like a true diode. Figure 8 nor on the ohmmeter
tests, it is best to have an inexpen-
This tester can also be used for
sive beta/leakage tester on hand.
testing tunnel diodes. The wave-
There are many available for
form is shown in Figure 10.
under $50.00 and some of the best
are available in kit form. If a leak-
age current tester is unavailable
you can always short out the
emitter-base junction simulta-
neously measuring the drop across TIP #9: When calculating the gain
the collector load resistor. For of a stage, be sure and include the
example, if you did this and mea- parallel loading effects of the next
sured 30 mV across a 10K load stage bias resistors and input
resistor (with the emitter shorted) impedance.
your leakage current would be
All of the above tips relate back to
I = - E - - or 3 4 which
- 30mV important characteristics of tran-
When testing tunnel diodes, make R 10K sistors. Of course, there are many
sure the switch is in the In-Circuit would be about right for a german- other tips that are common to NPN
WWW. H PARCHIVE.COM
A
and PNP transistors as well as to supply voltage. The leakage times shows up marginal "$
FET's and vacuum tubes but that current then equals the vol- operation.
may be good material for another tage across the load resistor H. Shorting collector to emitter
article. (RL) divided by RL. (Make simulates saturation as the
In summary, here is a list of impor- sure the collector is not DC transistor behaves like a
tant points relating to transistors coupled to the next stage.) closed switch.
which you may find useful in G. Abnormal increases in room Essentially the same material is
coming up with troubleshooting temperature leakage current, covered in a service video tape
tips of your own. say 10 times normal, often which you can purchase from
indicate contamination of the Hewlett-Packard. This tape is en-
A. NPN and PNP transistors are base-collector junction (possi- titled Troubleshoot Transistor Cir-
basicalIy "off" devices while bly due to a cracked or bro- cuits Faster (17 minutes), I.D.
vacuum tubes are basically ken hermetic seal). The result #800683.
"on" devices. is a shift in the normal bias
B. Transistors are made up of If you have some good trouble-
operating point. Trouble will shooting ideas, send them in and
two diodes: a base-emitter only be experienced if the
diode and a base-collector we will share them with all the
driving signal drives the tran- readers.
diode. In normal (amplifier) sistor to or near cutoff. The
operation, the base-emitter transistor, of course, will not This material is printed with the per-
diode is forward biased and properly turn off and the re- mission of the Hayden Book Co.,
the base-collector diode is sult may be clipping or dis- Inc., Rochelle Park, N.J. It will ap-
reversed biased. tortion due to the residual pear along with other troubleshoot-
C. Shorting the base to emitter leakage current flowing thru ing material in a revised edition of
turns off transistors while for- the external resistors. Heat- George Stanley's well-known book,
ward biasing base-emitter and cooling a transistor aggra- TRANSISTOR BASICS: A SHORT
junctions turns on transistors. vates this condition and some- COURSE. Watch for it.
D. All transistors have leakage
current across their reversed
biased base-collector diodes.
For surface passivated silicon
George Stanley, a member of
I..E.E., received his B.S. E. E.
tion, and is the author of Tran-
sistor Basics: A Short Course,
3
transistors, this current is degree from Stanford University. Hayden Book Co., and A Casebook
usually no more than several He is very interested and active of Basic Circuits for Electronics
nanoamperes. Since germa- in the area of technical educa- Instrumentation, Rhinehart Press.
nium transistors cannot be He also created a fifteen-part
surface passivated, this leak- video tape series entitled "Prac-
age current normally may be tical Transistors" for Hewlett-
Packard.
several microamperes.
E. Leakage current increases Prior to becoming involved in
with heat (a law of physics) technical education, George was
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