A pushpull amplifier circuit diagram is shown above. In figure 1, a current source i s having shunt source resistance r s drives the amplifier through a transformer tr 1. A multistage amplifier circuit that is coupled to the next stage directly, can be called as a direct coupled amplifier. The total power of the three power amplifiers should some up to a total of 100w each driving a resistive load of 8 ohms. We present a mathematical analysis of the commonmode instability and power backoff feature of a transformercoupled class ab differential power amplifier pa. Transformer coupled class a amps are twice as efficient as their capacitive coupled counterparts in theory. Class b amplifier the inefficiency of class a amplifiers is largely due to the transistor bias conditions.
Benha university faculty of engineering at shoubra l. Transformer coupled popular in power amplifiers to minimize power wastage. The aboveshown circuit is a directly coupled class a amplifier. The coupling transformer provides good impedance matching between the output and load and. Since the circuit uses a transformer to step voltage or. Plate current in a class ab, b, or c amplifier does not flow for the complete period. Generally, in class c amplifier, load is a tuned circuit, which selects the fundamental or the desired harmonic rejecting all other frequency components. We will refer to this configuration as a seriesfed class a amplifier, and we will consider capacitor and transformer coupled loads in a later discussion.
Quite often lc coupled and transformer coupled amplifiers cannot be distinguished as a transformer is some kind of inductor. The class c amplifier conducts for less than half a cycle and has a theoretical. In class a amplifier, the load can be either connected directly in the collector or it can be transformer coupled. Untuned large signal amplifier power amplifier of class ab and b type invariably use transformer coupled stages while those of class a type also generally use transformer coupled stage. It may be noted that maximum power is dissipated in the transistor in the zero signal. They use low distortion class ab, are not a class a distorted heater. Class b power amplifier if the collector current flows during the half cycle of the signal only, the power amplifier is known as class b amplifier. The class ab amplifier circuit is the combination of both class a and class b amplifier.
It means that the maximum 50% dc supply power is converted into ac power. As the output swings positive or negative, the voltage swing is coupled via cb. Power amplifier classes are, in electronics, letter symbols applied to different power amplifier. Using transformer coupling the efficiency of the amplifier can be improved to a great extend. Maximum collector efficiency of transformer coupled class a. Rc coupled stages are not used because the quiescent current flawing through the load resistor results in large wastage of dc power in it. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. Transformer coupled class b amplifier crossover distortion. Transformercoupled power amplifier stability and power. Transformer coupled audio power amplifier engineering projects. Understanding amplifier operating classes electronic. Transistor audio power amplifiers questions and answers pdf free download mcqs interview objective type questions for eee ece electronics engineer students skip to content engineering interview questions,mcqs,objective questions, class notes,seminor topics,lab viva pdf free download. So they were only used if the gain was specified down to dc e. Push pull amplifier circuit, operation, advantages and.
The input stage is normally a cathode coupled amplifier. The latter method is often preferred for two main reasons. This gives the class a maximum efficiency of between 50% and 78. In general, where amount of power involved is 1w or more, the amplifier is termed as power amplifier. A threestage, transformercoupled class ab power amplifier pa and a super sourcefollowerbased iq upconversion mixer are implemented in 40nm lpcmos technology. By adding the diode, transistors are biased in slightly conducting state even when no signal is present at base terminal, thus removing the crossover distortion problem. The output of a class c amplifier is biased for operation at less than 180 of the cycle and will operate only with a tuned resonant circuit, which provides a full. The output conducts between 180 and360 of the ac input signal. We will refer to this configuration as a seriesfed class a amplifier, and we will consider capacitor and transformer coupled loads. The transformer is subject to various power losses. Transformer coupled power amplifier the other class a power amplifier configuration were going to be considering is the transformer coupled circuit shown to the right a modified version of figure 8.
Transformer coupled class a amplifiers the offset volt. A form of class a amplifier having a maximum efficiency of 50% uses a transformer to couple the output signal to the load as shown in the below figure. My 1971 philips colour tv used a class a transistor amplifier and an output transformer. The image of the conduction angle derives from amplifying a sinusoidal signal. Mar 16, 2019 multiple choice questions and answers on transistor audio power amplifiers. Class a the amplifier conducts through the full 360 of the input. Dec 02, 2019 mcq in introductiondefinitions and amplifier types. Electronics tutorial about the class ab amplifier circuit that is forward biased to. A basic class b amplifier consists of a pair of transistors, driven in antiphase but driving a common output load, as shown in figure 3a. However, in a transformer, though there is magnification of input voltage or current, the. Class ab amplifiers are the most popular choice for broadband amplifiers. The main objective of this project was to design a 100w power amplifier with feedbackpair complementary symmetry output power transistors. Chitralekha mahanta, department of electronics and communication engineering,iit guwahati. The amplifier conducts somewhere between 180 and 360.
The demand for greater output power led to paralleled triodes and triodes in pushpull configurations with balanced output transformers. In order to realize the function of amplification, the transformer may appear to be a potential device. The class b amplifier circuit is biased in such a way that each transistor will work on one half cycle. Transformer coupled class a power amplifier the class a power amplifier as discussed in the previous chapter, is the circuit in which the output current flows. We saw how a transformer coupled, singleended amplifier could attach to an impedancemultiplier circuit imc, which would augment the amplifiers output wattage by threefold and still reflect the desired load impedance to the single 300b. Maximum collector efficiency of transformer coupled class. Multiple choice questions and answers on transistor audio. The pushpull action is best illustrated by drawing the ac. You can also move it down into class ab, which increas4es power output slightly at the expense of higher distortion. Transformer coupled ce amplifier engineering projects.
Notice that resistance r is considered to be the load. To design three class ab power amplifiers with feedbackpair complimentary symmetry output driven by an active 3way crossover network and a pre amplifier. An amplifier may be biased at a dc level above the zerobasecurrent level of class b and above onehalf the supply voltage level of class a. Class ab amplifier output stage combines the advantages of the class a amplifier and the class b amplifier producing a better amplifier design the purpose of any amplifier is to produce an output which follows the characteristics of the input signal but is sufficiently large enough to supply the needs of the load connected to it. Jan 19, 2012 class ab amplifierthis amplifier is a compromise between theclass a and class b amplifierthe qpointis above that of the class b but below theclass a.
General amp use hammond puts a century of experience into manufacturing high quality. In addition to reading the questions and answers on my site, i would suggest you to check the following, on amazon, as well. The maximum power dissipation in the transistor occurs under zero signal condition. The limitations due to the efficiency of class a power amplifiers. Benha university faculty of engineering at shoubra december 2014. Class b amplifier is the actual pushpull amplifier. Power amplifier circuits output stages are classified as a, b, ab and c for analog designsand class d and e for switching designs. In the classb circuit, the two output transistors are said to be operating in push pull. The efficiency of class a amplifier is less than 50% due to power losses in the output transformer. So, the next type of class a amplifier is transformer coupled class a amplifier. Both the transistors are operated in class b operation i.
You can bias it at higher current, but it doesnt make a big difference. This is a simple circuit form to use in presenting a few basic concepts. On this channel you can get education and knowledge for general issues and topics. The correct load resistance, which will enable the amplifier to deliver its rated power, must be presented to its output. An amplifier where the load is coupled to the output using a transformer is called a transformer coupled amplifier. Circuit diagram and working of push pull amplifier. Transformer coupled class a power amplifier the class a power amplifier as discussed in the previous chapter, is the circuit in which the output current flows for the entire cycle of the ac input supply. Load resistance r l is fed from the collector output circuit through a transformer tr 2. Design and compensation of high performance class ab. Introduction a couple of years ago i met per lundahl of the swedish company lundahl transformers and his father at an exhibition in connection with an audio engineering society convention, and i tried.
To improve the full power efficiency of the class a amplifier it is possible to design the circuit with a transformer connected directly in the collector circuit to form a circuit called a transformer coupled amplifier. The efficient impedance matching of the transformer combiner and efficiency improvement at power backoff, a major benefit of this structure, are illustrated. Here we will be discussing transformer coupled class a amplifier. Due to power loss in the primary of the transformer, the collector efficiency of class a amplifier is always less than 50%. An understanding of these special techniques is necessary for the. Rc coupled stages are popular in audio frequencies. The classes a, b, ab and c refer to the way the amplifiers are biased, although class c is mainly used in oscillator circuits. The above expression shows that in class a amplifier, maximum 50% of the dc power supplied can be converted into ac power output. The class ab amplifier cores presented here improve the amplifier s power supply noise insensitivity at high frequencies and increase bandwidth efficiency when compared to the commonly used twostage miller compensated amplifier, enabling the construction of better buffers and more powerefficient and reliable low voltage mixed signal chips. Operational amplifiers incorporate circuit configurations that may be relatively unfamiliar to the circuit designer with a background in other areas.
Figure 1 gives the basic circuit of a transformer coupled ce amplifier stage. May, 2018 impedance matching transformer coupled audio power amplifier. Class a power amplifier theory, efficiency equation. Oct 18, 2011 transformercoupled class a power amplifier this is also sometimes referred to as single ended power amplifier. Maximum collector efficiency of transformer coupled class a power amplifier. The term single ended denoting only one transistor is used to distinguish it from the pushpull amplifier using two transistors.
Guitar replacement hammond manufactures direct replacement transformers for popular tube amps from fender, marshall, vox, ampeg, orange, and yorkville traynor. In this particular design, q1 and q2 are wired in the commonemitter mode and drive the loudspeaker via pushpull transformer t2, and the antiphase input drive is obtained via phasesplitting transformer t1. Oct 18, 2011 transformer coupled classa power amplifier. This is biased heavily into class a, at about ma per pair. The output stage of a multistage amplifier is also called mixer stage power stage detector stage f stage answer. Transformer coupled class a power amplifier electronic. This short video shows how this efficiency is achieved and what real world results would. Multiple choice questions and answers on transistor audio power amplifiers.
If a load impedance such as a loud speaker is used, then for maximum transfer of power from the amplifier having high output impedance to the low impedance of 5 to 15ohm of the voice coil of the loudspeaker, it is required to use step down output transformer. Note that with the transformer coupled driver, the 300b does go into a2 operation a little bit. This is also sometimes referred to as single ended power amplifier. Direct coupled amplifier, using no impedance and bias matching components this class of amplifier was very uncommon in the vacuum tube days when the anode output voltage was at greater than several hundred volts and the grid input voltage at a few volts minus. Class b amplifiers have the advantage over their class a amplifier cousins in that no current flowsthrough the transistors when they are in their quiescent state ie, with no input signal, therefore no poweris dissipated in the output transistors or transformer when there is no signal present unlike class aamplifier stages that require. The first tube amplifiers were constructed with triodes, directly coupled to high impedance loudspeakers, with interstage transformers, but without output transformers. We present a mathematical analysis of the commonmode instability and power backoff feature of a transformer coupled class ab differential power amplifier pa. Transformercoupled power amplifier stability and power back. In the above class ab biasing examples, the input signal is coupled directly. Push pull amplifier circuit diagram class a, class b and. A transformer coupled ce amplifier finds following applications. In the classb circuit, the two output transistors are said to be operating in pushpull. Efficiency of class b amplifier is higher than class a amplifier, as it consists of two transistors npn and pnp. Let us go into the constructional and operational details of a transformer coupled amplifier.
Transformer coupled class ab power amplifier v in r l q 1 q 2 v cc r 2 class ab power amplifiers d here dc power from v cc is converted into ac output power in r l. Class c amplifiers, although not used as audio amplifiers, do find use in tuned circuits as in communications. Transformer coupled audio power amplifier engineering. To have this kind of operation, q point must be located at cutoff. A multistage amplifier circuit that is coupled to the next stage, with the help of a transformer, can be called as a transformer coupled amplifier. Direct coupled amplifier, using no impedance and bias matching components this class of amplifier was very uncommon in the vacuum tube days when the anode output voltage was at greater than several hundred volts and. Powerboosted class a amplifiers my last post dealt with giving the venerable 300b a power boost. Class a amplifier circuit operation and applications. The class ab amplifier conducts for more than half a cycle, but less than a full cycle. An amplifier where the load is coupled to the output of the transistor using a transformer is called a direct coupled amplifier. Class ab this is a compromise between the class a and b amplifiers. The transformer improves the efficiency of the amplifier by matching the impedance of the load with that of the amplifiers.
Ee and ece important mcq pdf transistor audio power amplifiers 1. Aug, 2018 the circuit diagram for class a amplifier is given below. The class ab power amplifier is one which is created by. May 15, 2009 lecture series on basic electronics by dr. In a class b amplifier, the transistors are biased to cutoff, so that there is no transistor power dissipation when there is no input signal. Aug 11, 2016 transformer coupled class a amps are twice as efficient as their capacitive coupled counterparts in theory. Plate current in a class ab, b, or c amplifier does not flow for the complete period of an ac cycle and the waveform is maintained by the inertia of the tuned circuit. A transformer coupled class a power amplifier has a load of 100 o on the secondary. They are smaller lighter compared to transformer coupled stages. Mar 07, 2018 the transformer coupled class a power amplifier has maximum collector efficiency 50%. Module 5 lecture 2 transformer couple power amplifier. Class cthe output of the class cconducts for less than 180 of theac cycle.
Amplifiers are classified according to their circuit configurations and methods of operation. In case of a directcoupled class a power amplifier shown, the quiescent current flows through the collector resistive load and causes large. Classification of power amplifiers seriesfed class a. Transformer coupled class a power amplifier tutorialspoint. The amplifier circuit in which, the previous stage is connected to the next stage using a coupling transformer, is called as transformer coupled amplifier. What turn ratio npns of transformer is required to match 4 o speaker to a transistor having an output impedance of 8000 o. Transformer coupled class a amplifier free download as powerpoint presentation. A 60ghz transformer coupled amplifier in 65nm digital cmos.
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