The limitations due to the efficiency of class a power amplifiers. Crossover distortion is later explained in this article. A class ab power amplifier is, as the name suggests, a mix of class a and class b power amplifiers. The class b amplifier circuit above uses complimentary transistors for each half of the waveform and while class b amplifiers have a much high gain than the class a types, one of the main disadvantages of class b type pushpull amplifiers is that they suffer from an effect known commonly as crossover distortion. Electronics tutorial about class b amplifier and class b power amplifiers. Amplifiers are given a classification according to the way in which they are biassed and they operate. Pushpull amplifier is a power amplifier which is used to supply high power to the load. Comparison chart for class ab amplifier vs class d here, i am going to present a comparison chart between these two amplifiers, class ab and class d. From this level up the tubes start to saturate, and the current through the control grids forces the polarization level more negatively, out from class ab. This arrangement allows the valves to be operated in class ab, which is considerably more efficient than class a up to 80% compared with up to 50% respectively reducing the amount of power wasted as heat in power valves. Highpower amplifiers in particular are encumbered by the need for very large transformers capable of conducting large currents without saturating. In order to compensate these problems, the pushpull configuration is introduced in class b amplifier. Theyre great in a range of applications, and are the amp of choice for many car audio upgrades and hifi home theaters.
Single ended class ab configurations are not practical just because a major portion of one half cycle will be missing at the output. The total power of the three power amplifiers should some up to a total of 100w each driving a resistive load of 8 ohms. Q1 and q2 are two identical transistor and their emitter terminals are connected together. A push pull amplifier is a type of amplifier that can drive current in either direction through the load. Class b amplifier is the actual pushpull amplifier. The principal disadvantage of the push pull amplifier circuits we have discussed so far is the cost and bulk of their output transformers. Pushpull amplifier circuit diagram, working and application. Complementary pushpull amplifiers for active antennas. Types of amplifiers and their circuits with working. The forward bias causes the circuit to operate in classab mode, so both transistors are slightly on during crossover. When large amounts of output power are required more than about 15w the push pull output stage becomes the order of the day. Illustration of class b and class ab push pull, inverting amplifier output current and voltage characteristics of the push pull, inverting amplifier rl 1k.
In this arrangement, one transistor amplifies the positive half cycle whereas another. 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. Class b amplifier and the classb transistor amplifier. The class ab pushpull output circuit is slightly less efficient than class b because it uses a small. Circuit diagram of a typical class ab push pull amplifier is shown in the figure below. Pushpull class b and class ab amplifiers analog devices.
The information provided is and remains the sole property of kennedy tube audio a. In most applications where ac power is driving a load, a complementary symmetry push pull power amplifier is employed. Its not that its class b, its that its a commoncollector emitter follower push pull configuration. This amplifier usually has a voltage gain of one, and a large current gain. How is the gain controlled in a class bab amplifier. This form of amplification is technically known as class ab rather than class b, because each transistor is on for more than 50% of the time during a complete. Figure 1 shows a common configuration using a 2n3904 transistor and a 2n3906 transistor.
The construction of the class a power amplifier circuit in pushpull configuration is shown as in the figure below. Push pull amplifier circuit diagram class a, class b and. The class b amplifier circuit above uses complimentary transistors for each half of the waveform and while class b amplifiers have a much high gain than the class a types, one of the main disadvantages of class b type pushpull amplifiers is that they suffer from. So, in case of any kind of guidance related to these amplifiers you can check out this chart and if you do not know about these amplifiers and are confused between the selections, then this chart will help you anyway. The class ab push pull output circuit is slightly less efficient than class b because it uses a small quiescent current flowing, to bias the transistors just above cut off as shown in fig. A disadvantage of class b or class ab is that it is more difficult to implement the circuit in order to get a linear reproduction of the input waveform. However if we modify the class a amplifier circuit to operate in class b mode. An amplifier that has that property is called a class a amplifier. Push pull amplifier circuit diagram class a, class b and class ab. To design three class ab power amplifiers with feedbackpair complimentary symmetry output driven by an active 3way crossover network and a pre amplifier.
Pushpull class b and class ab amplifiers analog devices wiki. Electronics tutorial about the class ab amplifier circuit that is forward biased to. A hybrid of alwayson class a and push pull class b topology, ab amplifiers draw less power and reduce heat discharge, while delivering accurate and distortionfree sound. What class ab does is fill in the gap in the middle of class b push pull operation. Class ab amplifiers are often used as amplifier output stages in emitterfollower and commonemitter configurations. A kennedy audio labs, and is provided with the understanding that this information is for personal use only.
It is the most efficient configuration for transforming dc power from the power supply to the ac power. The amplifier changes smoothly from class ab to class f as the plate voltage rises. Following completion of this lab you should be able to explain the basic operation of push pull class b and class ab amplifiers that use bjts as the amplifying elements, describe crossover distortion, explain how and why it happens, and how it differs from distortion caused by other mechanisms, describe what level shifters do, and explain what. Efficiency of class b amplifier is higher than class a amplifier, as it consists of two transistors npn and pnp. Class c amplifiers are biased so that conduction occurs for much less than class c amplifiers are more efficient than either class a or push pull class b and class ab, which means that more output power can be obtained from class c operation. Amplifier classes including class a, class b, class ab, class c and the like are widely seen when dealing with amplifier specifications and their design. The image of the conduction angle derives from amplifying a sinusoidal signal. Power amplifiers purpose of a power amplifier generate high output power efficient conversion of dc power to rf power linear amplification generally pas will be common source cascode inductor is a choke to provide d apacitor is a ac coupling path to output james buckwalter 2. In the case of a class b ab amplifier, crossover distortion can be reduced by using a slight forward bias in the base circuit such that the transistors are idling at a small output current. This arrangement mainly reduces the harmonic distortion introduced by the nonlinearity of the transfer characteristics of a single transistor amplifier. This configuration allows linear class ab push pull operation until the amplifier reaches roughly 100 w.
The class b amplifier consists of two transistors in a push pull configuration. That gain of the commoncollector configuration, whether singleended or push pull, is less than 1. What is class a and how does it differ from class ab. For my lab project this semester i have to build an audio amplifier that has an pre amp stage using op amp circuits, and an output stage using a bjt power amplifier, all to be driven by 3 or 4 1. Class b audio amplifier example it is required to design a class b output stage to deliver an average power of 20 w to an 8 speaker load. The output amplitude is a nonlinear function of the. Classa designs can be simpler than other classes insofar as class ab and b designs require two connected devices in the circuit pushpull output, each to. Also, explain what practical purpose this push pull ampli. The term push pull refers to a common type of class b or class ab amplifier circuit in which two transistors are used on alternating half. What is class b power amplifier feedback amplifiers. A socalled bias network in a complementarysymmetry push pull output stage see illustration functions to maintain a constant voltage difference between the bases of the two transistors. What is class a push pull amplifier electronic devices electrical engineering duration. A bridge amplifier configuration effectively doubles the voltage swing at the load. It has smaller heat dissipation, so small heatsink is needed.
Understanding amplifier operating classes electronic. Using this approximation we see that a class ab amplifier acts much like a class b amplifier. A push pull amplifier can be built using amplifier ics, rather than discretes, as in the traditional class b amp. This then would give us what is commonly called an class ab amplifier circuit.
What is the main factor governing the maximum power rating ptot of a transistor. Ece 3274 power amplifier project push pull qty device. The class ab circuit, over most of its input signal range, operates as if the q n or q p transistor is conducting and the q p or q n transistor is cut off. The commonemitter class ab stage is used in railtorail operational amplifier opamp stages in order to allow the output voltage to swing very close to the power supply voltages. In electronic terms, class ab push pull is like two asymmetrically biased amplifiers that mirror image each. The voltage amplification must come from previous stages. Power amplifier circuits output stages are classified as a, b, ab and c for analog designsand class d and e for switching designs. The circuit functions as an amplifier in that the current levels at the output are larger than those at the input. Like the class b amplifier, it also uses 2 conducting elements transistors, but they both run at the same time. Trask, push pull amplifiers rev e 3 4 july 20 it is difficult to comprehend why anyone would resort to using class ab in a small signal application, especially in an active antenna amplifier where exceptional linearity and nf are highly desired. The circuit diagram of a typical class a push pull amplifier is shown above.
Class d amplifier is the highest power efficient amplifier class in the a, b, ab, and c and d segment. To avoid saturation of the transistors and nonlinear distortion, the power supply v cc is to be 5v greater than peak output voltage. Just like the class b configuration, push pull mechanism is essential for realizing practical class ab power amplifiers. The class ab pushpull output circuit is slightly less efficient than class b because it uses. An ideal class b amplifier has a conduction angle of 180 degrees, or one. Push pull amplifier circuit, operation, advantages and. The circuit requires various switching components like mosfets which has low on resistance. Class a, class b, class ab and class c amplifier scary. It eliminates the cutoff by ensuring that some current flows during idle and throughout the transition from one side of the push pull to the other.
It is a widely used topology in digital audio players or controlling the motors as well. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. Although the circuit would produce severe distortion as only half of the signal wave form produces a current in the load, because the load in this case is a tuned circuit resonating at the signal frequency, the resonating effect of the tuned circuit fills in the missing half cycles. All the smallsignal amplifiers have been designed so that output voltage can vary in response to both positive and negative inputs. In terms of distortion and performance, push pull amplifiers are more efficient than singleended amplifier because of its push pull arrangement. Pdf audio amplifier design group members can kayal. The class b amplifier circuit is biased in such a way that each transistor will work on one half cycle of the input waveform. A push pull amplifier can be made in class a, class b, class ab or class c configurations.
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