Category: Amplifiers

Amplifiers Feedback An amplifier circuit simply increases the signal strength. But while amplifying, it just increases the strength of its input signal whether it contains information or some noise along with information. This noise or some disturbance is introduced in the amplifiers because of their strong tendency to introduce hum due to sudden temperature changes or stray electric and magnetic fields. Therefore, every high gain amplifier tends to give noise along with signal in its output, which is very undesirable. The noise level in the amplifier circuits can be considerably reduced by using negative feedback done by injecting a fraction of output in phase opposition to the input signal. Principle of Feedback Amplifier A feedback amplifier generally consists of two parts. They are the amplifier and the feedback circuit. The feedback circuit usually consists of resistors. The concept of feedback amplifier can be understood from the following figure. From the above figure, the gain of the amplifier is represented as A. the gain of the amplifier is the ratio of output voltage Vo to the input voltage Vi. the feedback network extracts a voltage Vf = β Vo from the output Vo of the amplifier. This voltage is added for positive feedback and subtracted for negative feedback, from the signal voltage Vs. Now, $$V_i = V_s + V_f = V_s + beta V_o$$ $$V_i = V_s – V_f = V_s – beta V_o$$ The quantity β = Vf/Vo is called as feedback ratio or feedback fraction. Let us consider the case of negative feedback. The output Vo must be equal to the input voltage (Vs – βVo) multiplied by the gain A of the amplifier. Hence, $$(V_s – beta V_o)A = V_o$$ Or $$A V_s – A beta V_o = V_o$$ Or $$A V_s = V_o (1 + A beta)$$ Therefore, $$frac{V_o}{V_s} = frac{A}{1 + A beta}$$ Let Af be the overall gain (gain with the feedback) of the amplifier. This is defined as the ratio of output voltage Vo to the applied signal voltage Vs, i.e., $$A_f = frac{Output : voltage}{Input : signal : voltage} = frac{V_o}{V_s}$$ So, from the above two equations, we can understand that, The equation of gain of the feedback amplifier, with negative feedback is given by $$A_f = frac{A}{1 + A beta}$$ The equation of gain of the feedback amplifier, with positive feedback is given by $$A_f = frac{A}{1 – A beta}$$ These are the standard equations to calculate the gain of feedback amplifiers. Types of Feedbacks The process of injecting a fraction of output energy of some device back to the input is known as Feedback. It has been found that feedback is very useful in reducing noise and making the amplifier operation stable. Depending upon whether the feedback signal aids or opposes the input signal, there are two types of feedbacks used. Positive Feedback The feedback in which the feedback energy i.e., either voltage or current is in phase with the input signal and thus aids it is called as Positive feedback. Both the input signal and feedback signal introduces a phase shift of 180o thus making a 360o resultant phase shift around the loop, to be finally in phase with the input signal. Though the positive feedback increases the gain of the amplifier, it has the disadvantages such as Increasing distortion Instability It is because of these disadvantages the positive feedback is not recommended for the amplifiers. If the positive feedback is sufficiently large, it leads to oscillations, by which oscillator circuits are formed. This concept will be discussed in OSCILLATORS tutorial. Negative Feedback The feedback in which the feedback energy i.e., either voltage or current is out of phase with the input and thus opposes it, is called as negative feedback. In negative feedback, the amplifier introduces a phase shift of 180o into the circuit while the feedback network is so designed that it produces no phase shift or zero phase shift. Thus the resultant feedback voltage Vf is 180o out of phase with the input signal Vin. Though the gain of negative feedback amplifier is reduced, there are many advantages of negative feedback such as Stability of gain is improved Reduction in distortion Reduction in noise Increase in input impedance Decrease in output impedance Increase in the range of uniform application It is because of these advantages negative feedback is frequently employed in amplifiers. Learning working make money

Noise in Amplifier An Amplifier, while amplifying just increases the strength of its input signal whether it contains information or some noise along with information. This noise or some disturbance is introduced in the amplifiers because of their strong tendency to introduce hum due to sudden temperature changes or stray electric and magnetic fields. The performance of an amplifier mainly depends on this Noise. Noise is an unwanted signal that creates disturbance to the desired signal content in the system. This can be an additional signal that is produced within the system or can be some disturbance accompanied with the desired information of the input signal. However, it is unwanted and has to be removed. A good system is one in which the noise generated by the amplifier itself is small compared to noise from the incoming source. Noise Noise is an unwanted signal which interferes with the original message signal and corrupts the parameters of the message signal. This alteration in the communication process, makes the message to get altered after reaching. It is most likely to be entered at the channel or the receiver. The following graph shows the characteristics of a noise signal. Hence, it is understood that noise is some signal which has no pattern and no constant frequency or amplitude. It is quite random and unpredictable. Measures are usually taken to reduce it, though it can’t be completely eliminated. Most common examples of noise are − “Hiss” sound in radio receivers “Buzz” sound amidst of telephone conversations “Flicker” in television receivers etc. Effects of Noise Noise is an inconvenient feature which affects the system performance. The effects of noise include − Noise limits the operating range of the systems − Noise indirectly places a limit on the weakest signal that can be amplified by an amplifier. The oscillator in the mixer circuit may limit its frequency because of noise. A system’s operation depends on the operation of its circuits. Noise limits the smallest signal that a receiver is capable of processing. Noise affects the sensitivity of receivers − Sensitivity is the minimum amount of input signal necessary to obtain the specified quality output. Noise effects the sensitivity of a receiver system, which eventually effects the output. Signal to Noise Ratio When a signal is received and it has to be amplified, first the signal is filtered out to remove any unwanted noise if available. The ratio of the information signal present in the received signal to the noise present is called as Signal to Noise ratio. This ratio has to be higher for a system so that it produces pure information signal unaffected by the unwanted noise. The SNR can be understood as $$SNR = frac{P_{signal}}{P_{noise}}$$ SNR is expressed in logarithmic basis using decibels. $$SNR_{db} = 10 log_{10}left (frac{P_{signal}}{P_{noise}} right )$$ Signal-to-noise ratio is the ratio of the signal power to the noise power. The higher the value of SNR, the greater will be the quality of the received output. Types of Noise The classification of noise is done depending up on the type of source, the effect it shows or the relation it has with the receiver etc. There are two main ways of which noise gets produced. One is through some external source while the other is created by the internal source, within the receiver section. External Source This noise is produced by the external sources which may occur in the medium or channel of communication, usually. This noise can’t be completely eliminated. The best way is to avoid the noise from affecting the signal. Most common examples of this type of noise are − Atmospheric Noise (due to irregularities in atmosphere) Extra-terrestrial noise such as solar noise and cosmic noise Industrial noise Internal Source This noise is produced by the receiver components while functioning. The components in the circuits, due to continuous functioning, may produce few types of noise. This noise is quantifiable. A proper receiver design may lower the effect of this internal noise. Most common examples of this type of noise are − Thermal agitation noise (Johnson noise or Electrical noise) Shot noise (due to random movement of electrons and holes Transit-time noise (during transition) Miscellaneous noise is another type of noise which includes flicker, resistance effect and mixer generated noise, etc. Finally, this gives an overall idea on how a noise will be and how it can affect the amplifier, though present in transmitter or receiver section. The amplifiers that amplify low signals and hence amplify noise in a low level can be called as Low-noise amplifiers. All the types of amplifiers discussed are more or less subjected to noise in some way or the other. The performance of an amplifier determines its efficiency to deal with the unwanted factors. Learning working make money

Discuss Amplifiers 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. Learning working make money

Amplifiers – Quick Guide 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

Amplifiers – Useful Resources The following resources contain additional information on Amplifiers. Please use them to get more in-depth knowledge on this. Useful Links on Amplifiers − Wikipedia Reference for Amplifiers Useful Books on Amplifiers To enlist your site on this page, please drop an email to [email protected] Learning working make money

Amplifiers Negative Feedback Negative feedback in an amplifier is the method of feeding a portion of the amplified output to the input but in opposite phase. The phase opposition occurs as the amplifier provides 180o phase shift whereas the feedback network doesn’t. While the output energy is being applied to the input, for the voltage energy to be taken as feedback, the output is taken in shunt connection and for the current energy to be taken as feedback, the output is taken in series connection. There are two main types of negative feedback circuits. They are − Negative Voltage Feedback Negative Current Feedback Negative Voltage Feedback In this method, the voltage feedback to the input of amplifier is proportional to the output voltage. This is further classified into two types − Voltage-series feedback Voltage-shunt feedback Negative Current Feedback In this method, the voltage feedback to the input of amplifier is proportional to the output current. This is further classified into two types. Current-series feedback Current-shunt feedback Let us have a brief idea on all of them. Voltage-Series Feedback In the voltage series feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as shunt-driven series-fed feedback, i.e., a parallel-series circuit. The following figure shows the block diagram of voltage series feedback, by which it is evident that the feedback circuit is placed in shunt with the output but in series with the input. As the feedback circuit is connected in shunt with the output, the output impedance is decreased and due to the series connection with the input, the input impedance is increased. Voltage-Shunt Feedback In the voltage shunt feedback circuit, a fraction of the output voltage is applied in parallel with the input voltage through the feedback network. This is also known as shunt-driven shunt-fed feedback i.e., a parallel-parallel proto type. The below figure shows the block diagram of voltage shunt feedback, by which it is evident that the feedback circuit is placed in shunt with the output and also with the input. As the feedback circuit is connected in shunt with the output and the input as well, both the output impedance and the input impedance are decreased. Current-Series Feedback In the current series feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as series-driven series-fed feedback i.e., a series-series circuit. The following figure shows the block diagram of current series feedback, by which it is evident that the feedback circuit is placed in series with the output and also with the input. As the feedback circuit is connected in series with the output and the input as well, both the output impedance and the input impedance are increased. Current-Shunt Feedback In the current shunt feedback circuit, a fraction of the output voltage is applied in series with the input voltage through the feedback circuit. This is also known as series-driven shunt-fed feedback i.e., a series-parallel circuit. The below figure shows the block diagram of current shunt feedback, by which it is evident that the feedback circuit is placed in series with the output but in parallel with the input. As the feedback circuit is connected in series with the output, the output impedance is increased and due to the parallel connection with the input, the input impedance is decreased. Let us now tabulate the amplifier characteristics that get affected by different types of negative feedbacks. Characteristics Types of Feedback Voltage-Series Voltage-Shunt Current-Series Current-Shunt Voltage Gain Decreases Decreases Decreases Decreases Bandwidth Increases Increases Increases Increases Input resistance Increases Decreases Increases Decreases Output resistance Decreases Decreases Increases Increases Harmonic distortion Decreases Decreases Decreases Decreases Noise Decreases Decreases Decreases Decreases Learning working make money

Class AB and Class C Power Amplifiers The class A and class B amplifier so far discussed has got few limitations. Let us now try to combine these two to get a new circuit which would have all the advantages of both class A and class B amplifier without their inefficiencies. Before that, let us also go through another important problem, called as Cross over distortion, the output of class B encounters with. Cross-over Distortion In the push-pull configuration, the two identical transistors get into conduction, one after the other and the output produced will be the combination of both. When the signal changes or crosses over from one transistor to the other at the zero voltage point, it produces an amount of distortion to the output wave shape. For a transistor in order to conduct, the base emitter junction should cross 0.7v, the cut off voltage. The time taken for a transistor to get ON from OFF or to get OFF from ON state is called the transition period. At the zero voltage point, the transition period of switching over the transistors from one to the other, has its effect which leads to the instances where both the transistors are OFF at a time. Such instances can be called as Flat spot or Dead band on the output wave shape. The above figure clearly shows the cross over distortion which is prominent in the output waveform. This is the main disadvantage. This cross over distortion effect also reduces the overall peak to peak value of the output waveform which in turn reduces the maximum power output. This can be more clearly understood through the non-linear characteristic of the waveform as shown below. It is understood that this cross-over distortion is less pronounced for large input signals, where as it causes severe disturbance for small input signals. This cross over distortion can be eliminated if the conduction of the amplifier is more than one half cycle, so that both the transistors won’t be OFF at the same time. This idea leads to the invention of class AB amplifier, which is the combination of both class A and class B amplifiers, as discussed below. Class AB Power Amplifier As the name implies, class AB is a combination of class A and class B type of amplifiers. As class A has the problem of low efficiency and class B has distortion problem, this class AB is emerged to eliminate these two problems, by utilizing the advantages of both the classes. The cross over distortion is the problem that occurs when both the transistors are OFF at the same instant, during the transition period. In order to eliminate this, the condition has to be chosen for more than one half cycle. Hence, the other transistor gets into conduction, before the operating transistor switches to cut off state. This is achieved only by using class AB configuration, as shown in the following circuit diagram. Therefore, in class AB amplifier design, each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A. The conduction angle of class AB amplifier is somewhere between 180o to 360o depending upon the operating point selected. This is understood with the help of below figure. The small bias voltage given using diodes D1 and D2, as shown in the above figure, helps the operating point to be above the cutoff point. Hence the output waveform of class AB results as seen in the above figure. The crossover distortion created by class B is overcome by this class AB, as well the inefficiencies of class A and B don’t affect the circuit. So, the class AB is a good compromise between class A and class B in terms of efficiency and linearity having the efficiency reaching about 50% to 60%. The class A, B and AB amplifiers are called as linear amplifiers because the output signal amplitude and phase are linearly related to the input signal amplitude and phase. 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. The efficiency of class C amplifier is high while linearity is poor. The conduction angle for class C is less than 180o. It is generally around 90o, which means the transistor remains idle for more than half of the input signal. So, the output current will be delivered for less time compared to the application of input signal. The following figure shows the operating point and output of a class C amplifier. This kind of biasing gives a much improved efficiency of around 80% to the amplifier, but introduces heavy distortion in the output signal. Using the class C amplifier, the pulses produced at its output can be converted to complete sine wave of a particular frequency by using LC circuits in its collector circuit. Learning working make money

Push-Pull Class A Power Amplifier So far, we have seen two types of class A power amplifiers. The main problems that should be dealt with are low power output and efficiency. It is possible to obtain greater power output and efficiency than that of the Class A amplifier by using a combinational transistor pair called as Push-Pull configuration. In this circuit, we use two complementary transistors in the output stage with one transistor being an NPN or N-channel type while the other transistor is a PNP or P-channel (the complement) type connected in order to operate them like PUSH a transistor to ON and PULL another transistor to OFF at the same time. This push-pull configuration can be made in class A, class B, class C or class AB amplifiers. Construction of Push-Pull Class A Power Amplifier The construction of the class A power amplifier circuit in push-pull configuration is shown as in the figure below. This arrangement mainly reduces the harmonic distortion introduced by the non-linearity of the transfer characteristics of a single transistor amplifier. In Push-pull arrangement, the two identical transistors T1 and T2 have their emitter terminals shorted. The input signal is applied to the transistors through the transformer Tr1 which provides opposite polarity signals to both the transistor bases. The collectors of both the transistors are connected to the primary of output transformer Tr2. Both the transformers are center tapped. The VCC supply is provided to the collectors of both the transistors through the primary of the output transformer. The resistors R1 and R2 provide the biasing arrangement. The load is generally a loudspeaker which is connected across the secondary of the output transformer. The turns ratio of the output transformer is chosen in such a way that the load is well matched with the output impedance of the transistor. So maximum power is delivered to the load by the amplifier. Circuit Operation The output is collected from the output transformer Tr2. The primary of this transformer Tr2 has practically no dc component through it. The transistors T1 and T2 have their collectors connected to the primary of transformer Tr2 so that their currents are equal in magnitude and flow in opposite directions through the primary of transformer Tr2. When the a.c. input signal is applied, the base of transistor T1 is more positive while the base of transistor T2 is less positive. Hence the collector current ic1 of transistor T1 increases while the collector current ic2 of transistor T2 decreases. These currents flow in opposite directions in two halves of the primary of output transformer. Moreover, the flux produced by these currents will also be in opposite directions. Hence, the voltage across the load will be induced voltage whose magnitude will be proportional to the difference of collector currents i.e. $$(i_{c1} – i_{c2})$$ Similarly, for the negative input signal, the collector current ic2 will be more than ic1. In this case, the voltage developed across the load will again be due to the difference $$(i_{c1} – i_{c2})$$ As $i_{c2} > i_{c1}$ The polarity of voltage induced across load will be reversed. $$i_{c1} – i_{c2} = i_{c1} + (-i_{c2})$$ To have a better understanding, let us consider the below figure. The overall operation results in an a.c. voltage induced in the secondary of output transformer and hence a.c. power is delivered to that load. It is understood that, during any given half cycle of input signal, one transistor is being driven (or pushed) deep into conduction while the other being non-conducting (pulled out). Hence the name Push-pull amplifier. The harmonic distortion in Push-pull amplifier is minimized such that all the even harmonics are eliminated. Advantages The advantages of class A Push-pull amplifier are as follows High a.c. output is obtained. The output is free from even harmonics. The effect of ripple voltages are balanced out. These are present in the power supply due to inadequate filtering. Disadvantages The disadvantages of class A Push-pull amplifier are as follows The transistors are to be identical, to produce equal amplification. Center-tapping is required for the transformers. The transformers are bulky and costly. 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