Category: Amplifiers

RC Coupling Amplifier The resistance-capacitance coupling is, in short termed as RC coupling. This is the mostly used coupling technique in amplifiers. Construction of a Two-stage RC Coupled Amplifier The constructional details of a two-stage RC coupled transistor amplifier circuit are as follows. The two stage amplifier circuit has two transistors, connected in CE configuration and a common power supply VCC is used. The potential divider network R1 and R2 and the resistor Re form the biasing and stabilization network. The emitter by-pass capacitor Ce offers a low reactance path to the signal. The resistor RL is used as a load impedance. The input capacitor Cin present at the initial stage of the amplifier couples AC signal to the base of the transistor. The capacitor CC is the coupling capacitor that connects two stages and prevents DC interference between the stages and controls the shift of operating point. The figure below shows the circuit diagram of RC coupled amplifier. Operation of RC Coupled Amplifier When an AC input signal is applied to the base of first transistor, it gets amplified and appears at the collector load RL which is then passed through the coupling capacitor CC to the next stage. This becomes the input of the next stage, whose amplified output again appears across its collector load. Thus the signal is amplified in stage by stage action. The important point that has to be noted here is that the total gain is less than the product of the gains of individual stages. This is because when a second stage is made to follow the first stage, the effective load resistance of the first stage is reduced due to the shunting effect of the input resistance of the second stage. Hence, in a multistage amplifier, only the gain of the last stage remains unchanged. As we consider a two stage amplifier here, the output phase is same as input. Because the phase reversal is done two times by the two stage CE configured amplifier circuit. Frequency Response of RC Coupled Amplifier Frequency response curve is a graph that indicates the relationship between voltage gain and function of frequency. The frequency response of a RC coupled amplifier is as shown in the following graph. From the above graph, it is understood that the frequency rolls off or decreases for the frequencies below 50Hz and for the frequencies above 20 KHz. whereas the voltage gain for the range of frequencies between 50Hz and 20 KHz is constant. We know that, $$X_C = frac{1}{2 pi f_c}$$ It means that the capacitive reactance is inversely proportional to the frequency. At Low frequencies (i.e. below 50 Hz) The capacitive reactance is inversely proportional to the frequency. At low frequencies, the reactance is quite high. The reactance of input capacitor Cin and the coupling capacitor CC are so high that only small part of the input signal is allowed. The reactance of the emitter by pass capacitor CE is also very high during low frequencies. Hence it cannot shunt the emitter resistance effectively. With all these factors, the voltage gain rolls off at low frequencies. At High frequencies (i.e. above 20 KHz) Again considering the same point, we know that the capacitive reactance is low at high frequencies. So, a capacitor behaves as a short circuit, at high frequencies. As a result of this, the loading effect of the next stage increases, which reduces the voltage gain. Along with this, as the capacitance of emitter diode decreases, it increases the base current of the transistor due to which the current gain (β) reduces. Hence the voltage gain rolls off at high frequencies. At Mid-frequencies (i.e. 50 Hz to 20 KHz) The voltage gain of the capacitors is maintained constant in this range of frequencies, as shown in figure. If the frequency increases, the reactance of the capacitor CC decreases which tends to increase the gain. But this lower capacitance reactive increases the loading effect of the next stage by which there is a reduction in gain. Due to these two factors, the gain is maintained constant. Advantages of RC Coupled Amplifier The following are the advantages of RC coupled amplifier. The frequency response of RC amplifier provides constant gain over a wide frequency range, hence most suitable for audio applications. The circuit is simple and has lower cost because it employs resistors and capacitors which are cheap. It becomes more compact with the upgrading technology. Disadvantages of RC Coupled Amplifier The following are the disadvantages of RC coupled amplifier. The voltage and power gain are low because of the effective load resistance. They become noisy with age. Due to poor impedance matching, power transfer will be low. Applications of RC Coupled Amplifier The following are the applications of RC coupled amplifier. They have excellent audio fidelity over a wide range of frequency. Widely used as Voltage amplifiers Due to poor impedance matching, RC coupling is rarely used in the final stages. Learning working make money

Based on Configurations Any transistor amplifier, uses a transistor to amplify the signals which is connected in one of the three configurations. For an amplifier it is a better state to have a high input impedance, in order to avoid loading effect in Multi-stage circuits and lower output impedance, in order to deliver maximum output to the load. The voltage gain and power gain should also be high to produce a better output. Let us now study different configurations to understand which configuration suits better for a transistor to work as an amplifier. CB Amplifier The amplifier circuit that is formed using a CB configured transistor combination is called as CB amplifier. Construction The common base amplifier circuit using NPN transistor is as shown below, the input signal being applied at emitter base junction and the output signal being taken from collector base junction. The emitter base junction is forward biased by VEE and collector base junction is reverse biased by VCC. The operating point is adjusted with the help of resistors Re and Rc. Thus the values of Ic, Ib and Icb are decided by VCC, VEE, Re and Rc. Operation When no input is applied, the quiescent conditions are formed and no output is present. As Vbe is at negative with respect to ground, the forward bias is decreased, for the positive half of the input signal. As a result of this, the base current IB also gets decreased. The below figure shows the CB amplifier with self-bias circuit. As we know that, $$I_C cong I_E cong beta I_B$$ Both the collector current and emitter current get decreased. The voltage drop across RC is $$V_C = I_C R_C$$ This VC also gets decreased. As ICRC decreases, VCB increases. It is because, $$V_{CB} = V_{CC} – I_C R_C$$ Thus, a positive half cycle output is produced. In CB configuration, a positive input produces a positive output and hence input and output are in phase. So, there is no phase reversal between input and output in a CB amplifier. If CB configuration is considered for amplification, it has low input impedance and high output impedance. The voltage gain is also low compared to CE configuration. Hence CB configured amplifiers are used at high frequency applications. CE Amplifier The amplifier circuit that is formed using a CE configured transistor combination is called as CE amplifier. Construction The common emitter amplifier circuit using NPN transistor is as shown below, the input signal being applied at emitter base junction and the output signal being taken from collector base junction. The emitter base junction is forward biased by VEE and collector base junction is reverse biased by VCC. The operating point is adjusted with the help of resistors Re and Rc. Thus the values of Ic, Ib and Icb are decided by VCC, VEE, Re and Rc. Operation When no input is applied, the quiescent conditions are formed and no output is present. When positive half of the signal is being applied, the voltage between base and emitter Vbe is increased because it is already positive with respect to ground. As forward bias increases, the base current too increases accordingly. Since IC = βIB, the collector current increases as well. The following circuit diagram shows a CE amplifier with self-bias circuit. The collector current when flows through RC, the voltage drop increases. $$V_C = I_C R_C$$ As a consequence of this, the voltage between collector and emitter decreases. Because, $$V_{CB} = V_{CC} – I_C R_C$$ Thus, the amplified voltage appears across RC. Therefore, in a CE amplifier, as the positive going signal appears as a negative going signal, it is understood that there is a phase shift of 180o between input and output. CE amplifier has a high input impedance and lower output impedance than CB amplifier. The voltage gain and power gain are also high in CE amplifier and hence this is mostly used in Audio amplifiers. CC Amplifier The amplifier circuit that is formed using a CC configured transistor combination is called as CC amplifier. Construction The common collector amplifier circuit using NPN transistor is as shown below, the input signal being applied at base collector junction and the output signal being taken from emitter collector junction. The emitter base junction is forward biased by VEE and collector base junction is reverse biased by VCC. The Q-values of Ib and Ie are adjusted by Rb and Re. Operation When no input is applied, the quiescent conditions are formed and no output is present. When positive half of the signal is being applied, the forward bias is increased because Vbe is positive with respect to collector or ground. With this, the base current IB and the collector current IC are increased. The following circuit diagram shows a CC amplifier with self-bias circuit. Consequently, the voltage drop across Re i.e. the output voltage is increased. As a result, positive half cycle is obtained. As the input and output are in phase, there is no phase reversal. If CC configuration is considered for amplification, though CC amplifier has better input impedance and lower output impedance than CE amplifier, the voltage gain of CC is very less which limits its applications to impedance matching only. Comparison between CB CE CC Amplifiers Let us compare the characteristic details of CB, CE, and CC amplifiers. Characteristic CE CB CC Input resistance Low (1K to 2K) Very low (30-150 &ohm;) High (20-500 K&ohm;) Output resistance Large (≈ 50 K) High (≈ 500 K) Low (50-1000 K&ohm;) Current gain B high α < 1 High (1 + β) Voltage gain High (≈ 1500) High (≈ 1500) Less than one Power gain High (≈ 10,000) High (≈ 7500) Low (250-500) Phase between input and output reversed same same Due to the compatibility and characteristic features, the common-emitter configuration is mostly used in amplifier circuits. Learning working make money

Materials – Introduction Every material in nature has certain properties. These properties define the behavior of the materials. Material Science is a branch of electronics that deals with the study of flow of electrons in various materials or spaces, when they are subjected to various conditions. Due to the intermixing of atoms in solids, instead of single energy levels, there will be bands of energy levels formed. These set of energy levels, which are closely packed are called as Energy bands. Types of Materials The energy band in which valence electrons are present is called Valence band, while the band in which conduction electrons are present is called Conduction band. The energy gap between these two bands is called as Forbidden energy gap. Electronically, the materials are broadly classified as Insulators, Semiconductors, and Conductors. Insulators − Insulators are such materials in which the conduction cannot take place, due to the large forbidden gap. Examples: Wood, Rubber. Semiconductors − Semiconductors are such materials in which the forbidden energy gap is small and the conduction takes place if some external energy is applied. Examples: Silicon, Germanium. Conductors − Conductors are such materials in which the forbidden energy gap disappears as the valence band and conduction band become very close that they overlap. Examples: Copper, Aluminum. Of all the three, insulators are used where resistivity to electricity is desired and conductors are used where the conduction has to be high. The semiconductors are the ones which give rise to a specific interest of how they are used. Semiconductors A Semiconductor is a substance whose resistivity lies between the conductors and insulators. The property of resistivity is not the only one that decides a material as a semiconductor, but it has few properties as follows. Semiconductors have the resistivity which is less than insulators and more than conductors. Semiconductors have negative temperature co-efficient. The resistance in semiconductors, increases with the decrease in temperature and vice versa. The Conducting properties of a Semi-conductor changes, when a suitable metallic impurity is added to it, which is a very important property. The Semiconductor devices are extensively used in the field of electronics. The transistor has replaced the bulky vacuum tubes, from which the size and cost of the devices got decreased and this revolution has kept on increasing its pace leading to the new inventions like integrated electronics. Semiconductors can be classified as shown below. A semiconductor in its extremely pure form is said to be an intrinsic semiconductor. But the conduction capability of this pure form is too low. In order to increase the conduction capability of intrinsic semiconductor, it is better to add some impurities. This process of adding impurities is called as Doping. Now, this doped intrinsic semiconductor is called as an Extrinsic Semiconductor. The impurities added, are generally pentavalent and trivalent impurities. Depending upon these types of impurities, another classification is done. When a pentavalent impurity is added to a pure semiconductor, it is called as N-type extrinsic Semiconductor. As well, when a trivalent impurity is added to a pure semiconductor, it is called as P-type extrinsic Semiconductor. P-N Junction When an electron moves from its place, a hole is said to be formed there. So, a hole is the absence of an electron. If an electron is said to be moved from negative to positive terminal, it means that a hole is being moved from positive to negative terminal. The materials mentioned above are the basics of semiconductor technology. The N-type material formed by adding pentavalent impurities has electrons as its majority carriers and holes as minority carriers. While, the P-type material formed by adding trivalent impurities has holes as its majority carriers and electrons as minority carriers. Let us try to understand what happens when the P and N materials are joined together. If a P-type and an N-type material are brought close to each other, both of them join to form a junction, as shown in the figure below. A P-type material has holes as the majority carriers and an N-type material has electrons as the majority carriers. As opposite charges attract, few holes in P-type tend to go to n-side, whereas few electrons in N-type tend to go to P-side. As both of them travel towards the junction, holes and electrons recombine with each other to neutralize and forms ions. Now, in this junction, there exists a region where the positive and negative ions are formed, called as PN junction or junction barrier as shown in the figure. The formation of negative ions on P-side and positive ions on N-side results in the formation of a narrow charged region on either side of the PN junction. This region is now free from movable charge carriers. The ions present here have been stationary and maintain a region of space between them without any charge carriers. As this region acts as a barrier between P and N type materials, this is also called as Barrier junction. This has another name called as Depletion region meaning it depletes both the regions. There occurs a potential difference VD due to the formation of ions, across the junction called as Potential Barrier as it prevents further movement of holes and electrons through the junction. This formation is called as a Diode. Biasing of a Diode When a diode or any two terminal components are connected in a circuit, it has two biased conditions with the given supply. They are Forward biased condition and Reverse biased condition. Forward Biased Condition When a diode is connected in a circuit, with its anode to the positive terminal and cathode to the negative terminal of the supply, then such a connection is said to be forward biased condition. This kind of connection makes the circuit more and more forward biased and helps in more conduction. A diode conducts well in forward biased condition. Reverse Biased Condition When a diode is connected in a circuit, with its anode to the negative terminal and cathode to the positive terminal

Amplifiers Tutorial Job Search An electronic signal contains some information which cannot be utilized if doesn’t have proper strength. The process of increasing the signal strength is called as Amplification. Almost all electronic equipment must include some means for amplifying the signals. We find the use of amplifiers in medical devices, scientific equipment, automation, military tools, communication devices, and even in household equipment. In this tutorial, we will discuss all the important concepts from the introduction of transistors along with the amplifier action of transistor. In addition, we will cover all the topics related to all the major types of transistor amplifiers in detail. Audience This tutorial will suit all beginners who want to learn the fundamental concepts of transistors and transistor amplifier circuits. Prerequisites Though this tutorial is intended for beginners in the field of Electronics and Communications, we expect the readers to have some prior knowledge regarding the functioning of different electronic components. Therefore, we suggest that you first go through our tutorial on Learning working make money

Transistor Biasing Biasing is the process of providing DC voltage which helps in the functioning of the circuit. A transistor is based in order to make the emitter base junction forward biased and collector base junction reverse biased, so that it maintains in active region, to work as an amplifier. In the previous chapter, we explained how a transistor acts as a good amplifier, if both the input and output sections are biased. Transistor Biasing The proper flow of zero signal collector current and the maintenance of proper collectoremitter voltage during the passage of signal is known as Transistor Biasing. The circuit which provides transistor biasing is called as Biasing Circuit. Need for DC biasing If a signal of very small voltage is given to the input of BJT, it cannot be amplified. Because, for a BJT, to amplify a signal, two conditions have to be met. The input voltage should exceed cut-in voltage for the transistor to be ON. The BJT should be in the active region, to be operated as an amplifier. If appropriate DC voltages and currents are given through BJT by external sources, so that BJT operates in active region and superimpose the AC signals to be amplified, then this problem can be avoided. The given DC voltage and currents are so chosen that the transistor remains in active region for entire input AC cycle. Hence DC biasing is needed. The below figure shows a transistor amplifier that is provided with DC biasing on both input and output circuits. For a transistor to be operated as a faithful amplifier, the operating point should be stabilized. Let us have a look at the factors that affect the stabilization of operating point. Factors affecting the operating point The main factor that affect the operating point is the temperature. The operating point shifts due to change in temperature. As temperature increases, the values of ICE, β, VBE gets affected. ICBO gets doubled (for every 10o rise) VBE decreases by 2.5mv (for every 1o rise) So the main problem which affects the operating point is temperature. Hence operating point should be made independent of the temperature so as to achieve stability. To achieve this, biasing circuits are introduced. Stabilization The process of making the operating point independent of temperature changes or variations in transistor parameters is known as Stabilization. Once the stabilization is achieved, the values of IC and VCE become independent of temperature variations or replacement of transistor. A good biasing circuit helps in the stabilization of operating point. Need for Stabilization Stabilization of the operating point has to be achieved due to the following reasons. Temperature dependence of IC Individual variations Thermal runaway Let us understand these concepts in detail. Temperature Dependence of IC As the expression for collector current IC is $$I_C = beta I_B + I_{CEO}$$ $$= beta I_B + (beta + 1) I_{CBO}$$ The collector leakage current ICBO is greatly influenced by temperature variations. To come out of this, the biasing conditions are set so that zero signal collector current IC = 1 mA. Therefore, the operating point needs to be stabilized i.e. it is necessary to keep IC constant. Individual Variations As the value of β and the value of VBE are not same for every transistor, whenever a transistor is replaced, the operating point tends to change. Hence it is necessary to stabilize the operating point. Thermal Runaway As the expression for collector current IC is $$I_C = beta I_B + I_{CEO}$$ $$= beta I_B + (beta + 1)I_{CBO}$$ The flow of collector current and also the collector leakage current causes heat dissipation. If the operating point is not stabilized, there occurs a cumulative effect which increases this heat dissipation. The self-destruction of such an unstabilized transistor is known as Thermal run away. In order to avoid thermal runaway and the destruction of transistor, it is necessary to stabilize the operating point, i.e., to keep IC constant. Stability Factor It is understood that IC should be kept constant in spite of variations of ICBO or ICO. The extent to which a biasing circuit is successful in maintaining this is measured by Stability factor. It denoted by S. By definition, the rate of change of collector current IC with respect to the collector leakage current ICO at constant β and IB is called Stability factor. $S = frac{d I_C}{d I_{CO}}$ at constant IB and β Hence we can understand that any change in collector leakage current changes the collector current to a great extent. The stability factor should be as low as possible so that the collector current doesn’t get affected. S=1 is the ideal value. The general expression of stability factor for a CE configuration can be obtained as under. $$I_C = beta I_B + (beta + 1)I_{CO}$$ Differentiating above expression with respect to IC, we get $$1 = beta frac{d I_B}{d I_C} + (beta + 1)frac{d I_{CO}}{dI_C}$$ Or $$1 = beta frac{d I_B}{d I_C} + frac{(beta + 1)}{S}$$ Since $frac{d I_{CO}}{d I_C} = frac{1}{S}$ Or $$S = frac{beta + 1}{1 – beta left (frac{d I_B}{d I_C} right )}$$ Hence the stability factor S depends on β, IB and IC. Learning working make money

Methods of Transistor Biasing The biasing in transistor circuits is done by using two DC sources VBB and VCC. It is economical to minimize the DC source to one supply instead of two which also makes the circuit simple. The commonly used methods of transistor biasing are Base Resistor method Collector to Base bias Biasing with Collector feedback resistor Voltage-divider bias All of these methods have the same basic principle of obtaining the required value of IB and IC from VCC in the zero signal conditions. Base Resistor Method In this method, a resistor RB of high resistance is connected in base, as the name implies. The required zero signal base current is provided by VCC which flows through RB. The base emitter junction is forward biased, as base is positive with respect to emitter. The required value of zero signal base current and hence the collector current (as IC = βIB) can be made to flow by selecting the proper value of base resistor RB. Hence the value of RB is to be known. The figure below shows how a base resistor method of biasing circuit looks like. Let IC be the required zero signal collector current. Therefore, $$I_B = frac{I_C}{beta}$$ Considering the closed circuit from VCC, base, emitter and ground, while applying the Kirchhoff’s voltage law, we get, $$V_{CC} = I_B R_B + V_{BE}$$ Or $$I_B R_B = V_{CC} – V_{BE}$$ Therefore $$R_B = frac{V_{CC} – V_{BE}}{I_B}$$ Since VBE is generally quite small as compared to VCC, the former can be neglected with little error. Then, $$R_B = frac{V_{CC}}{I_B}$$ We know that VCC is a fixed known quantity and IB is chosen at some suitable value. As RB can be found directly, this method is called as fixed bias method. Stability factor $$S = frac{beta + 1}{1 – beta left ( frac{d I_B}{d I_C} right )}$$ In fixed-bias method of biasing, IB is independent of IC so that, $$frac{d I_B}{d I_C} = 0$$ Substituting the above value in the previous equation, Stability factor, $S = beta + 1$ Thus the stability factor in a fixed bias is (β+1) which means that IC changes (β+1) times as much as any change in ICO. Advantages The circuit is simple. Only one resistor RE is required. Biasing conditions are set easily. No loading effect as no resistor is present at base-emitter junction. Disadvantages The stabilization is poor as heat development can’t be stopped. The stability factor is very high. So, there are strong chances of thermal run away. Hence, this method is rarely employed. Collector to Base Bias The collector to base bias circuit is same as base bias circuit except that the base resistor RB is returned to collector, rather than to VCC supply as shown in the figure below. This circuit helps in improving the stability considerably. If the value of IC increases, the voltage across RL increases and hence the VCE also increases. This in turn reduces the base current IB. This action somewhat compensates the original increase. The required value of RB needed to give the zero signal collector current IC can be calculated as follows. Voltage drop across RL will be $$R_L = (I_C + I_B)R_L cong I_C R_L$$ From the figure, $$I_C R_L + I_B R_B + V_{BE} = V_{CC}$$ Or $$I_B R_B = V_{CC} – V_{BE} – I_C R_L$$ Therefore $$R_B = frac{V_{CC} – V_{BE} – I_C R_L}{I_B}$$ Or $$R_B = frac{(V_{CC} – V_{BE} – I_C R_L)beta}{I_C}$$ Applying KVL we have $$(I_B + I_C)R_L + I_B R_B + V_{BE} = V_{CC}$$ Or $$I_B(R_L + R_B) + I_C R_L + V_{BE} = V_{CC}$$ Therefore $$I_B = frac{V_{CC} – V_{BE} – I_C R_L}{R_L + R_B}$$ Since VBE is almost independent of collector current, we get $$frac{d I_B}{d I_C} = – frac{R_L}{R_L + R_B}$$ We know that $$S = frac{1 + beta}{1 – beta (d I_B / d I_C)}$$ Therefore $$S = frac{1 + beta}{1 + beta left ( frac{R_L}{R_L + R_B} right )}$$ This value is smaller than (1+β) which is obtained for fixed bias circuit. Thus there is an improvement in the stability. This circuit provides a negative feedback which reduces the gain of the amplifier. So the increased stability of the collector to base bias circuit is obtained at the cost of AC voltage gain. Biasing with Collector Feedback resistor In this method, the base resistor RB has its one end connected to base and the other to the collector as its name implies. In this circuit, the zero signal base current is determined by VCB but not by VCC. It is clear that VCB forward biases the base-emitter junction and hence base current IB flows through RB. This causes the zero signal collector current to flow in the circuit. The below figure shows the biasing with collector feedback resistor circuit. The required value of RB needed to give the zero signal current IC can be determined as follows. $$V_{CC} = I_C R_C + I_B R_B + V_{BE}$$ Or $$R_B = frac{V_{CC} – V_{BE} – I_C R_C}{I_B}$$ $$= frac{V_{CC} – V_{BE} – beta I_B R_C}{I_B}$$ Since $I_C = beta I_B$ Alternatively, $$V_{CE} = V_{BE} + V_{CB}$$ Or $$V_{CB} = V_{CE} – V_{BE}$$ Since $$R_B = frac{V_{CB}}{I_B} = frac{V_{CE} – V_{BE}}{I_B}$$ Where $$I_B = frac{I_C}{beta}$$ Mathematically, Stability factor, $S < (beta + 1)$ Therefore, this method provides better thermal stability than the fixed bias. The Q-point values for the circuit are shown as $$I_C = frac{V_{CC} – V_{BE}}{R_B/ beta + R_C}$$ $$V_{CE} = V_{CC} – I_C R_C$$ Advantages The circuit is simple as it needs only one resistor. This circuit provides some stabilization, for lesser changes. Disadvantages The circuit doesn’t provide good stabilization. The circuit provides negative feedback. Voltage Divider Bias Method Among all the methods of providing biasing and stabilization, the voltage divider bias method is the most prominent one. Here, two resistors R1 and R2 are employed, which are connected to VCC and provide biasing. The resistor RE employed in the emitter provides stabilization. The name voltage divider comes from the voltage

Transistor Load Line Analysis Till now we have discussed different regions of operation for a transistor. But among all these regions, we have found that the transistor operates well in active region and hence it is also called as linear region. The outputs of the transistor are the collector current and collector voltages. Output Characteristics When the output characteristics of a transistor are considered, the curve looks as below for different input values. In the above figure, the output characteristics are drawn between collector current IC and collector voltage VCE for different values of base current IB. These are considered here for different input values to obtain different output curves. Load Line When a value for the maximum possible collector current is considered, that point will be present on the Y-axis, which is nothing but the Saturation point. As well, when a value for the maximum possible collector emitter voltage is considered, that point will be present on the X-axis, which is the Cutoff point. When a line is drawn joining these two points, such a line can be called as Load line. This is called so as it symbolizes the output at the load. This line, when drawn over the output characteristic curve, makes contact at a point called as Operating point or quiescent point or simply Q-point. The concept of load line can be understood from the following graph. The load line is drawn by joining the saturation and cut off points. The region that lies between these two is the linear region. A transistor acts as a good amplifier in this linear region. If this load line is drawn only when DC biasing is given to the transistor, but no input signal is applied, then such a load line is called as DC load line. Whereas the load line drawn under the conditions when an input signal along with the DC voltages are applied, such a line is called as an AC load line. DC Load Line When the transistor is given the bias and no signal is applied at its input, the load line drawn under such conditions, can be understood as DC condition. Here there will be no amplification as the signal is absent. The circuit will be as shown below. The value of collector emitter voltage at any given time will be $$V_{CE} = V_{CC} – I_C R_C$$ As VCC and RC are fixed values, the above one is a first degree equation and hence will be a straight line on the output characteristics. This line is called as D.C. Load line. The figure below shows the DC load line. To obtain the load line, the two end points of the straight line are to be determined. Let those two points be A and B. To obtain A When collector emitter voltage VCE = 0, the collector current is maximum and is equal to VCC/RC. This gives the maximum value of VCE. This is shown as $$V_{CE} = V_{CC} – I_C R_C$$ $$0 = V_{CC} – I_C R_C$$ $$I_C = V_{CC}/R_C$$ This gives the point A (OA = VCC/RC) on collector current axis, shown in the above figure. To obtain B When the collector current IC = 0, then collector emitter voltage is maximum and will be equal to the VCC. This gives the maximum value of IC. This is shown as $$V_{CE} = V_{CC} – I_C R_C$$ $$= V_{CC}$$ (AS IC = 0) This gives the point B, which means (OB = VCC) on the collector emitter voltage axis shown in the above figure. Hence we got both the saturation and cutoff point determined and learnt that the load line is a straight line. So, a DC load line can be drawn. AC Load Line The DC load line discussed previously, analyzes the variation of collector currents and voltages, when no AC voltage is applied. Whereas the AC load line gives the peak-to-peak voltage, or the maximum possible output swing for a given amplifier. We shall consider an AC equivalent circuit of a CE amplifier for our understanding. From the above figure, $$V_{CE} = (R_C // R_1) times I_C$$ $$r_C = R_C // R_1$$ For a transistor to operate as an amplifier, it should stay in active region. The quiescent point is so chosen in such a way that the maximum input signal excursion is symmetrical on both negative and positive half cycles. Hence, $V_{max} = V_{CEQ}$ and $V_{min} = -V_{CEQ}$ Where VCEQ is the emitter-collector voltage at quiescent point The following graph represents the AC load line which is drawn between saturation and cut off points. From the graph above, the current IC at the saturation point is $$I_{C(sat)} = I_{CQ} + (V_{CEQ}/r_C)$$ The voltage VCE at the cutoff point is $$V_{CE(off)} = V_{CEQ} + I_{CQ}r_C$$ Hence the maximum current for that corresponding VCEQ = VCEQ / (RC // R1) is $$I_{CQ} = I_{CQ} * (R_C // R_1)$$ Hence by adding quiescent currents the end points of AC load line are $$I_{C(sat)} = I_{CQ} + V_{CEQ}/ (R_C // R_1)$$ $$V_{CE(off)} = V_{CEQ} + I_{CQ} * (R_C // R_1)$$ AC and DC Load Line When AC and DC Load lines are represented in a graph, it can be understood that they are not identical. Both of these lines intersect at the Q-point or quiescent point. The endpoints of AC load line are saturation and cut off points. This is understood from the figure below. From the above figure, it is understood that the quiescent point (the dark dot) is obtained when the value of base current IB is 10mA. This is the point where both the AC and DC load lines intersect. In the next chapter, we will discuss the concept of quiescent point or the operating point in detail. Learning working make money

Operating Point When a line is drawn joining the saturation and cut off points, such a line can be called as Load line. This line, when drawn over the output characteristic curve, makes contact at a point called as Operating point. This operating point is also called as quiescent point or simply Q-point. There can be many such intersecting points, but the Q-point is selected in such a way that irrespective of AC signal swing, the transistor remains in the active region. The following graph shows how to represent the operating point. The operating point should not get disturbed as it should remain stable to achieve faithful amplification. Hence the quiescent point or Q-point is the value where the Faithful Amplification is achieved. Faithful Amplification The process of increasing the signal strength is called as Amplification. This amplification when done without any loss in the components of the signal, is called as Faithful amplification. Faithful amplification is the process of obtaining complete portions of input signal by increasing the signal strength. This is done when AC signal is applied at its input. In the above graph, the input signal applied is completely amplified and reproduced without any losses. This can be understood as Faithful Amplification. The operating point is so chosen such that it lies in the active region and it helps in the reproduction of complete signal without any loss. If the operating point is considered near saturation point, then the amplification will be as under. If the operation point is considered near cut off point, then the amplification will be as under. Hence the placement of operating point is an important factor to achieve faithful amplification. But for the transistor to function properly as an amplifier, its input circuit (i.e., the base-emitter junction) remains forward biased and its output circuit (i.e., collector-base junction) remains reverse biased. The amplified signal thus contains the same information as in the input signal whereas the strength of the signal is increased. Key factors for Faithful Amplification To ensure faithful amplification, the following basic conditions must be satisfied. Proper zero signal collector current Minimum proper base-emitter voltage (VBE) at any instant. Minimum proper collector-emitter voltage (VCE) at any instant. The fulfillment of these conditions ensures that the transistor works over the active region having input forward biased and output reverse biased. Proper Zero Signal Collector Current In order to understand this, let us consider a NPN transistor circuit as shown in the figure below. The base-emitter junction is forward biased and the collector-emitter junction is reverse biased. When a signal is applied at the input, the base-emitter junction of the NPN transistor gets forward biased for positive half cycle of the input and hence it appears at the output. For negative half cycle, the same junction gets reverse biased and hence the circuit doesn’t conduct. This leads to unfaithful amplification as shown in the figure below. Let us now introduce a battery VBB in the base circuit. The magnitude of this voltage should be such that the base-emitter junction of the transistor should remain in forward biased, even for negative half cycle of input signal. When no input signal is applied, a DC current flows in the circuit, due to VBB. This is known as zero signal collector current IC. During the positive half cycle of the input, the base-emitter junction is more forward biased and hence the collector current increases. During the negative half cycle of the input, the input junction is less forward biased and hence the collector current decreases. Hence both the cycles of the input appear in the output and hence faithful amplification results, as shown in the below figure. Hence for faithful amplification, proper zero signal collector current must flow. The value of zero signal collector current should be at least equal to the maximum collector current due to the signal alone. Proper Minimum VBE at any instant The minimum base to emitter voltage VBE should be greater than the cut-in voltage for the junction to be forward biased. The minimum voltage needed for a silicon transistor to conduct is 0.7v and for a germanium transistor to conduct is 0.5v. If the base-emitter voltage VBE is greater than this voltage, the potential barrier is overcome and hence the base current and collector currents increase sharply. Hence if VBE falls low for any part of the input signal, that part will be amplified to a lesser extent due to the resultant small collector current, which results in unfaithful amplification. Proper Minimum VCE at any instant To achieve a faithful amplification, the collector emitter voltage VCE should not fall below the cut-in voltage, which is called as Knee Voltage. If VCE is lesser than the knee voltage, the collector base junction will not be properly reverse biased. Then the collector cannot attract the electrons which are emitted by the emitter and they will flow towards base which increases the base current. Thus the value of β falls. Therefore, if VCE falls low for any part of the input signal, that part will be multiplied to a lesser extent, resulting in unfaithful amplification. So if VCE is greater than VKNEE the collector-base junction is properly reverse biased and the value of β remains constant, resulting in faithful amplification. Learning working make money

Transistor as an Amplifier For a transistor to act as an amplifier, it should be properly biased. We will discuss the need for proper biasing in the next chapter. Here, let us focus how a transistor works as an amplifier. Transistor Amplifier A transistor acts as an amplifier by raising the strength of a weak signal. The DC bias voltage applied to the emitter base junction, makes it remain in forward biased condition. This forward bias is maintained regardless of the polarity of the signal. The below figure shows how a transistor looks like when connected as an amplifier. The low resistance in input circuit, lets any small change in input signal to result in an appreciable change in the output. The emitter current caused by the input signal contributes the collector current, which when flows through the load resistor RL, results in a large voltage drop across it. Thus a small input voltage results in a large output voltage, which shows that the transistor works as an amplifier. Example Let there be a change of 0.1v in the input voltage being applied, which further produces a change of 1mA in the emitter current. This emitter current will obviously produce a change in collector current, which would also be 1mA. A load resistance of 5k&ohm; placed in the collector would produce a voltage of 5 k&ohm; × 1 mA = 5V Hence it is observed that a change of 0.1v in the input gives a change of 5v in the output, which means the voltage level of the signal is amplified. Performance of Amplifier As the common emitter mode of connection is mostly adopted, let us first understand a few important terms with reference to this mode of connection. Input Resistance As the input circuit is forward biased, the input resistance will be low. The input resistance is the opposition offered by the base-emitter junction to the signal flow. By definition, 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, $R_i = frac{Delta V_{BE}}{Delta I_B}$ Where Ri = input resistance, VBE = base-emitter voltage, and IB = base current. Output Resistance The output resistance of a transistor amplifier is very high. The collector current changes very slightly with the change in collector-emitter voltage. By definition, 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 = $R_o = frac{Delta V_{CE}}{Delta I_C}$ Where Ro = Output resistance, VCE = Collector-emitter voltage, and IC = Collector-emitter voltage. Effective Collector Load The load is connected at the collector of a transistor and for a single-stage amplifier, the output voltage is taken from the collector of the transistor and for a multi-stage amplifier, the same is collected from a cascaded stages of transistor circuit. By definition, it is the total load as seen by the a.c. collector current. In case of single stage amplifiers, the effective collector load is a parallel combination of RC and Ro. Effective Collector Load, $R_{AC} = R_C // R_o$ $$= frac{R_C times R_o}{R_C + R_o} = R_{AC}$$ Hence for a single stage amplifier, effective load is equal to collector load RC. In a multi-stage amplifier (i.e. having more than one amplification stage), the input resistance Ri of the next stage also comes into picture. Effective collector load becomes parallel combination of RC, Ro and Ri i.e, Effective Collector Load, $R_{AC} = R_C // R_o // R_i$ $$R_C // R_i = frac{R_C R_i}{R_C + R_i}$$ As input resistance Ri is quite small, therefore effective load is reduced. Current Gain The gain in terms of current when the changes in input and output currents are observed, is called as Current gain. By definition, it is the ratio of change in collector current (ΔIC) to the change in base current (ΔIB). Current gain, $beta = frac{Delta I_C}{Delta I_B}$ The value of β ranges from 20 to 500. The current gain indicates that input current becomes β times in the collector current. Voltage Gain The gain in terms of voltage when the changes in input and output currents are observed, is called as Voltage gain. By definition, it is the ratio of change in output voltage (ΔVCE) to the change in input voltage (ΔVBE). Voltage gain, $A_V = frac{Delta V_{CE}}{Delta V_{BE}}$ $$= frac{Change : in: output : current times effective: load}{Change : in: input : current times input : resistance}$$ $$= frac{Delta I_C times R_{AC}}{Delta I_B times R_i} = frac{Delta I_C}{Delta I_B} times frac{R_{AC}}{R_i} = beta times frac{R_{AC}}{R_i}$$ For a single stage, RAC = RC. However, for Multistage, $$R_{AC} = frac{R_C times R_i}{R_C + R_i}$$ Where Ri is the input resistance of the next stage. Power Gain The gain in terms of power when the changes in input and output currents are observed, is called as Power gain. By definition, it is the ratio of output signal power to the input signal power. Power gain, $A_P = frac{(Delta I_C)^2 times R_{AC}}{(Delta I_B)^2 times R_i}$ $$= left ( frac{Delta I_C}{Delta I_B} right ) times frac{Delta I_C times R_{AC}}{Delta I_B times R_i}$$ = Current gain × Voltage gain Hence these are all the important terms which refer the performance of amplifiers. Learning working make money

Transistor Configurations Any transistor has three terminals, the emitter, the base, and the collector. Using these 3 terminals the transistor can be connected in a circuit with one terminal common to both input and output in three different possible configurations. The three types of configurations are Common Base, Common Emitter and Common Collector configurations. In every configuration, the emitter junction is forward biased and the collector junction is reverse biased. Common Base (CB) Configuration The name itself implies that the Base terminal is taken as common terminal for both input and output of the transistor. The common base connection for both NPN and PNP transistors is as shown in the following figure. For the sake of understanding, let us consider NPN transistor in CB configuration. When the emitter voltage is applied, as it is forward biased, the electrons from the negative terminal repel the emitter electrons and current flows through the emitter and base to the collector to contribute collector current. The collector voltage VCB is kept constant throughout this. In the CB configuration, the input current is the emitter current IE and the output current is the collector current IC. Current Amplification Factor (α) The ratio of change in collector current (ΔIC) to the change in emitter current (ΔIE) when collector voltage VCB is kept constant, is called as Current amplification factor. It is denoted by α. $alpha = frac{Delta I_C}{Delta I_E}$ at constant VCB Expression for Collector current With the above idea, let us try to draw some expression for collector current. Along with the emitter current flowing, there is some amount of base current IB which flows through the base terminal due to electron hole recombination. As collector-base junction is reverse biased, there is another current which is flown due to minority charge carriers. This is the leakage current which can be understood as Ileakage. This is due to minority charge carriers and hence very small. The emitter current that reaches the collector terminal is $$alpha I_E$$ Total collector current $$I_C = alpha I_E + I_{leakage}$$ If the emitter-base voltage VEB = 0, even then, there flows a small leakage current, which can be termed as ICBO (collector-base current with output open). The collector current therefore can be expressed as $$I_C = alpha I_E + I_{CBO}$$ $$I_E = I_C + I_B$$ $$I_C = alpha (I_C + I_B) + I_{CBO}$$ $$I_C (1 – alpha) = alpha I_B + I_{CBO}$$ $$I_C = frac{alpha}{1 – alpha}I_B + frac{I_{CBO}}{1 – alpha}$$ $$I_C = left ( frac{alpha}{1 – alpha} right )I_B + left ( frac{1}{1 – alpha} right )I_{CBO}$$ Hence the above derived is the expression for collector current. The value of collector current depends on base current and leakage current along with the current amplification factor of that transistor in use. Characteristics of CB configuration This configuration provides voltage gain but no current gain. Being VCB constant, with a small increase in the Emitter-base voltage VEB, Emitter current IE gets increased. Emitter Current IE is independent of Collector voltage VCB. Collector Voltage VCB can affect the collector current IC only at low voltages, when VEB is kept constant. The input resistance Ri is the ratio of change in emitter-base voltage (ΔVEB) to the change in emitter current (ΔIE) at constant collector base voltage VCB. $R_i = frac{Delta V_{EB}}{Delta I_E}$ at constant VCB As the input resistance is of very low value, a small value of VEB is enough to produce a large current flow of emitter current IE. The output resistance Ro is the ratio of change in the collector base voltage (ΔVCB) to the change in collector current (ΔIC) at constant emitter current IE. $R_o = frac{Delta V_{CB}}{Delta I_C}$ at constant IE As the output resistance is of very high value, a large change in VCB produces a very little change in collector current IC. This Configuration provides good stability against increase in temperature. The CB configuration is used for high frequency applications. Common Emitter (CE) Configuration The name itself implies that the Emitter terminal is taken as common terminal for both input and output of the transistor. The common emitter connection for both NPN and PNP transistors is as shown in the following figure. Just as in CB configuration, the emitter junction is forward biased and the collector junction is reverse biased. The flow of electrons is controlled in the same manner. The input current is the base current IB and the output current is the collector current IC here. Base Current Amplification factor (β) The ratio of change in collector current (ΔIC) to the change in base current (ΔIB) is known as Base Current Amplification Factor. It is denoted by β. $$beta = frac{Delta I_C}{Delta I_B}$$ Relation between β and α Let us try to derive the relation between base current amplification factor and emitter current amplification factor. $$beta = frac{Delta I_C}{Delta I_B}$$ $$alpha = frac{Delta I_C}{Delta I_E}$$ $$I_E = I_B + I_C$$ $$Delta I_E = Delta I_B + Delta I_C$$ $$Delta I_B = Delta I_E – Delta I_C$$ We can write $$beta = frac{Delta I_C}{Delta I_E – Delta I_C}$$ Dividing by ΔIE $$beta = frac{Delta I_C/Delta I_E}{frac{Delta I_E}{Delta I_E} – frac{Delta I_C}{Delta I_E}}$$ We have $$alpha = Delta I_C / Delta I_E$$ Therefore, $$beta = frac{alpha}{1 – alpha}$$ From the above equation, it is evident that, as α approaches 1, β reaches infinity. Hence, the current gain in Common Emitter connection is very high. This is the reason this circuit connection is mostly used in all transistor applications. Expression for Collector Current In the Common Emitter configuration, IB is the input current and IC is the output current. We know $$I_E = I_B + I_C$$ And $$I_C = alpha I_E + I_{CBO}$$ $$= alpha(I_B + I_C) + I_{CBO}$$ $$I_C(1 – alpha) = alpha I_B + I_{CBO}$$ $$I_C = frac{alpha}{1 – alpha}I_B + frac{1}{1 – alpha}I_{CBO}$$ If base circuit is open, i.e. if IB = 0, The collector emitter current with base open is ICEO $$I_{CEO} = frac{1}{1 – alpha}I_{CBO}$$ Substituting the value of this in the