What is transistor?
A transistor is a semiconductor device used to amplify or switch electronic signals and electrical power.Or A transistor is a device that regulates current or voltage flow and acts as a switch or gate for electronic signals. Transistors consist of three layers of a semiconductor material, each capable of carrying a current.
Bipolar Junction Transistor
the basic structure of the bipolar junction transistor (BJT) determines its operating characteristic.In this section, you will see how semi-conductive materials are used to form a BJT, and you will learn the standard BJT symbols.
The BJT is constructed with three doped semiconductor regions separated by two pn junctions, are shown in the epitaxial planar structure. The three regions are called emitter,base and collector . Physical representations of the two types of BJT are shown in above Fig. One type consists of two n regions separated by a p region (npn), and the other type consists of two p regions separated by an n region (pnp). The term bipolar refers to the use of both holes and electron as current carriers in the transistor structure.
The pn junction joining the base region and the emitter region is called base-emitter junction. The pn junction joining the base region and the collector region is called the base-collector junction. A wire lead connects to each of the three regions,are shown. These leads are labeled E,B and C for emitter,base and collector,respectively. The base region is lightly doped and very thin compared to the heavily doped emitter and the moderately doped collector region. (The reason for this is discussed in the next section). The schematic symbols for the npn and pnp bipolar junction transistors.
How does a bipolar junction transistor work?
In order for a BJT to operate properly as an amplifier, the two pn junctions must be correctly biased with external de voltages. In this section, we mainly use the npn transistor for illustration. The operation of the pnp is the same as for the npn except that the roles of the electrons and holes,the bias voltage polarities,and the current directions are all reversed.
A bias arrangement for both npn and pnp BJT for operation as an amplifier . Notice that the both cases the base-emitter (BE) junction is forward-biased and the base collector (BC) junction is reverse-biased. This condition is called forward-reverse bias.
To understand how a transistor operates,let’s examine what happens inside the npn structure. The heavily doped n-type emitter region has a very high density of conduction-band (free) electrons. These free electrons easily diffuse through the forward-based BE junction into the lightly doped and very thin p-type base region,as indicated by the wide arrow. The base has a low density of holes, which are the majority carriers,as represented by the white circles. A small percentage of the total number of free electrons injected into the base region recombine with the holes and move as valence electrons through the base region and into the emitter region as hole current,indicated by the red arrows.
When the electrons that have recombined with holes as valence electrons leave the crystalline structure of the base,they become free electrons in the metallic base lead and produce the external base current. Most of the free electrons that have entered the base do not recombine with holes because the base is very thin. A s the free electrons move toward the reverse-biased BC junction,they are swept across into the collector region by the attraction of the positive collector supply voltage. The free electrons move through the collector region, into the external circuit,and then return into the emitter region along with the base current,as indicated. The emitter current is slightly greater than the collector current because of the small base current that splits off from the total current injected into the base region from the emitter. Watch also how transistor work? .
The directions of the currents in an npn transistor and its schematic symbol are shown in above Fig; these for a pnp transistor are shown in above Fig. Notice that the arrow on the emitter inside the transistor symbols points in the direction of conventional current. These diagrams show that the emitter current (IE) is the sum of the collector current (IC) and the base current (IB), expressed as follow:
IE = IC + IB
As mentioned before, IB is very small compared to IE or IC. The capital-letter subscripts indicate dc values.
BJT Characteristics And Parameters
Two important parameters, βDC (dc current gain) and αDC are introduced and used to analyze a BJT circuit. Also,transistor characteristic curves are covered,and you will learn how a BJT’s operation can be determined from these curves. Finally, maximum ratings of a BJT are discussed.
When a transistor is connected to de bias voltages for both npn and pnp types, VBB forward-biases the base-emitter junction, and VCC reverse biases the base-collector junction. Although in this chapter we are using separate battery symbols to re[present the bias voltages, in practice the voltages are often derived from a single de power supply.For example, VCC is normally taken directly from the power supply output and VBB (which is smaller) can be produced with a voltage divider.
DC Beta (βDC ) And DC Alpha (αDC ):
The DC current gain of a transistor is the ratio of the de collector current (IC) to the de base current (IB) and is designed de beta (βDC ).
Typically values of βDC range from less than 20 to 200 or higher. βDC is usually designated as an equivalent hybrid (h) parameter hFE, on transistor datasheets. All you need to know now is that:
hFE = βDC
The ratio of the de collector current (IC) to the de emitter current (IE) is the de alpha ( αDC ). The alpha is a less-used parameter than beta in transistor circuits.
Typically,values of αDC range from 0.95 to 0.99 or greater,but αDC is always less than 1. The reason is that that IC is always slightly less than IE by the amount of IB . For example, if IE = 100 mA and IB = 1 mA , then IC = 99 mA and αDC = 0.99.
Transistor Dc Model:
You can view the understand BJT as a device with a current input and the dependent current source in the output circuits for an npn . The input circuit is a forward-biased diode through which there is base current. The output circuit is a dependent current source (diamond-shaped element) with a value that is dependent on the base current,IB and equal to βDC IB. Recall that independent current source symbols have a circular shape.
BJT Circuit Analysis:
Consider the basic transistor bias circuit configuration. The transistor de currents and three de voltages can be identified.
IB: dc base current IE: dc emitter current IC: dc collector current VBE: dc voltage at base with respect to emitter VCB: dc voltage at collector with respect to base VCE: dc voltage at collector with respect to emitter
The base-bias voltage source, V BB , ‘forward-biases the base-emitter junction, and the collector-bias voltage source,V CC, reverse biases the base-collector junction. When the base-emitter junction is forward-biased,it is like a forward-biased diode and has a nominal forward voltage drop of.
V BE ≅ 0.7 V
Although in an actual transistor V BE can be as high as 0.9 V and is dependent on current, we will use 0.7 V throughout this text in order to simplify the analysis of the basic concepts. Keep in mind that the characteristic of the base-emitter junction is the same as a normal diode curve like the one in the above Figure.
V RB = V BB – V BB
Collector Characteristics Curves:
Using a circuit like that shown in above Fig, a set of collector characteristic curves can be generated that show how the collector current, I C , varies with the collector-to-emitter voltage, V CE , for specified values of base current, I B. Notice in the circuit diagram the both VBB and VCC are variable sources of voltage.
Assume that V BB is set to produce a certain value of I B and V CC is zero. For this condition, both the base-emitter junction and the base-collector junction are forward-biased because the base is at approximately 0.7 V while the emitter and the collector are at 0 V. The base current is through the base-emitter junction because of the low impedance path to ground and,therefore, I C is zero. When both junctions are forward-biased the transistor is in the saturation region of its operation. Saturation is the state of a BJT in which the collector current has reached a maximum and is independent of the base current.
As V CC is increased, V CE increases as the collector current increases. This i indicated by the portion of the characteristic curve between points A and B. IC increases as VCC is increased because VCE remains less than 0.7 V due to the forward-biased base-collector junction.
Ideally,when VCE exceeds 0.7 V, the base-collector junction becomes reverse-biased and the transistor goes into the active, or linear ,region of its operation. Once the base-collector junction is reversed-biased,IC levels off and remains essentially constant for a given value of IB as VCE continuous to increase. Actually, IC increase very slightly as VCE increases due to widening of the base-collector depletion region. This results in fewer holes for recombination in the base region which effectively causes a slight increase in βDC . This is shown by the portion of the characteristic curve between point B and C. For this portion of the characteristic curve, the value of IC is determined only by the relationship expressed as IC = βDC IB.
When VCE reaches a sufficiently high voltage, the reverse-biased base-collector junction goes into breakdown; and the collector current increases rapidly as indicated by the part of the curve to the right of point C. A transistor should never be operated in this breakdown region.
A family of collector characteristic curves is produced when IC versus VCE is plotted for several values of IB . When IB = o, the transistor is in the cutoff region although there is a very small collector leakage current as indicated. Cutoff is the nonconducting state of a transistor. The amount of collector leakage current for IB = o is exaggerated on the graph for illustration.
As previously mentioned, when IB =0, the transistor is in the cutoff region for its operation. With the base lead open, resulting in a base current of zero. Under this condition, there is a very small amount of collector leakage current ICEO , due mainly to thermally produced carriers. Because ICEO is extremely small, it will usually be neglected in circuit analysis so that VCE = V CC . In cutoff, neither the base-emitter nor the base-collector junctions are forward-biased. The subscript CEO represents collector- to-emitter with the base open.
When the base-emitter junction becomes forward-biased and the base current is increased, the collector current also increases ( IC = βDC IB ) and V CE decreases as a result of more drop across the collector resistor (VCE = VCC – ICRC). When VCE reaches its saturation value,VCE(sat) ,the base collector junction becomes forward-biased and IC can increase no further even with a continued increase in IB . At the point of saturation, the relation IC = βDC IB is no longer valid VCE(sat) for a transistor occurs somewhere below the knee of the collector curves, and it is usually only a few tenths of a volt.
DC Load Line:
Cutoff and saturation can be illustrated in relation to the collector characteristic curves by the use of a load line. A de load line drawn on a family of curves connecting the cutoff point and the saturation point. The bottom of the load line is at ideal cutoff where IC = 0 and VCE = VCC. The top of the load line is at saturation along the load line is the active region of the transistor’s operation.
More About βDC :
The βDC or hFE Is an important BJT parameter that we need to examine further. βDC is not truly constant but varies with both collector current and with temperature. Keeping the junction temperature constant and increasing IC causes βDC to increase to a maximum. A further increase in IC beyond this maximum point βDC to decrease. If IC is held constant and the temperature is varied, βDC changes directly with the temperature. If the temperature goes up,βDC goes up and vice versa. The variation of βDC with IC and junction temperature (TJ) for a typical BJT.
A transistor datasheet usually specifies βDC (hFE) at specific IC values. Even at fixed values of IC and temperature, βDC varies from one device to another for an given type of transistor due to inconsistencies in the manufacturing process that are unavoidable. The βDC specified at a certain value of IC is usually the minimum value.βDC(min) , although the maximum and typical values are also sometimes specified.
Maximum Transistor Ratings:
A BJT, like any other electronic device,has limitations on its operation. These limitations are stated in the form of maximum ratings and are normally specified on the manufacture’s datasheet. Typically, maximum ratings are given for collector-to-base voltage, collector-to-emitter voltage,emitter-to-base voltage, collector current,and power dissipation. The produce of V CE and IC must not exceed the maximum power dissipation. Both VCE and IC cannot be maximum the same time. If V CE is maximum,IC can be calculated as.
If IC is maximum, VCE can be calculated by rearranging the previous equation as follows:
Derating P D (max) :
P D (max) is usually specified at 25°C . For higher temperature, P D (max) is less. Datasheets often give derating factors for determinating P D (max) at any temperature above 25°C. For example, a derating factor of 2 mW/°C indicates that the maximum power dissipation is reduced 2 mW for each degree Celsius increase in temperature.
DC and AC Quantities:
Before discussing the concept of transistor amplification, the designations that we will use for the circuit quantities of current, voltage,and resistance must be explained because amplifier circuits have both dc and ac quantities.
In this text, italic capital letters are used for both dc and ac currents (I) and voltages (V). This rule applies to rms, average, peak, and peak-to-peak ac values. AC current and voltage values are always rms unless stated otherwise. Although some texts use lowercase i and v for ac current and voltage, we reserve the use of lowercase i and v only for instantaneous values. In this text, the distinction between a dc current or voltage and an ac current or voltage is in the subscript.
DC quantities always carry an uppercase roman (nonitalic) subscript. For example, IB, IC, and IE are the dc transistor currents. VBE, VCB, and VCE are the dc voltages from one transistor terminal to another. Single subscripted voltages such as VB, VC, and VE are dc voltages from the transistor terminals to ground.
AC and all time-varying quantities always carry a lowercase italic subscript. For example,Ib, Ic, and Ie are the ac transistor currents. Vbe, Vcb, and Vce are the ac voltages from one transistor terminal to another. Single subscripted voltages such as Vb, Vc, and Ve are ac voltages from the transistor terminals to ground.
The rule is different for internal transistor resistances. As you will see later, transistors have internal ac resistances that are designated by lowercase r¿ with an appropriate subscript. For example, the internal ac emitter resistance is designated as r¿e .
Circuit resistances external to the transistor itself use the standard italic capital R with a subscript that identifies the resistance as dc or ac (when applicable), just as for current and voltage. For example RE is an external dc emitter resistance and Re is an external ac emitter resistance.
As you have learned, a transistor amplifies current because the collector current is equal to the base current multiplied by the current gain, β. The base current in a transistor is very small compared to the collector and emitter currents. Because of this, the collector current is approximately equal to the emitter current.
With this in mind, let’s look at the circuit. An ac voltage, Vs, is superimposed on the dc bias voltage VBB by capacitive coupling as shown. The dc bias voltage VCC is connected to the collector through the collector resistor,RC.
The ac input voltage produces an ac base current, which results in a much larger ac collector current. The ac collector current produces an ac voltage across RC, thus producing an amplified, but inverted, reproduction of the ac input voltage in the active region of operation.
The forward-biased base-emitter junction presents a very low resistance to the ac signal. This internal ac emitter resistance is designated r¿e and appears in series with RB. The ac base voltage is:
The ac collector voltage, Vc, equals the ac voltage drop across RC.
Since IC ≅ Ie, the ac collector voltage is:
Vb can be considered the transistor ac input voltage where Vb = Vs – IbRB. VC can be considered the transistor ac output voltage. Since voltage gain is defined as the ratio of the output voltage to the input voltage, the ratio of VC to Vb is the ac voltage gain, AV, of the transistor. Substituting IeRC for VC and Ier¿e for Vb yields:
The Ie terms cancel; therefore,
This equation shows that the transistor provides amplification in the form of voltage gain, which is dependent on the values of RC and r¿e.
The BJT As A Switch
In the previous section, you saw how a BJT can be used as a linear amplifier. The second major application area is switching applications. When used as an electronic switch, a BJT is normally operated alternately in cutoff and saturation. Many digital circuits use the BJT as a switch.
illustrates the basic operation of a BJT as a switching device. In part (a), the transistor is in the cutoff region because the base-emitter junction is not forward-biased. In this condition, there is, ideally, an open between collector and emitter, as indicated by the switch equivalent. In part (b), the transistor is in the saturation region because the base-emitter junction and the base-collector junction are forward-biased and the base current is made large enough to cause the collector current to reach its saturation value. In this condition,there is, ideally, a short between collector and emitter, as indicated by the switch equivalent. Actually, a small voltage drop across the transistor of up to a few tenths of a volt normally occurs, which is the saturation voltage, VCE(sat).
Conditions in Cutoff:
As mentioned before, a transistor is in the cutoff region when the base-emitter junction is not forward-biased. Neglecting leakage current, all of the currents are zero, and VCE is equal to VCC.
VCE(cutoff) = V CC
Conditions in Saturation:
As you have learned, when the base-emitter junction is forward-biased and there is enough base current to produce a maximum collector current, the transistor is saturated. The formula for collector saturation current is: Since VCE(sat) is very small compared to V CC, it can usually be neglected.The minimum value of base current needed to produce saturation is: Normally, IB should be significantly greater than IB(min) to ensure that the transistor is saturated.
A Simple Application of a Transistor Switch
The transistor is used as a switch to turn the LED on and off. For example, a square wave input voltage with a period of 2 s is applied to the input as indicated. When The square wave is at 0 V, the transistor is in cutoff; and since there is no collector current,the LED does not emit light. When the square wave goes to its high level, the transistor saturates. This forward-biases the LED, and the resulting collector current through the LED causes it to emit light. Thus, the LED is on for 1 second and off for 1 second.
In a Photo-transistor the base current is produced when light strikes the photosensitive semiconductor base region. The collector-base pn junction is exposed to incident light through a lens opening in the transistor package. When there is no incident light,there is only a small thermally generated collector-to-emitter leakage current,I CEO ; this dark current, I λ,is produced that is directly proportional to the light intensity. This action produces a collector current that increases with I λ . Except for the way base current is generated, the phototransistor behaves as a conventional BJT. In many cases,there is no electrical connection to the base.
The relationship between the collector current and the light-generated base current in a phototransistor is:
I C = β DC I λ
The schematic symbol and some typical photo-transistors are shown in above Fig. Since the actual photo-generation of base current occurs in the collector-base region,the larger the physical area of this region,the more base current is generated. Thus,a typical photo-transistor is designed to offer a large area to the incident light, as the simplified structure diagram in above Figure:
Typical phototransistor structure.
A phototransistor can be either a two-lead or a three-lead device. In the three-lead configuration,the base lead is brought out so that the device can be used as a conventional BJT with or without the additional light-sensitivity feature. In the two-lead configuration, the base is not electrically available, and the device can be used only with light as the input. In many applications, the phototransistor is used in the two-lead version.
a phototransistor with a biasing circuit and typical collector characteristic curves. Notice that each individual curve on the graph corresponds to a certain value of light intensity (in this case, the units are mW/cm 2) and that the collector current increases with light intensity.
Phototransistors are not sensitive to all light but only to light within a certain range of wavelengths. They are most sensitive to particular wavelengths in the red and infrared part of the spectrum, as shown by the peak of the infrared spectral response curve in above Figure:
Applications of phototransistor
Phototransistors are used in a variety of applications. A light-operated relay circuit is The phototransistor Q1 drives the BJT Q2. When there is sufficient incident light on Q1, transistor Q2 is driven into saturation, and collector current through the relay coil energizes the relay. The diode across the relay coil prevents, by its limiting action, a large voltage transient from occurring at the collector of Q2 when the transistor turns off.
A circuit in which a relay is deactivated by incident light on the phototransistor. When there is insufficient light, transistor Q2 is biased on, keeping the relay energized. When there is sufficient light, phototransistor Q1 turns on; this pulls the base of Q2 low, thus turning Q2 off and de-energizing the relay.
An optocouplers use an LED optically coupled to a photodiode or a phototransistor in a single package. Two basic types are LED -to-photodiode and LED-to-phototransistor,as shown in above Figure. Examples of typical packages are shown Figure:
A key parameter in optocouplers is the CTR (current transfer ration). The CTR is an indication of how efficiently a signal is coupled from input to output and id expressed as the ratio of a change in the LED current to the corresponding change in the photodiode or phototransistor current. It is usually expressed as a percentage.
Examples of optocoupler packages:
A key parameter in optocouplers is the CTR (current transfer ratio). The CTR is an indirection of how efficiently a signal is coupled from input to output and is expressed as the ratio of a change in the LED current to the corresponding change in the photodiode or phototransistor current. It is usually expressed as a percentage.
CTR versus IF for a typical optocoupler:
Typical graph of CTR versus forward LED current. For this case, it varies from about 50% to about 110%.
Optocouplers are used to isolate sections of a circuit that are incompatible in terms of the voltage levels or currents required. For example, they are used to protect hospital patients from shock when they are connected to monitoring instruments or other devices. They are also used to isolate low-current control or signal circuits from noisy power supply circuits or higher-current motor and machine circuits.
Transistor Categories and Packaging:
BJT s are available in a wide range of package types for various applications. Those with mounting studs or heat sinks are usually power transistors. Low-power and medium-power transistors are usually found in smaller metal or plastic cases. Still another package classification is for high-frequency devices. You should be familiar with common transistor packages and be able to identify the emitter, base, and collector terminals.
Manufacturers generally classify bipolar junction transistors into three broad categories: general-purpose/small-signal devices, power devices, and RF (radio frequency/microwave) devices. Although each of these categories, to a large degree, has its own unique package types, you will find certain types of packages used in more than one device category. Let’s look at transistor packages for each of the three categories so that you will be able to recognize a transistor when you see one on a circuit board and have a good idea of what general category it is in.
General-purpose/small-signal transistors are generally used for low- or medium-power amplifiers or switching circuits. The packages are either plastic or metal cases. Certain types of packages contain multiple transistors. two common plastic cases and a metal can package. multiple-transistor packages. Some of the multiple-transistor packages such as the dual in-line (DIP) and the small-outline (SO) are the same as those used for many integrated circuits. Typical pin connections are shown so you can identify the emitter, base, and collector.
Power transistors are used to handle large currents (typically more than 1 A) and/or large voltages. For example, the final audio stage in a stereo system uses a power transistor amplifier to drive the speakers. Some common package are shown in Figure: Plastic and metal cases for general-purpose/small-signal transistors. Pin configurations may vary. Examples of multiple-transistor packages: Examples of power transistor and packages: Greatly enlarged cutaway view of tiny transistor chip mounted in the encapsulated package. The metal tab or the metal case is common to the collector and is thermally connected to a heat sink for heat dissipation. Notice in part (e) how the small transistor chip is mounted inside the much larger package.
RF transistors are designed to operate at extremely high frequencies and are commonly used for various purposes in communications systems and other high-frequency applications. Their unusual shapes and lead configurations are designed to optimize certain high-frequency parameters.
As you already know, a critical skill in electronics work is the ability to identify a circuit malfunction and to isolate the failure to a single component if necessary. In this section, the basics of troubleshooting transistor bias circuits and testing individual transistors are covered.
Troubleshooting a Biased Transistor:
Several faults can occur in a simple transistor bias circuit. Possible faults are open bias resistors,open or resistive connections, shorted connections, and opens or shorts internal to the transistor itself. A basic transistor bias circuit with all voltages referenced to ground. The two bias voltages are V BB 3 V and V CC 9 V. The correct voltage measurements at the base and collector are shown. Analytically, these voltages are verified as follows. A βDC = 200 is taken as midway between the minimum and maximum values of h FE given on the datasheet for the 2N3904. A different h FE (βDC),of course, will produce different results for the given circuit.
A basic transistor bias circuit.
Several faults that can occur in the circuit and the accompanying symptoms. Symptoms are shown in terms of measured voltages that are incorrect.If a transistor circuit is not operating correctly, it is a good idea to verify that V CC and ground are connected and operating. A simple check at the top of the collector resistor and at the collector itself will quickly ascertain if V CC is present and if the transistor is conducting normally or is in cutoff or saturation. If it is in cutoff, the collector voltage will equal V CC; if it is in saturation, the collector voltage will be near zero. Another faulty measurement can be seen if there is an open in the collector path. The term floating point refers to a point in the circuit that is not electrically connected to ground or a “solid” voltage. Normally, very small and sometimes fluctuating voltages in the μV to low mV range are generally measured at floating points. The faults are typical but do not represent all possible faults that may occur.
Testing a Transistor with a DMM:
A digital multi-meter can be used as a fast and simple way to check a transistor for open or shorted junctions. For this test, you can view the transistor as two diodes connected for both npn and pnp transistors. The base-collector junction is one diode and the base-emitter junction is the other. Examples of faults and symptoms in the basic transistor bias circuit. A transistor viewed as two diodes:
Recall that a good diode will show an extremely high resistance (or open) with reverse bias and a very low resistance with forward bias. A defective open diode will show an extremely high resistance (or open) for both forward and reverse bias. A defective shorted or resistive diode will show zero or a very low resistance for both forward and reverse bias. An open diode is the most common type of failure. Since the transistor pn junctions are, in effect diodes, the same basic characteristics apply.
The DMM Diode Test Position:
Many digital multi-meters (DMMs) have a diode test position that provides a convenient way to test a transistor. A typical DMM, has a small diode symbol to mark the position of the function switch.When set to diode test, the meter provides an internal voltage sufficient to forward-bias and reverse-bias a transistor junction. Typical DMM test of a properly functioning npn transistor. Leads are reversed for a pnp transistor.
When the Transistor Is Not Defective:
The red (positive) lead of the meter is connected to the base of an npn transistor and the black (negative) lead is connected to the emitter to forward-bias the base-emitter junction. If the junction is good, you will get a reading of between approximately 0.6 V and 0.8 V, with 0.7 V being typical for forward bias.
The leads are switched to reverse-bias the base-emitter junction, as shown. If the transistor is working properly, you will typically get an OL indication.
The process just described is repeated for the base-collector junction. For a pnp transistor, the polarity of the meter leads are reversed for each test.
When the Transistor Is Defective:
When a transistor has failed with an open junction or internal connection, you get an open circuit voltage reading (OL) for both the forward-bias and the reverse-bias conditions for that junction. If a junction is shorted, the meter reads 0 V in both forward- and reverse-bias tests, as indicated in part (b). Some DMMs provide a test socket on their front panel for testing a transistor for the h FE (β DC) values. If the transistor is inserted improperly in the socket or if it is not functioning properly due to a faulty junction or internal connection, a typical meter will flash a 1 or display a 0. If a value of β DC within the normal range for the specific transistor is displayed, the device is functioning properly. The normal range of βDC can be determined from the datasheet.
Checking a Transistor with the OHMs Function:
DMMs that do not have a diode test position or an h FE socket can be used to test a transistor for open or shorted junctions by setting the function switch to an OHMs range. For the forward-bias check of a good transistor pn junction, you will get a resistance reading that can vary depending on the meter’s internal battery. Many DMMs do not have sufficient voltage on the OHMs range to fully forward-bias a junction, and you may get a reading of from several hundred to several thousand ohms. For the reverse-bias check of a good transistor, you will get an out-of-range indication on most DMMs because the reverse resistance is too high to measure. An out-of-range indication may be a flashing 1 or a display of dashes, depending on the particular DMM. Even though you may not get accurate forward and reverse resistance readings on a DMM, the relative readings are sufficient to indicate a properly functioning transistor pn junction. The out-of-range indication shows that the reverse resistance is very high, as you expect. The reading of a few hundred to a few thousand ohms for forward bias indicates that the forward resistance is small compared to the reverse resistance, as you expect. Testing a defective npn transistor.Leads are reversed for a pnp transistor.
An individual transistor can be tested either in-circuit or out-of-circuit with a transistor tester. For example, let’s say that an amplifier on a particular printed circuit (PC) board has malfunctioned. Good troubleshooting practice dictates that you do not unsolder a component from a circuit board unless you are reasonably sure that it is bad or you simply cannot isolate the problem down to a single component. When components are removed, there is a risk of damage to the PC board contacts and traces.
You can perform an in-circuit check of the transistor using a transistor tester similar to the one. The three clip-leads are connected to the transistor terminals and the tester gives a positive indication if the transistor is good. Transistor tester (courtesy of B + K Precision).