Category: Amplifiers

Basic Amplifier We hope that you have gained sufficient knowledge on operating point, its stability and the compensation techniques in the previous chapter. Let us now try to understand the fundamental concepts of a basic amplifier circuit. 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. Amplification in practical applications is done using Multi-stage amplifiers. A number of single-stage amplifiers are cascaded to form a Multi-stage amplifier. Let us see how a single-stage amplifier is built, which is the basic for a Multi-stage amplifier. Single-stage Transistor Amplifier When only one transistor with associated circuitry is used for amplifying a weak signal, the circuit is known as single-stage amplifier. Analyzing the working of a Single-stage amplifier circuit, makes us easy to understand the formation and working of Multi-stage amplifier circuits. A Single stage transistor amplifier has one transistor, bias circuit and other auxiliary components. The following circuit diagram shows how a single stage transistor amplifier looks like. When a weak input signal is given to the base of the transistor as shown in the figure, a small amount of base current flows. Due to the transistor action, a larger current flows in the collector of the transistor. (As the collector current is β times of the base current which means IC = βIB). Now, as the collector current increases, the voltage drop across the resistor RC also increases, which is collected as the output. Hence a small input at the base gets amplified as the signal of larger magnitude and strength at the collector output. Hence this transistor acts as an amplifier. Practical Circuit of a Transistor Amplifier The circuit of a practical transistor amplifier is as shown below, which represents a voltage divider biasing circuit. The various prominent circuit elements and their functions are as described below. Biasing Circuit The resistors R1, R2 and RE form the biasing and stabilization circuit, which helps in establishing a proper operating point. Input Capacitor Cin This capacitor couples the input signal to the base of the transistor. The input capacitor Cin allows AC signal, but isolates the signal source from R2. If this capacitor is not present, the input signal gets directly applied, which changes the bias at R2. Coupling Capacitor CC This capacitor is present at the end of one stage and connects it to the other stage. As it couples two stages it is called as coupling capacitor. This capacitor blocks DC of one stage to enter the other but allows AC to pass. Hence it is also called as blocking capacitor. Due to the presence of coupling capacitor CC, the output across the resistor RL is free from the collector’s DC voltage. If this is not present, the bias conditions of the next stage will be drastically changed due to the shunting effect of RC, as it would come in parallel to R2 of the next stage. Emitter by-pass capacitor CE This capacitor is employed in parallel to the emitter resistor RE. The amplified AC signal is by passed through this. If this is not present, that signal will pass through RE which produces a voltage drop across RE that will feedback the input signal reducing the output voltage. The Load resistor RL The resistance RL connected at the output is known as Load resistor. When a number of stages are used, then RL represents the input resistance of the next stage. Various Circuit currents Let us go through various circuit currents in the complete amplifier circuit. These are already mentioned in the above figure. Base Current When no signal is applied in the base circuit, DC base current IB flows due to biasing circuit. When AC signal is applied, AC base current ib also flows. Therefore, with the application of signal, total base current iB is given by $$i_B = I_B + i_b$$ Collector Current When no signal is applied, a DC collector current IC flows due to biasing circuit. When AC signal is applied, AC collector current ic also flows. Therefore, the total collector current iC is given by $$i_C = I_C + i_c$$ Where $I_C = beta I_B$ = zero signal collecor current $i_c = beta i_b$ = collecor current due to signal Emitter Current When no signal is applied, a DC emitter current IE flows. With the application of signal, total emitter current iE is given by $$i_E = I_E + i_e$$ It should be remembered that $$I_E = I_B + I_C$$ $$i_e = i_b + i_c$$ As base current is usually small, it is to be noted that $I_E cong I_C$ and $i_e cong i_c$ These are the important considerations for the practical circuit of transistor amplifier. Now let us know about the classification of Amplifiers. Learning working make money

Transformer Coupled Amplifier We have observed that the main drawback of RC coupled amplifier is that the effective load resistance gets reduced. This is because, the input impedance of an amplifier is low, while its output impedance is high. When they are coupled to make a multistage amplifier, the high output impedance of one stage comes in parallel with the low input impedance of next stage. Hence, effective load resistance is decreased. This problem can be overcome by a transformer coupled amplifier. In a transformer-coupled amplifier, the stages of amplifier are coupled using a transformer. Let us go into the constructional and operational details of a transformer coupled amplifier. Construction of Transformer Coupled Amplifier The amplifier circuit in which, the previous stage is connected to the next stage using a coupling transformer, is called as Transformer coupled amplifier. The coupling transformer T1 is used to feed the output of 1st stage to the input of 2nd stage. The collector load is replaced by the primary winding of the transformer. The secondary winding is connected between the potential divider and the base of 2nd stage, which provides the input to the 2nd stage. Instead of coupling capacitor like in RC coupled amplifier, a transformer is used for coupling any two stages, in the transformer coupled amplifier circuit. The figure below shows the circuit diagram of transformer coupled amplifier. The potential divider network R1 and R2 and the resistor Re together 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. Operation of Transformer Coupled Amplifier When an AC signal is applied to the input of the base of the first transistor then it gets amplified by the transistor and appears at the collector to which the primary of the transformer is connected. The transformer which is used as a coupling device in this circuit has the property of impedance changing, which means the low resistance of a stage (or load) can be reflected as a high load resistance to the previous stage. Hence the voltage at the primary is transferred according to the turns ratio of the secondary winding of the transformer. This transformer coupling provides good impedance matching between the stages of amplifier. The transformer coupled amplifier is generally used for power amplification. Frequency Response of Transformer Coupled Amplifier The figure below shows the frequency response of a transformer coupled amplifier. The gain of the amplifier is constant only for a small range of frequencies. The output voltage is equal to the collector current multiplied by the reactance of primary. At low frequencies, the reactance of primary begins to fall, resulting in decreased gain. At high frequencies, the capacitance between turns of windings acts as a bypass condenser to reduce the output voltage and hence gain. So, the amplification of audio signals will not be proportionate and some distortion will also get introduced, which is called as Frequency distortion. Advantages of Transformer Coupled Amplifier The following are the advantages of a transformer coupled amplifier − An excellent impedance matching is provided. Gain achieved is higher. There will be no power loss in collector and base resistors. Efficient in operation. Disadvantages of Transformer Coupled Amplifier The following are the disadvantages of a transformer coupled amplifier − Though the gain is high, it varies considerably with frequency. Hence a poor frequency response. Frequency distortion is higher. Transformers tend to produce hum noise. Transformers are bulky and costly. Applications The following are the applications of a transformer coupled amplifier − Mostly used for impedance matching purposes. Used for Power amplification. Used in applications where maximum power transfer is needed. Learning working make money

Multi-Stage Transistor Amplifier In practical applications, the output of a single state amplifier is usually insufficient, though it is a voltage or power amplifier. Hence they are replaced by Multi-stage transistor amplifiers. In Multi-stage amplifiers, the output of first stage is coupled to the input of next stage using a coupling device. These coupling devices can usually be a capacitor or a transformer. This process of joining two amplifier stages using a coupling device can be called as Cascading. The following figure shows a two-stage amplifier connected in cascade. The overall gain is the product of voltage gain of individual stages. $$A_V = A_{V1} times A_{V2} = frac{V_2}{V_1} times frac{V_0}{V_2} = frac{V_0}{V_1}$$ Where AV = Overall gain, AV1 = Voltage gain of 1st stage, and AV2 = Voltage gain of 2nd stage. If there are n number of stages, the product of voltage gains of those n stages will be the overall gain of that multistage amplifier circuit. Purpose of coupling device The basic purposes of a coupling device are To transfer the AC from the output of one stage to the input of next stage. To block the DC to pass from the output of one stage to the input of next stage, which means to isolate the DC conditions. Types of Coupling Joining one amplifier stage with the other in cascade, using coupling devices form a Multi-stage amplifier circuit. There are four basic methods of coupling, using these coupling devices such as resistors, capacitors, transformers etc. Let us have an idea about them. Resistance-Capacitance Coupling This is the mostly used method of coupling, formed using simple resistor-capacitor combination. The capacitor which allows AC and blocks DC is the main coupling element used here. The coupling capacitor passes the AC from the output of one stage to the input of its next stage. While blocking the DC components from DC bias voltages to effect the next stage. Let us get into the details of this method of coupling in the coming chapters. Impedance Coupling The coupling network that uses inductance and capacitance as coupling elements can be called as Impedance coupling network. In this impedance coupling method, the impedance of coupling coil depends on its inductance and signal frequency which is jwL. This method is not so popular and is seldom employed. Transformer Coupling The coupling method that uses a transformer as the coupling device can be called as Transformer coupling. There is no capacitor used in this method of coupling because the transformer itself conveys the AC component directly to the base of second stage. The secondary winding of the transformer provides a base return path and hence there is no need of base resistance. This coupling is popular for its efficiency and its impedance matching and hence it is mostly used. Direct Coupling If the previous amplifier stage is connected to the next amplifier stage directly, it is called as direct coupling. The individual amplifier stage bias conditions are so designed that the stages can be directly connected without DC isolation. The direct coupling method is mostly used when the load is connected in series, with the output terminal of the active circuit element. For example, head-phones, loud speakers etc. Role of Capacitors in Amplifiers Other than the coupling purpose, there are other purposes for which few capacitors are especially employed in amplifiers. To understand this, let us know about the role of capacitors in Amplifiers. The Input Capacitor Cin The input capacitor Cin present at the initial stage of the amplifier, couples AC signal to the base of the transistor. This capacitor Cin if not present, the signal source will be in parallel to resistor R2 and the bias voltage of the transistor base will be changed. Hence Cin allows, the AC signal from source to flow into input circuit, without affecting the bias conditions. The Emitter By-pass Capacitor Ce The emitter by-pass capacitor Ce is connected in parallel to the emitter resistor. It offers a low reactance path to the amplified AC signal. In the absence of this capacitor, the voltage developed across RE will feedback to the input side thereby reducing the output voltage. Thus in the presence of Ce the amplified AC will pass through this. Coupling Capacitor CC The capacitor CC is the coupling capacitor that connects two stages and prevents DC interference between the stages and controls the operating point from shifting. This is also called as blocking capacitor because it does not allow the DC voltage to pass through it. In the absence of this capacitor, RC will come in parallel with the resistance R1 of the biasing network of the next stage and thereby changing the biasing conditions of the next stage. Amplifier Consideration For an amplifier circuit, the overall gain of the amplifier is an important consideration. To achieve maximum voltage gain, let us find the most suitable transistor configuration for cascading. CC Amplifier Its voltage gain is less than unity. It is not suitable for intermediate stages. CB Amplifier Its voltage gain is less than unity. Hence not suitable for cascading. CE Amplifier Its voltage gain is greater than unity. Voltage gain is further increased by cascading. The characteristics of CE amplifier are such that, this configuration is very suitable for cascading in amplifier circuits. Hence most of the amplifier circuits use CE configuration. In the subsequent chapters of this tutorial, we will explain the types of coupling amplifiers. Learning working make money

Tuned Amplifiers The types of amplifiers that we have discussed so far cannot work effectively at radio frequencies, even though they are good at audio frequencies. Also, the gain of these amplifiers is such that it will not vary according to the frequency of the signal, over a wide range. This allows the amplification of the signal equally well over a range of frequencies and does not permit the selection of particular desired frequency while rejecting the other frequencies. So, there occurs a need for a circuit which can select as well as amplify. So, an amplifier circuit along with a selection, such as a tuned circuit makes a Tuned amplifier. What is a Tuned Amplifier? Tuned amplifiers are the amplifiers that are employed for the purpose of tuning. Tuning means selecting. Among a set of frequencies available, if there occurs a need to select a particular frequency, while rejecting all other frequencies, such a process is called Selection. This selection is done by using a circuit called as Tuned circuit. When an amplifier circuit has its load replaced by a tuned circuit, such an amplifier can be called as a Tuned amplifier circuit. The basic tuned amplifier circuit looks as shown below. The tuner circuit is nothing but a LC circuit which is also called as resonant or tank circuit. It selects the frequency. A tuned circuit is capable of amplifying a signal over a narrow band of frequencies that are centered at resonant frequency. When the reactance of the inductor balances the reactance of the capacitor, in the tuned circuit at some frequency, such a frequency can be called as resonant frequency. It is denoted by fr. The formula for resonance is $$2 pi f_L = frac{1}{2 pi f_c}$$ $$f_r = frac{1}{2 pi sqrt{LC}}$$ Types of Tuned Circuits A tuned circuit can be Series tuned circuit (Series resonant circuit) or Parallel tuned circuit (parallel resonant circuit) according to the type of its connection to the main circuit. Series Tuned Circuit The inductor and capacitor connected in series make a series tuned circuit, as shown in the following circuit diagram. At resonant frequency, a series resonant circuit offers low impedance which allows high current through it. A series resonant circuit offers increasingly high impedance to the frequencies far from the resonant frequency. Parallel Tuned Circuit The inductor and capacitor connected in parallel make a parallel tuned circuit, as shown in the below figure. At resonant frequency, a parallel resonant circuit offers high impedance which does not allow high current through it. A parallel resonant circuit offers increasingly low impedance to the frequencies far from the resonant frequency. Characteristics of a Parallel Tuned Circuit The frequency at which parallel resonance occurs (i.e. reactive component of circuit current becomes zero) is called the resonant frequency fr. The main characteristics of a tuned circuit are as follows. Impedance The ratio of supply voltage to the line current is the impedance of the tuned circuit. Impedance offered by LC circuit is given by $$frac{Supply : voltage}{Line equation} = frac{V}{I}$$ At resonance, the line current increases while the impedance decreases. The below figure represents the impedance curve of a parallel resonance circuit. Impedance of the circuit decreases for the values above and below the resonant frequency fr. Hence the selection of a particular frequency and rejection of other frequencies is possible. To obtain an equation for the circuit impedance, let us consider Line Current $I = I_L cos phi$ $$frac{V}{Z_r} = frac{V}{Z_L} times frac{R}{Z_L}$$ $$frac{1}{Z_r} = frac{R}{Z_L^2}$$ $$frac{1}{Z_r} = frac{R}{L/C} = frac{C R}{L}$$ Since, $Z_L^2 = frac{L}{C}$ Therefore, circuit impedance Zr is obtained as $$Z_R = frac{L}{C R}$$ Thus at parallel resonance, the circuit impedance is equal to L/CR. Circuit Current At parallel resonance, the circuit or line current I is given by the applied voltage divided by the circuit impedance Zr i.e., Line Current $I = frac{V}{Z_r}$ Where $Z_r = frac{L}{C R}$ Because Zr is very high, the line current I will be very small. Quality Factor For a parallel resonance circuit, the sharpness of the resonance curve determines the selectivity. The smaller the resistance of the coil, the sharper the resonant curve will be. Hence the inductive reactance and resistance of the coil determine the quality of the tuned circuit. The ratio of inductive reactance of the coil at resonance to its resistance is known as Quality factor. It is denoted by Q. $$Q = frac{X_L}{R} = frac{2 pi f_r L}{R}$$ The higher the value of Q, the sharper the resonance curve and the better the selectivity will be. Advantages of Tuned Amplifiers The following are the advantages of tuned amplifiers. The usage of reactive components like L and C, minimizes the power loss, which makes the tuned amplifiers efficient. The selectivity and amplification of desired frequency is high, by providing higher impedance at resonant frequency. A smaller collector supply VCC would do, because of its little resistance in parallel tuned circuit. It is important to remember that these advantages are not applicable when there is a high resistive collector load. Frequency Response of Tuned Amplifier For an amplifier to be efficient, its gain should be high. This voltage gain depends upon β, input impedance and collector load. The collector load in a tuned amplifier is a tuned circuit. The voltage gain of such an amplifier is given by Voltage gain = $frac{beta Z_C}{Z_{in}}$ Where ZC = effective collector load and Zin = input impedance of the amplifier. The value of ZC depends upon the frequency of the tuned amplifier. As ZC is maximum at resonant frequency, the gain of the amplifier is maximum at this resonant frequency. Bandwidth The range of frequencies at which the voltage gain of the tuned amplifier falls to 70.7% of the maximum gain is called its Bandwidth. The range of frequencies between f1 and f2 is called as bandwidth of the tuned amplifier. The bandwidth of a tuned amplifier depends upon the Q of the LC circuit i.e., upon the sharpness of the frequency response.

Class B Power Amplifier When the collector current flows only during the positive half cycle of the input signal, the power amplifier is known as class B power amplifier. Class B Operation The biasing of the transistor in class B operation is in such a way that at zero signal condition, there will be no collector current. The operating point is selected to be at collector cut off voltage. So, when the signal is applied, only the positive half cycle is amplified at the output. The figure below shows the input and output waveforms during class B operation. When the signal is applied, the circuit is forward biased for the positive half cycle of the input and hence the collector current flows. But during the negative half cycle of the input, the circuit is reverse biased and the collector current will be absent. Hence only the positive half cycle is amplified at the output. As the negative half cycle is completely absent, the signal distortion will be high. Also, when the applied signal increases, the power dissipation will be more. But when compared to class A power amplifier, the output efficiency is increased. Well, in order to minimize the disadvantages and achieve low distortion, high efficiency and high output power, the push-pull configuration is used in this class B amplifier. Class B Push-Pull Amplifier Though the efficiency of class B power amplifier is higher than class A, as only one half cycle of the input is used, the distortion is high. Also, the input power is not completely utilized. In order to compensate these problems, the push-pull configuration is introduced in class B amplifier. Construction The circuit of a push-pull class B power amplifier consists of two identical transistors T1 and T2 whose bases are connected to the secondary of the center-tapped input transformer Tr1. The emitters are shorted and the collectors are given the VCC supply through the primary of the output transformer Tr2. The circuit arrangement of class B push-pull amplifier, is same as that of class A push-pull amplifier except that the transistors are biased at cut off, instead of using the biasing resistors. The figure below gives the detailing of the construction of a push-pull class B power amplifier. The circuit operation of class B push pull amplifier is detailed below. Operation The circuit of class B push-pull amplifier shown in the above figure clears that both the transformers are center-tapped. When no signal is applied at the input, the transistors T1 and T2 are in cut off condition and hence no collector currents flow. As no current is drawn from VCC, no power is wasted. When input signal is given, it is applied to the input transformer Tr1 which splits the signal into two signals that are 180o out of phase with each other. These two signals are given to the two identical transistors T1 and T2. For the positive half cycle, the base of the transistor T1 becomes positive and collector current flows. At the same time, the transistor T2 has negative half cycle, which throws the transistor T2 into cutoff condition and hence no collector current flows. The waveform is produced as shown in the following figure. For the next half cycle, the transistor T1 gets into cut off condition and the transistor T2 gets into conduction, to contribute the output. Hence for both the cycles, each transistor conducts alternately. The output transformer Tr3 serves to join the two currents producing an almost undistorted output waveform. Power Efficiency of Class B Push-Pull Amplifier The current in each transistor is the average value of half sine loop. For half sine loop, Idc is given by $$I_{dc} = frac{(I_C)_{max}}{pi}$$ Therefore, $$(p_{in})_{dc} = 2 times left [ frac{(I_C)_{max}}{pi} times V_{CC} right ]$$ Here factor 2 is introduced as there are two transistors in push-pull amplifier. R.M.S. value of collector current = $(I_C)_{max}/ sqrt{2}$ R.M.S. value of output voltage = $V_{CC} / sqrt{2}$ Under ideal conditions of maximum power Therefore, $$(P_O)_{ac} = frac{(I_C)_{max}}{sqrt{2}} times frac{V_{CC}}{sqrt{2}} = frac{(I_C)_{max} times V_{CC}}{2}$$ Now overall maximum efficiency $$eta_{overall} = frac{(P_O)_{ac}}{(P_{in})_{dc}}$$ $$= frac{(I_C)_{max} times V_{CC}}{2} times frac{pi}{2 (I_C)_{max} times V_{CC}}$$ $$= frac{pi}{4} = 0.785 = 78.5%$$ The collector efficiency would be the same. Hence the class B push-pull amplifier improves the efficiency than the class A push-pull amplifier. Complementary Symmetry Push-Pull Class B Amplifier The push pull amplifier which was just discussed improves efficiency but the usage of center-tapped transformers makes the circuit bulky, heavy and costly. To make the circuit simple and to improve the efficiency, the transistors used can be complemented, as shown in the following circuit diagram. The above circuit employs a NPN transistor and a PNP transistor connected in push pull configuration. When the input signal is applied, during the positive half cycle of the input signal, the NPN transistor conducts and the PNP transistor cuts off. During the negative half cycle, the NPN transistor cuts off and the PNP transistor conducts. In this way, the NPN transistor amplifies during positive half cycle of the input, while PNP transistor amplifies during negative half cycle of the input. As the transistors are both complement to each other, yet act symmetrically while being connected in push pull configuration of class B, this circuit is termed as Complementary symmetry push pull class B amplifier. Advantages The advantages of Complementary symmetry push pull class B amplifier are as follows. As there is no need of center tapped transformers, the weight and cost are reduced. Equal and opposite input signal voltages are not required. Disadvantages The disadvantages of Complementary symmetry push pull class B amplifier are as follows. It is difficult to get a pair of transistors (NPN and PNP) that have similar characteristics. We require both positive and negative supply voltages. Learning working make money

Emitter Follower & Darlington Amplifier Emitter follower and darlington amplifier are the most common examples for feedback amplifiers. These are the mostly used ones with a number of applications. Emitter Follower Emitter follower circuit has a prominent place in feedback amplifiers. Emitter follower is a case of negative current feedback circuit. This is mostly used as a last stage amplifier in signal generator circuits. The important features of Emitter Follower are − It has high input impedance It has low output impedance It is ideal circuit for impedance matching All these ideal features allow many applications for the emitter follower circuit. This is a current amplifier circuit that has no voltage gain. Construction The constructional details of an emitter follower circuit are nearly similar to a normal amplifier. The main difference is that the load RL is absent at the collector terminal, but present at the emitter terminal of the circuit. Thus the output is taken from the emitter terminal instead of collector terminal. The biasing is provided either by base resistor method or by potential divider method. The following figure shows the circuit diagram of an Emitter Follower. Operation The input signal voltage applied between base and emitter, develops an output voltage Vo across RE, which is in the emitter section. Therefore, $$V_o = I_E R_E$$ The whole of this output current is applied to the input through feedback. Hence, $$V_f = V_o$$ As the output voltage developed across RL is proportional to the emitter current, this emitter follower circuit is a current feedback circuit. Hence, $$beta = frac{V_f}{V_o} = 1$$ It is also noted that the input signal voltage to the transistor (= Vi) is equal to the difference of Vs and Vo i.e., $$V_i = V_s – V_o$$ Hence the feedback is negative. Characteristics The major characteristics of the emitter follower are as follows − No voltage gain. In fact, the voltage gain is nearly 1. Relatively high current gain and power gain. High input impedance and low output impedance. Input and output ac voltages are in phase. Voltage Gain of Emitter Follower As the Emitter Follower circuit is a prominent one, let us try to get the equation for the voltage gain of an emitter follower circuit. Our Emitter Follower circuit looks as follows − If an AC equivalent circuit of the above circuit is drawn, it would look like the below one, as the emitter by pass capacitor is absent. The AC resistance rE of the emitter circuit is given by $$r_E = r’_E + R_E$$ Where $$r’_E = frac{25 mV}{I_E}$$ In order to find the voltage gain of the amplifier, the above figure can be replaced by the following figure. Note that input voltage is applied across the ac resistance of the emitter circuit i.e., (r’E + RE). Assuming the emitter diode to be ideal, the output voltage Vout will be $$V_{out} = i_e R_E$$ Input voltage Vin will be $$V_{in} = i_e(r’_e + R_E)$$ Therefore, the Voltage Gain of emitter follower is $$A_V = frac{V_{out}}{V_{in}} = frac{i_e R_E}{i_e(r’_e + R_E)} = frac{R_E}{(r’_e + R_E)}$$ Or $$A_V = frac{R_E}{(r’_e + R_E)}$$ In most practical applications, $$R_E gg r’_e$$ So, AV ≈ 1. In practice, the voltage gain of an emitter follower is between 0.8 and 0.999. Darlington Amplifier The emitter follower circuit which was just discussed lacks to meet the requirements of the circuit current gain (Ai) and the input impedance (Zi). In order to achieve some increase in the overall values of circuit current gain and input impedance, two transistors are connected as shown in the following circuit diagram, which is known as Darlington configuration. As shown in the above figure, the emitter of the first transistor is connected to the base of the second transistor. The collector terminals of both the transistors are connected together. Biasing Analysis Because of this type of connection, the emitter current of the first transistor will also be the base current of the second transistor. Therefore, the current gain of the pair is equal to the product of individual current gains i.e., $$beta = beta _1 beta _2$$ A high current gain is generally achieved with a minimum number of components. As two transistors are used here, two VBE drops are to be considered. The biasing analysis is otherwise similar for one transistor. Voltage across R2, $$V_2 = frac{V_CC}{R_1 + R_2} times R_2$$ Voltage across RE, $$V_E = V_2 – 2 V_{BE}$$ Current through RE, $$I_{E2} = frac{V_2 – 2 V_{BE}}{R_E}$$ Since the transistors are directly coupled, $$I_{E1} = I_{B2}$$ Now $$I_{B2} = frac{I_{E2}}{beta _2}$$ Therefore $$I_{E1} = frac{I_{E2}}{beta _2}$$ Which means $$I_{E1} = I_{E1} beta _2$$ We have $I_{E1} = beta _1 I_{B1}$ since $I_{E1} cong I_{C1}$ Hence, as $$I_{E2} = I_{E1} beta _2$$ We can write $$I_{E2} = beta _1 beta _2 I_{B1}$$ Therefore, Current Gain can be given as $$beta = frac{I_{E2}}{I_{B1}} = frac{beta _1 beta _2 I_{B1}}{I_{B1}} = beta _1 beta_2$$ Input impedance of the darling ton amplifier is $Z_{in} = beta_1 beta_2 R_E …..$ neglecting r’e In practice, these two transistors are placed in a single transistor housing and the three terminals are taken out of the housing as shown in the following figure. This three terminal device can be called as Darling ton transistor. The darling ton transistor acts like a single transistor that has high current gain and high input impedance. Characteristics The following are the important characteristics of Darling ton amplifier. Extremely high input impedance (MΩ). Extremely high current gain (several thousands). Extremely low output impedance (a few Ω). Since the characteristics of the Darling ton amplifier are basically the same as those of the emitter follower, the two circuits are used for similar applications. Till now we have discussed amplifiers based on positive feedback. The negative feedback in transistor circuits is helpful in the working of oscillators. The topic of oscillators is entirely covered in Oscillators tutorial. Learning working make money

Classification of Power Amplifiers The Power amplifiers amplify the power level of the signal. This amplification is done in the last stage in audio applications. The applications related to radio frequencies employ radio power amplifiers. But the operating point of a transistor, plays a very important role in determining the efficiency of the amplifier. The main classification is done based on this mode of operation. The classification is done based on their frequencies and also based on their mode of operation. Classification Based on Frequencies Power amplifiers are divided into two categories, based on the frequencies they handle. They are as follows. Audio Power Amplifiers − The audio power amplifiers raise the power level of signals that have audio frequency range (20 Hz to 20 KHz). They are also known as Small signal power amplifiers. Radio Power Amplifiers − Radio Power Amplifiers or tuned power amplifiers raise the power level of signals that have radio frequency range (3 KHz to 300 GHz). They are also known as large signal power amplifiers. Classification Based on Mode of Operation On the basis of the mode of operation, i.e., the portion of the input cycle during which collector current flows, the power amplifiers may be classified as follows. Class A Power amplifier − When the collector current flows at all times during the full cycle of signal, the power amplifier is known as class A power amplifier. Class B Power amplifier − When the collector current flows only during the positive half cycle of the input signal, the power amplifier is known as class B power amplifier. Class C Power amplifier − When the collector current flows for less than half cycle of the input signal, the power amplifier is known as class C power amplifier. There forms another amplifier called Class AB amplifier, if we combine the class A and class B amplifiers so as to utilize the advantages of both. Before going into the details of these amplifiers, let us have a look at the important terms that have to be considered to determine the efficiency of an amplifier. Terms Considering Performance The primary objective of a power amplifier is to obtain maximum output power. In order to achieve this, the important factors to be considered are collector efficiency, power dissipation capability and distortion. Let us go through them in detail. Collector Efficiency This explains how well an amplifier converts DC power to AC power. When the DC supply is given by the battery but no AC signal input is given, the collector output at such a condition is observed as collector efficiency. The collector efficiency is defined as $$eta = frac{average: a.c : power : output}{average : d.c : power: input: to : transistor}$$ For example, if the battery supplies 15W and AC output power is 3W. Then the transistor efficiency will be 20%. The main aim of a power amplifier is to obtain maximum collector efficiency. Hence the higher the value of collector efficiency, the efficient the amplifier will be. Power Dissipation Capacity Every transistor gets heated up during its operation. As a power transistor handles large currents, it gets more heated up. This heat increases the temperature of the transistor, which alters the operating point of the transistor. So, in order to maintain the operating point stability, the temperature of the transistor has to be kept in permissible limits. For this, the heat produced has to be dissipated. Such a capacity is called as Power dissipation capability. Power dissipation capability can be defined as the ability of a power transistor to dissipate the heat developed in it. Metal cases called heat sinks are used in order to dissipate the heat produced in power transistors. Distortion A transistor is a non-linear device. When compared with the input, there occur few variations in the output. In voltage amplifiers, this problem is not pre-dominant as small currents are used. But in power amplifiers, as large currents are in use, the problem of distortion certainly arises. Distortion is defined as the change of output wave shape from the input wave shape of the amplifier. An amplifier that has lesser distortion, produces a better output and hence considered efficient. Learning working make money

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

Power Amplifiers In practice, any amplifier consists of few stages of amplification. If we consider audio amplification, it has several stages of amplification, depending upon our requirement. Power Amplifier After the audio signal is converted into electrical signal, it has several voltage amplifications done, after which the power amplification of the amplified signal is done just before the loud speaker stage. This is clearly shown in the below figure. While the voltage amplifier raises the voltage level of the signal, the power amplifier raises the power level of the signal. Besides raising the power level, it can also be said that a power amplifier is a device which converts DC power to AC power and whose action is controlled by the input signal. The DC power is distributed according to the relation, DC power input = AC power output + losses Power Transistor For such Power amplification, a normal transistor would not do. A transistor that is manufactured to suit the purpose of power amplification is called as a Power transistor. A Power transistor differs from the other transistors, in the following factors. It is larger in size, in order to handle large powers. The collector region of the transistor is made large and a heat sink is placed at the collector-base junction in order to minimize heat generated. The emitter and base regions of a power transistor are heavily doped. Due to the low input resistance, it requires low input power. Hence there is a lot of difference in voltage amplification and power amplification. So, let us now try to get into the details to understand the differences between a voltage amplifier and a power amplifier. Difference between Voltage and Power Amplifiers Let us try to differentiate between voltage and power amplifier. Voltage Amplifier The function of a voltage amplifier is to raise the voltage level of the signal. A voltage amplifier is designed to achieve maximum voltage amplification. The voltage gain of an amplifier is given by $$A_v = beta left (frac{R_c}{R_{in}} right )$$ The characteristics of a voltage amplifier are as follows − The base of the transistor should be thin and hence the value of β should be greater than 100. The resistance of the input resistor Rin should be low when compared to collector load RC. The collector load RC should be relatively high. To permit high collector load, the voltage amplifiers are always operated at low collector current. The voltage amplifiers are used for small signal voltages. Power Amplifier The function of a power amplifier is to raise the power level of input signal. It is required to deliver a large amount of power and has to handle large current. The characteristics of a power amplifier are as follows − The base of transistor is made thicken to handle large currents. The value of β being (β > 100) high. The size of the transistor is made larger, in order to dissipate more heat, which is produced during transistor operation. Transformer coupling is used for impedance matching. Collector resistance is made low. The comparison between voltage and power amplifiers is given below in a tabular form. S.No Particular Voltage Amplifier Power Amplifier 1 β High (>100) Low (5 to 20) 2 RC High (4-10 KΩ) Low (5 to 20 Ω) 3 Coupling Usually R-C coupling Invariably transformer coupling 4 Input voltage Low (a few m V) High (2-4 V) 5 Collector current Low (≈ 1 mA) High (> 100 mA) 6 Power output Low High 7 Output impendence High (≈ 12 K Ω) Low (200 Ω) Learning working make money

Types of Tuned Amplifiers There are two main types of tuned amplifiers. They are − Single tuned amplifier Double tuned amplifier Single Tuned Amplifier An amplifier circuit with a single tuner section being at the collector of the amplifier circuit is called as Single tuner amplifier circuit. Construction A simple transistor amplifier circuit consisting of a parallel tuned circuit in its collector load, makes a single tuned amplifier circuit. The values of capacitance and inductance of the tuned circuit are selected such that its resonant frequency is equal to the frequency to be amplified. The following circuit diagram shows a single tuned amplifier circuit. The output can be obtained from the coupling capacitor CC as shown above or from a secondary winding placed at L. Operation The high frequency signal that has to be amplified is applied at the input of the amplifier. The resonant frequency of the parallel tuned circuit is made equal to the frequency of the signal applied by altering the capacitance value of the capacitor C, in the tuned circuit. At this stage, the tuned circuit offers high impedance to the signal frequency, which helps to offer high output across the tuned circuit. As high impedance is offered only for the tuned frequency, all the other frequencies which get lower impedance are rejected by the tuned circuit. Hence the tuned amplifier selects and amplifies the desired frequency signal. Frequency Response The parallel resonance occurs at resonant frequency fr when the circuit has a high Q. the resonant frequency fr is given by $$f_r = frac{1}{2 pi sqrt{LC}}$$ The following graph shows the frequency response of a single tuned amplifier circuit. At resonant frequency fr the impedance of parallel tuned circuit is very high and is purely resistive. The voltage across RL is therefore maximum, when the circuit is tuned to resonant frequency. Hence the voltage gain is maximum at resonant frequency and drops off above and below it. The higher the Q, the narrower will the curve be. Double Tuned Amplifier An amplifier circuit with a double tuner section being at the collector of the amplifier circuit is called as Double tuner amplifier circuit. Construction The construction of double tuned amplifier is understood by having a look at the following figure. This circuit consists of two tuned circuits L1C1 and L2C2 in the collector section of the amplifier. The signal at the output of the tuned circuit L1C1 is coupled to the other tuned circuit L2C2 through mutual coupling method. The remaining circuit details are same as in the single tuned amplifier circuit, as shown in the following circuit diagram. Operation The high frequency signal which has to be amplified is given to the input of the amplifier. The tuning circuit L1C1 is tuned to the input signal frequency. At this condition, the tuned circuit offers high reactance to the signal frequency. Consequently, large output appears at the output of the tuned circuit L1C1 which is then coupled to the other tuned circuit L2C2 through mutual induction. These double tuned circuits are extensively used for coupling various circuits of radio and television receivers. Frequency Response of Double Tuned Amplifier The double tuned amplifier has the special feature of coupling which is important in determining the frequency response of the amplifier. The amount of mutual inductance between the two tuned circuits states the degree of coupling, which determines the frequency response of the circuit. In order to have an idea on the mutual inductance property, let us go through the basic principle. Mutual Inductance As the current carrying coil produces some magnetic field around it, if another coil is brought near this coil, such that it is in the magnetic flux region of the primary, then the varying magnetic flux induces an EMF in the second coil. If this first coil is called as Primary coil, the second one can be called as a Secondary coil. When the EMF is induced in the secondary coil due to the varying magnetic field of the primary coil, then such phenomenon is called as the Mutual Inductance. The figure below gives an idea about this. The current is in the figure indicate the source current while iind indicates the induced current. The flux represents the magnetic flux created around the coil. This spreads to the secondary coil also. With the application of voltage, the current is flows and flux gets created. When the current is varies the flux gets varied, producing iind in the secondary coil, due to the Mutual inductance property. Coupling Under the concept of mutual inductance coupling will be as shown in the figure below. When the coils are spaced apart, the flux linkages of primary coil L1 will not link the secondary coil L2. At this condition, the coils are said to have Loose coupling. The resistance reflected from the secondary coil at this condition is small and the resonance curve will be sharp and the circuit Q is high as shown in the figure below. On the contrary, when the primary and secondary coils are brought close together, they have Tight coupling. Under such conditions, the reflected resistance will be large and the circuit Q is lower. Two positions of gain maxima, one above and the other below the resonant frequency are obtained. Bandwidth of Double Tuned Circuit The above figure clearly states that the bandwidth increases with the degree of coupling. The determining factor in a double tuned circuit is not Q but the coupling. We understood that, for a given frequency, the tighter the coupling the greater the bandwidth will be. The equation for bandwidth is given as $$BW_{dt} = k f_r$$ Where BWdt = bandwidth for double tuned circuit, K = coefficient of coupling, and fr = resonant frequency. We hope that now you have gained sufficient knowledge regarding the functioning of tuned amplifiers. In the next chapter, we will learn about feedback amplifiers. Learning working make money