Transistor PDF

# 8 Transistors ## 8.1 Transistor - When a third doped element is added to a crystal diode in such a way that two pn junctions are formed, the resulting device is known as a transistor. - The transistor is an entirely new type of electronic device that is capable of achieving amplification of weak...

# 8 Transistors ## 8.1 Transistor - When a third doped element is added to a crystal diode in such a way that two pn junctions are formed, the resulting device is known as a transistor. - The transistor is an entirely new type of electronic device that is capable of achieving amplification of weak signals in a fashion comparable and often superior to vacuum tubes. - Transistors are far smaller than vacuum tubes, have no filament and hence need no heating power, and can be operated in any position. - They are mechanically strong, have practically unlimited life, and can do some jobs better than vacuum tubes. ## 8.2 Naming the Transistor Terminals - A transistor (pnp or npn) has three sections of doped semiconductors. - The section on one side is the emitter. - The section on the opposite side is the collector. - The middle section is called the base and forms two junctions between the emitter and collector. - **Emitter**: The section on one side that supplies charge carriers (electrons or holes) is called the emitter. - The emitter is always forward biased w.r.t. base so that it can supply a large number of majority carriers. - In a pnp transistor, the emitter (p-type) is forward biased and supplies hole charges to its junction with the base. - In an npn transistor, the emitter (n-type) has a forward bias and supplies free electrons to its junction with the base. - **Collector**: The section on the other side that collects the charges is called the collector. - The collector is always reverse biased and its function is to remove charges from its junction with the base. - In a pnp transistor, the collector (p-type) has a reverse bias and receives hole charges that flow in the output circuit. - In an npn transistor, the collector (n-type) has reverse bias and receives electrons. - **Base**: The middle section which forms two pn-junctions between the emitter and collector is called the base. - The base-emitter junction is forward biased, allowing low resistance for the emitter circuit. - The base-collector junction is reverse biased and provides high resistance in the collector circuit. ## 8.3 Some Facts about the Transistor - The transistor has three regions: emitter, base, and collector. - The base is much thinner than the emitter, while the collector is wider than both. - The emitter is heavily doped so that it can inject a large number of charge carriers (electrons or holes) into the base. - The base is lightly doped and very thin; it passes most of the emitter-injected charge carriers to the collector. - The collector is moderately doped. ## 8.4 Transistor Action - The emitter-base junction of a transistor is forward biased whereas collector-base junction is reverse biased. - If the emitter-base junction is present, then forward bias causes the emitter current to flow. - This emitter current almost entirely flows in the collector circuit. - Therefore, the current in the collector circuit depends upon the emitter current. - If the emitter current is zero, then the collector current is nearly zero. - However, if the emitter current is 1 mA, then the collector current is also about 1 mA. ### 8.4 (i) Working of npn transistor - The forward bias causes the electrons in the n-type emitter to flow towards the base, constituting the emitter current IF. - As these electrons flow through the p-type base, they tend to combine with holes. - The base is lightly doped and very thin, therefore, only a few electrons combine with holes to constitute base current IB. - The remainder (more than 95%) cross over into the collector region to constitute collector current Ic. - It is clear that emitter current is the sum of collector and base currents i.e. IE = IB + IC ### 8.4 (ii) Working of pnp transistor - The forward bias causes the holes in the p-type emitter to flow towards the base, constituting the emitter current IF. - As these holes cross into n-type base, they tent to combine with the with electrons. - The base is lightly doped and very thin, therefore, only a few holes combine with the electrons. - The remainder (more than 95%) cross into the collector region to constitute collector current Ic. - Almost the entire emitter current flows in the collector circuit. ## 8.5 Transistor Symbols - Transistors are represented by schematic diagrams. - The symbols used for npn and pnp transistors are as follows: - **npn**: Emitter is shown by an arrow indicating the direction of conventional current flow with forward bias. - Current flows out of the emitter. - **pnp**: The conventional current flows into the emitter. ## 8.6 Transistor Circuit as an Amplifier - A transistor acts as an amplifier. - The weak signal is applied between emitter-base junction and the output is taken across the load connected in the collector circuit. - In order to achieve faithful amplification, the input circuit should always remain forward biased. - To do so, a d.c. voltage VEE is applied in the input circuit in addition to the signal as shown. - This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the input circuit forward biased regardless of the polarity of the signal. - A small change in signal voltage causes an appreciable change in emitter current. - Almost the same change in collector current is produced, which flows through a high load resistance producing a large voltage across it. - Thus, a weak signal applied in the input circuit appears in an amplified form in the collector circuit. ## 8.7 Transistor Connections - There are three leads in a transistor: emitter, base, and collector terminals. - To connect a transistor in a circuit, we need four terminals: two for the input and two for the output. - The difficulty of needing four terminals is overcome by making one terminal of the transistor common to both input and output terminals. - The input leads are connected between this common terminal and one of the other two terminals and the output is obtained between the common terminal and the remaining terminal. There are three ways to connect a transistor in a circuit: - **Common base connection** - **Common emitter connection** - **Common collector connection** - In each circuit connection, the emitter is always biased in the forward direction, while the collector always has a reverse bias. ## 8.8 Common Base Connection - The input is applied between emitter and base, and output is taken from collector and base. - The base of the transistor is common to both input and output circuits, hence the name common base connection. - In npn transistor, the current flows out of the emitter. - In pnp transistor, the current flows into the emitter. ### 8.8 (1) Current amplification factor (α) - It is the ratio of output current to input current. - In a common base connection, the input current is the emitter current IE, and the output current is the collector current Ic. - The ratio of change in collector current to the change in emitter current at constant collector-base voltage VCB is known as current amplification factor. * α=ΔΙc/ΔΙe at constant VCB - α is always less than unity. - It can be increased by decreasing the base current, which is achieved by making the base thin and doping it lightly. - Practical values of α are in commercial transistors between 0.9 and 0.99, where β ≥ 1. ### 8.8 (2) Expression for collector current - The whole of the emitter current does not reach the collector. - This is because a small percentage of it, as a result of electron-hole combinations occurring in the base area, gives rise to base current. - As the collector-base junction is reverse-biased, some leakage current flows due to minority carriers. - The total collector current consists of : - The part of the emitter current that reaches the collector terminal (αIE). - Leakage current, ICBO * Ic = αIE + ICEO = α(IB + IC) + ICEO - If IE = 0 (i.e. base circuit is open), then the collector current will be the current to the emitter, which is abbreviated as ICEO, meaning collector-emitter current with the base open. * ICEO= (1/(1−α))ICBO * Ic= (α/(1−α))IB + ICEO - When IE = 0, a leakage current (ICBO) flows in the collector circuit. - ICBO is the collector-base current with the emitter open. ## 8.9 Characteristics of Common Base Connection - The complete electrical behavior of a transistor can be described by the interrelation of the currents and voltages. - These relationships can be conveniently displayed graphically and are known as the characteristics of the transistor. - The important characteristics of the common base connection are the input characteristics and the output characteristics. ### 8.9 (1) Input characteristic - It is the curve between emitter current and emitter-base voltage at constant collector-base voltage VCB. - The input resistance of CB circuit is very small because I is almost independent of VCB. - Input resistance is the ratio of change in emitter-base voltage to the resulting change in emitter current at constant VCB. * rIN=ΔVBE/ΔIE at constant VCB ### 8.9 (2) Output characteristic - It is the curve between collector current and collector-base voltage at constant emitter current IF. - The output resistance of the CB circuit is very high because a very large change in collector-base produces only a tiny change in collector current. - Output resistance is the ratio of change in collector-base voltage to the resulting change in collector current at constant IE. * ro =ΔICB/ΔIC at constant IE ## 8.10 Common Emitter Connection - The input is applied between base and emitter, and the output is taken from collector and emitter. - The emitter of the transistor is common to both input and output circuits. - In npn transistor, the current flows out of the emitter. - In pnp transistor, the current flows into the emitter. ### 8.10 (1) Base current amplification factor (β) - It is the ratio of change in collector current (ΔIc) to the change in base current (ΔIB) * β* = ΔIc/ΔIB - The value of β is generally greater than 20; usually, its value ranges from 20 to 500. - This type of connection is frequently used as it gives appreciable current gain as well as voltage gain. ## 8.11 Measurement of leakage current - A very small leakage current flows in all transistor circuits. - In most cases, it is quite small and can be neglected. ### 8.11 (1) Circuit for ICBO test - The base is open (IB = 0) and the transistor is in a cut-off state. - Ideally, IC = 0 but actually, there is a small current from collector to emitter due to minority carriers, called ICBO (collector-to-base current with emitter open). - This current is usually in the nA range for silicon transistors. - Faulty transistors often have excessive leakage current. ### 8.11 (2) Circuit for ICEO test - The emitter is open (IE = 0) and the transistor is in a cut-off state. - A small current flows from the collector to the base because the base-collector junction is reverse biased, called ICBO (collector-to-base current with emitter open). - This current is also very small. - If ICBO is excessive in a measurement, then it is likely that the collector-base junction is shorted. * ICEO = (β+1)*ICBO ## 8.12 Characteristics of Common Emitter Connection - The important characteristics of this circuit arrangement are the input characteristics and output characteristics. ### 8.12 (1) Input Characteristic - It is the curve between base current and base-emitter voltage at constant collector-emitter voltage, VCE. - The value of input resistance is quite small because the input circuit is always forward biased. - It ranges from 500 Ω for small transistors to 5 Ω for high-power transistors. ### 8.12 (2) Output Characteristic - It is the curve between collector current and collector-emitter voltage at constant base current, IB. - The output resistance of a transistor is high because the collector-base junction is reverse biased. ### 8.12 (3) Effective Collector Load - It is the total load seen by the ac collector current. - It is the parallel combination of Rc and Ro for a single-stage amplifier. - However, in a multistage amplifier, it is also the input resistance of the following state. ### (4) Current Gain - It is the ratio of change in collector current (ΔIc) to the change in base current (ΔIB). * β = ΔIc /ΔIB - The value of β ranges from 20 to 500. ## 8.13 Common Collector Connection - The input is applied between the base and the collector, and the output is taken between the emitter and the collector. - The collector of the transistor is common to both input and output circuits, hence the name **common collector connection.** The current flows out the emitter in an npn transistor and flows into the emitter in a pnp transistor. ### 8.13 (1) Current Amplification factor (γ) - It is the ratio of change in emitter current (ΔIE) to the change in base current (ΔIB). *γ = ΔIE /ΔIB - This circuit provides about the same current gain as the common emitter circuit as ΔIE ≃ ΔIC. ## 8.14 Comparison of Transistor Connections |S. No.|Characteristic | Common base| Common emitter| Common collector| |:---|:---|:---|:---|:---| | 1 |Input resistance |Low (about 100 Ω)|Low (about 750 Ω)|Very high (about 750 ΚΩ)| | 2 |Output resistance |Very high (about 450 ΚΩ)| High(about 45 ΚΩ)|Low (about 50 Ω)| | 3 |Voltage gain |about 150 |about 500 |less than 1| | 4 | Applications |For high frequency applications |For audio frequency applications |For impedance matching| | 5 |Current gain |No (less than 1) |High (β)|Appreciable| ## 8.15 Commonly Used Transistor Connection - Out of the three transistor connections, the common emitter circuit is the most efficient. - It is used in about 90 to 95 per cent of all transistor applications. - The main reasons for its use are : - **High current gain** : Ic = BIB + ICEO - **High voltage and power gain** : The ratio of output impedance to input impedance is small (around 50). - **Moderate output to input impedance ratio** : This makes this circuit arrangement an ideal one for coupling between various transistor stages. ## 8.16 Transistor as an Amplifier in CE Arrangement - Figure 8.33 shows the common emitter npn amplifier circuit. - *A battery VBB is connected in the input circuit. This d.c. voltage is known as bias voltage and its magnitude is such that it always keeps the emitter-base junction forward biased regardless of the polarity of the signal source.* - During the positive half-cycle of the signal, the forward bias across the emitter-base junction is increased, which causes an increase in collector current and a greater voltage drop across the collector load resistance. - During the negative half-cycle of the signal, the forward bias across the emitter-base junction is decreased, which causes a decrease in collector current and a smaller voltage drop across the collector load resistance. - This results in an amplified output. ## 8.17 Transistor Load Line Analysis - In the transistor circuit analysis, it is generally required to determine the collector current for various collector-emitter voltages. - This is done by plotting the output characteristics and determining the collector current for the desired collector-emitter voltage. - However, a simpler method known as the **load line method** can be used to solve these problems. - This method is frequently used in the analysis of transistor applications. ## 8.18 Operating Point - The zero signal values of Ic and VCE are known as the operating point. - It is called the operating point because it is where variations of Ic and VCE take place when signal is applied. - It is also called the quiescent (silent) point or the Q-point because it is the point on the IC-VCE characteristic when the transistor is silent. - The operating point is defined by where the d.c. load line intersects the proper base current curve. ## 8.19 Practical Way of Drawing CE Circuit - The common emitter circuits can be shown in a different way to simplify their representation. - Figure 8.45 shows the practical way of drawing a CE circuit. ## 8.20 Output from Transistor Amplifier - A transistor acts as an amplifier. - There are two methods of taking output from a transistor connected in a common emitter configuration. - The output can be taken across RC (i.e. from the collector load) - Output= iRc - The output can be taken across terminals 1 and 2 - Output = - iRc ## 8.21 Performance of Transistor Amplifier - The performance of a transistor amplifier depends upon input resistance, output resistance, effective collector load, current gain, voltage gain, and power gain. ### 8.21 (1) Input Resistance - It is the ratio of small change in base-emitter voltage (ΔVBE) to the resulting change in base current (ΔIB) at constant collector-emitter voltage. * Input resistance, RIN= ΔVBE/ΔIB at constant VCE - The value of input resistance is quite small because the input circuit is always forward biased. - It ranges from 500 Ω for small transistors to 5 Ω for high-powered transistors. ### 8.21 (2) Output Resistance - It is the ratio of change in collector-emitter voltage (ΔVCE) to the resulting change in collector current (ΔIC) at constant base current. * Output resistance, Ro= ΔVCE/ΔIC at constant IB - The output resistance is high due to the fact that the collector-base junction is reverse biased. ### 8.21 (3) Effective Collector Load - Effective collector load is the total load seen by the ac collector current. - It is the parallel combination of Rc and Ro. - In a multistage amplifier, it’s the parallel combination of Rc, Ro, and the input resistance of the next stage. ### 8.21 (4) Current Gain - It is the ratio of change in collector current (ΔIc) to the change in base current (ΔIB). * β = ΔIc /ΔIB - The current gain indicates that the input current becomes β times in the collector circuit. ### 8.21 (5) Voltage Gain - It is the ratio of change in output voltage (ΔVCE) to the change in input voltage (ΔVBE) * A = ΔVCE /ΔVBE ### 8.21 (6) Power Gain - It is the ratio of output signal power to the input signal power. * Ap= (ΔIc)²×RAC/ (ΔIB)²×R ## 8.22 Cut-off and Saturation Points - **Cut-off**: The point where the load line intersects the IB = 0 curve. - At the cut-off point, IB =0 and only a small collector leakage current, ICEO, exists. - The base-emitter junction no longer remains forward biased, and normal transistor action is lost. - The collector-emitter voltage is nearly equal to Vcc, i.e. VCE(cut-off)= VCC - **Saturation**: The point where the load line intersects the IB = IB(sat) curve. - At this point, the base current is maximum and so is the collector current. - The collector-base junction no longer remains reverse biased, and normal transistor action is lost. ## 8.23 Power Rating of Transistor - The maximum power that a transistor can handle without destruction is known as the **power rating** of the transistor. - Almost all the power is dissipated at the reverse biased collector-base junction. * PD(max)= IC * VCB = IC * VCE - When connecting a transistor in a circuit, it should be ensured that its power rating is not exceeded, otherwise, the transistor will be destroyed due to excessive heat. - The maximum VCE allowed can be calculated for the power rating (PD(max)) and the collector current (Ic). * PD(max)= IC * VCE(max) - If VCE exceeds this value, the transistor will be destroyed. ## 8.24 Maximum Power Dissipation Curve - It is a curve that shows the maximum power dissipation of a transistor. - To draw the curve, know the power rating of the transistor. - Using convenient VCE values, the corresponding collector currents are calculated for the maximum power dissipation. - Many points are located on the output characteristics, and a curve is drawn through the above points to obtain the maximum power dissipation curve. - The transistor voltage and current (i.e. VCE and Ic) conditions must at all times be maintained in the portion of the characteristics below the maximum power dissipation curve. ## 8.25 Characteristics of Transistor - Transistor can act in two states: activated and deactivated. - Transistor consists of two junctions: base-emitter junction and collector-base junction. ### 8.25 (1) Cut-off - Both base-emitter junction and collector-base junction are off. - No base current, collector current, or emitter current is flowing. ### 8.25 (2) Active State - Base-emitter junction is on, and collector-base junction is off. - Collector current is β times the base current. ### 8.25 (3) Saturated - Both base-emitter junction and collector-base junction are on. - The transistor acts as a switch and the collector and emitter are shorted. ## 8.26 Measurement of Transistor Parameters - There are three common measurement methods used to determine the transistor’s parameters (α, β, and ICBO). - *The common emitter connection is generally used for measuring β because it provides the highest gain and is the most widely used amplifier configuration.* - The measured data is plotted on characteristic curves. ## 8.27 Practical Applications of Transistors - Transistors are used in many electronic devices: - Amplifiers - Switches - Oscillators - Digital circuits - Power supplies - Sensors - To select the best transistor configuration for a particular application depends on the specific requirements of the application, such as the necessary current gain, voltage gain, power gain, and input and output impedance. ## 8.28 Understanding Load Lines in Transistor Circuits - A d.c. load line is a straight line that shows the operating points of the transistor circuit. - It is plotted on the output characteristics of the transistor. - The end points of the load line are determined by the maximum collector-emitter voltage (Vcc) and the maximum collector current (Vcc/Rc), where Rc is the collector resistance. - The operating point (Q point) is where the load line intersects the base current curve. - The Q point determines the operating conditions of the transistor. - The d.c. load line is essential for understanding the operation of a transistor in a circuit. - It helps engineers to determine the operating point and predict the performance of the circuit under specific conditions. ## 8.29 Saturation and Cutoff in Transistors - **Saturation**: occurs when the transistor is turned on as hard as possible and the transistor is conducting as much current as possible. - **Cutoff**: occurs when the transistor is turned off and it is not conducting any current. - Understanding saturation and cutoff is crucial for designing and troubleshooting transistor circuits. - By properly biasing a transistor, you can ensure that it operates in the active region, where it can be used effectively as an amplifier or switch. ## 8.30 Practical Considerations - **Bias Stability**: Using the correct resistor values for the base, collector, and emitter is crucial but requires careful attention to ensure that the transistor operates in the active region. - **Power Dissipation**: Excessive power dissipation could overheat the transistor. - **Temperature Effects**: The temperature affects the transistor's performance. - **Transistor Characteristics**: It is important to understand the transistor's parameters such as current gain (β), leakage current (ICBO), and input and output resistances. ## 8.31 Troubleshooting Transistor Circuits - **Symptoms**: The first step is to identify the symptoms of the malfunction. Is the circuit not working at all, or is it exhibiting unexpected behavior? - **Measurement**: Use a multimeter to measure the voltages and currents in the circuit. - **Visual Inspection**: Look for any signs of physical damage, such as broken connections, burnt resistors or capacitors. - **Transistor Testing**: Check for proper transistor function by testing its parameters. - **Biasing**: Verify that the transistor is properly biased. - **Load Line Analysis**: Use load line analysis to diagnose operating point issues. - **Component Replacement**: If a component test reveals a fault, then replace it. - **Testing**: Retest the circuit after making repairs. ## 8.32 Transistor Applications - Transistors are used extensively in a wide range of electronic devices. - The choice of transistor depends on the specific application. - The most widely used transistor configuration is the common emitter connection. - The main reason for the popularity of the common emitter configuration is high current gain. - Other factors to consider when choosing a transistor include collector current (IC), collector-emitter voltage (VCE), power rating, and frequency. - The transistor’s configuration and characteristics are essential to ensure the proper functioning of the circuit and achieve the desired performance.