An ohmmeter can be used for the transistor testing i.e., whether the transistor is good or not. We know that base-emitter junction of a transistor is forward biased while collector-base junction is reverse biased. Therefore, forward biased base-emitter junction should have low resistance and reverse biased collector-base junction should register a much higher resistance. Fig. 8.65 shows the process of testing an npn transistor with an ohmmeter.
(i) Transistor testing with the helip of forward biased base-emitter junction (biased by internal supply) should read a low resistance, typically 100 Ω to 1 kΩ as shown in Fig. 8.65 (i). If that is so, the transistor is good. However, if it fails this check, the transistor is faulty and it must be replaced.
(ii) Transistor testing with the help of reverses biased collector-base junction (again reverse biased by internal supply) should be checked as shown in Fig. 8.65 (ii). If the reading of the ohmmeter is 100 kΩ or higher, the transistor is good. If the ohmmeter registers a small resistance, the transistor is faulty and requires replacement.
Note. When testing a pnp transistor, the ohmmeter leads must be reversed. The results of the transistor testing, however, will be the same.
There are three leads in a transistor viz. collector, emitter and base. When a transistor is to be connected in a circuit, it is necessary to know which terminal is which. The identification of the leads of transistor varies with manufacturer. However, there are three systems in general use as shown in Fig. 8.64.
(i) When the leads of a transistor are in the same plane and unevenly spaced [See Fig. 8.64 (i)],
they are identified by the positions and spacings of leads. The central lead is the base lead. The
collector lead is identified by the larger spacing existing between it and the base lead. The remaining lead is the emitter.
(ii) When the leads of a transistor are in the same plane but evenly spaced [See Fig. 8.64 (ii)],
the central lead is the base, the lead identified by dot is the collector and the remaining lead is the
(iii) When the leads of a transistor are spaced around the circumference of a circle [See Fig. 8.64 (iii)], the three leads are generally in E-B-C order clockwise from a gap.
In practical circuits, you must be able to tell whether a given transistor is connected as a common emitter, common base or common collector. There is an easy way to ascertain it. Just locate the terminals where the input a.c. signal is applied to the transistor and where the a.c output is taken from the transistor. The remaining third terminal is the common terminal. For instance, if the a.c input is applied to the base and the a.c output is taken from the collector, then common terminal is the emitter. Hence the transistor is connected in common emitter configuration. If the a.c. input is applied to the base and a.c output is taken from the emitter, then common terminal is the collector. Therefore, the transistor is connected in common collector configuration.
From the time semiconductor engineering came to existence, several numbering systems were adopted by different countries. However, the accepted numbering system is that announced by Pro electron Standardisation Authority in Belgium. According to this system of numbering semiconductor devices :
(i) Every semiconductor device is numbered by five alpha-numeric symbols, comprising either
two letters and three numbers (e.g. BF194) or three letters and two numbers (e.g. BFX63). When two numbers are included in the symbol (e.g. BFX63), the device is intended for industrial and professional equipment. When the symbol contains three numbers (e.g. BF194), the device is intended for entertainment or consumer equipment.
(ii) The first letter indicates the nature of semiconductor material. For example :
A = germanium, B = silicon, C = gallium arsenide, R = compound material (e.g. cadmium sulphide)
Thus AC125 is a germanium transistor whereas BC149 is a silicon transistor.
(iii) The second letter indicates the device and circuit function.
A = diode B = Variable capacitance diode
C = A.F. low powered transistor D = A.F. power transistor
E = Tunnel diode F = H.F. low power transistor
G = Multiple device H = Magnetic sensitive diode
K = Hall-effect device L = H.F. power transistor
M = Hall-effect modulator P = Radiation sensitive diode
Q = Radiation generating diode R = Thyristor (SCR or triac)
S = Low power switching transistor T = Thyristor (power)
U = Power switching transistor X = diode, multiplier
Y = Power device Z = Zener diode
Common base amplifiers are not used as frequently as the CE amplifiers. The two important
applications of CB amplifiers are : (i) to provide voltage gain without current gain and (ii) for impedance matching in high-frequency applications. Out of the two, the high frequency applications are far more common.
(i) To provide voltage gain without current gain. We know that a CB amplifier has a high voltage gain while the current gain is nearly 1 (i.e. Aij 1). Therefore, this circuit can be used to provide high voltage gain without increasing the value of circuit current. For instance, consider the case where the output current from an amplifier has sufficient value for the required application but the voltage gain needs to be increased. In that case, CB amplifier will serve the purpose because it would increase the voltage without increasing the current. This is illustrated in Fig. 8.66. The CB amplifier will provide voltage gain without any current gain.
(ii) For impedance matching in high frequency applications. Most high-frequency voltage sources have a very low output impedance. When such a low-impedance source is to be connected to a high-impedance load, you need a circuit to match the source impedance to the load impedance.
Since a common-base amplifier has low input impedance and high output impedance, the common-base circuit will serve well in this situation. Let us illustrate this point with a numerical example. Suppose a high-frequency source with internal resistance 25 Ω is to be connected to a load of 8 kΩ as shown in Fig. 8.67. If the source is directly connected to the load, small source power will be transferred to the load due to mismatching. However, it is possible to design a CB amplifier that has an input impedance of nearly 25 Ω and output impedance of nearly 8 kΩ. If such a CB circuit is placed between the source and the load, the source will be matched to the load as shown in Fig. 8.68.
Note that source impedance very closely matches the input impedance of CB amplifier. Therefore, there is a maximum power transfer from the source to input of CB amplifier. The high output impedance of the amplifier very nearly matches the load resistance. As a result, there is a maximum
power transfer from the amplifier to the load. The net result is that maximum power has been transferred from the original source to the original load. A common-base amplifier that is used for this purpose is called a buffer amplifier.
A transistor is a solid-state device that performs the same functions as the grid-controlled vacuum
tube. However, due to the following advantages, the transistors have upstaged the vacuum tubes in most areas of electronics :
(i) High voltage gain. We can get much more voltage gain with a transistor than with a vacuum
tube. Triode amplifiers normally have voltage gain of less than 75. On the other hand, transistor
amplifiers can provide a voltage gain of 300 or more. This is a distinct advantage of transistors over
(ii) Lower supply voltage. Vacuum tubes require much higher d.c. voltages than transistors. Vacuum tubes generally run at d.c. voltages ranging from 200V to 400V whereas transistors require
much smaller d.c. voltages for their operation. The low voltage requirement permits us to build portable, light-weight transistor equipment instead of heavier vacuum-tube equipment.
(iii) No heating. A transistor does not require a heater whereas the vacuum tube can only operate with a heater. The heater requirement in vacuum tubes poses many problems. First, it makes the power supply bulky. Secondly, there is a problem of getting rid of heat. The heater limits the tube’s
useful life to a few thousand hours. Transistors, on the other hand, last for many years. This is the
reason that transistors are permanently soldered into a circuit whereas tubes are plugged into sockets.
(iv) Miscellaneous. Apart from the above salient advantages, the transistors have superior edge
over the tubes in the following respects :
(a) transistors are much smaller than vacuum tubes. This means that transistor circuits can be
more compact and light-weight.
(b) transistors are mechanically strong due to solid-state.
(c) transistors can be integrated along with resistors and diodes to produce ICs which are
extremely small in size.
Although transistors are constantly maintaining superiority over the vacuum tubes, yet they suffer
from the following drawbacks :
(i) Lower power dissipation. Most power transistors have power dissipation below 300W while vacuum tubes can easily have power dissipation in kW. For this reason, transistors cannot be used in high power applications e.g. transmitters, industrial control systems, microwave systems etc. In such areas, vacuum tubes find wide applications.
(ii) Lower input impedance. A transistors has low input impedance. A vacuum tube, on the other hand, has very high input impedance (of the order of MΩ) because the control grid draws negligible current. There are many electronic applications where we required high input impedance
e.g. electronic voltmeter, oscilloscope etc. Such areas of application need vacuum tubes. It may be
noted here that field-effect transistor (FET) has a very high input impedance and can replace a vacuum tube in almost all applications.
(iii) Temperature dependence. Solid-state devices are very much temperature dependent. A slight change in temperature can cause a significant change in the characteristics of such devices. On the other hand, small variations in temperature hardly affect the performance of tubes. It is a distinct disadvantage of transistors.
(iv) Inherent variation of parameters. The manufacture of solid-state devices is indeed a very
difficult process. Inspite of best efforts, the parameters of transistors (e.g. β, VBE etc.) are not the same even for the transistors of the same batch. For example, β for BC 148 transistors may vary between 100 and 600.
Reference: Principles Of Electronics By V K Mehta And Rohit Mehta
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