The disadvantage of class a is low efficiency and maximum. To improve the full power efficiency of the previous class a amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage producing what is commonly termed as a class b amplifier also known as a pushpull amplifier configuration. 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. An ideal classb amplifier has a theoretical maximum efficiency of 78%. Using this approximation we see that a class ab amplifier acts much like a class b amplifier. Input power the input power, pin, is given by pinvps. Class c operation, especially for highefficiency, highpower radio frequency rf amplifier applications. Class a efficiency vs rf power construct efficiency in terms of minimum device output voltage and d. Class c amplifier design july 29, 2012 8 output voltage waveform of idealized class c amplifier 1 for t class b pushpull amplifier. Due to power loss in the primary of the transformer, the collector efficiency of class a amplifier is always less than 50%. Thus, the maximum collector efficiency of class b power amplifier is 78.
Why is the efficiency of a class b amplifier greater than a. Its a traditional amplifier which works differently than the other amplifiers classes. Fig 3 below shows typical efficiency curves for linear and class d amplifiers. Abstract the work presented in this paper provides a theoretical analysis and estimation of most the important power parameters of the class b. The above expression shows that in class a amplifier, maximum 50% of the dc power supplied can be converted into ac power output. The qpoint is the point at which a transistor operates without a signal, and it pretty much determines the amount o. Complementary symmetry class b and class ab power amplifiers the circuit for a typical pushpull power amplifier is shown to the right figure 8. In order to compensate these problems, the pushpull configuration is introduced in class b amplifier. Class b amplifier and the classb transistor amplifier. This question already includes the reason why is it so, and if you understand the qpoint, then you will understand why it is so. The class ab amplifier is a blend of class a and class b, and strives to offer a compromise in efficiency and performance. How can i calculate the efficiency of a class ab amplifier circuit. Class ab and class c power amplifiers tutorialspoint.
Pdf on the design and efficiency of class a, b, ab, g, and. Ab amplifier, whereas a class d amplifier is much more efficient, with values in the order of 90% in practical designs. Class c amplifier is tuned amplifier which works in two different operating modes, tuned or untuned. Power conversion efficiency similar to class b disadvantages. A singletransistor class c amplifier 50 watts output for 30m.
Class a power amplifier is the simplest of all power amplifier configurations. Power parameters and efficiency of class b amplifier. Power dissipation for low signal levels higher than class b. Aug 09, 2016 the class a amplifier has great fidelity, it stays true to the original signal. Obviously, the output is not a faithful reproduction of the input if only one halfcycle is present. Class a waveforms average average class a waveforms class a with more efficient waveform theoretical maximum at device efficiency 50% practical rf amplifiers require a network between active device and load. Its efficiency is about 75% because the active device is biased beyond cutoff. 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 of a pushpull class b power amplifier consists of two identical transistors t 1 and t 2 whose bases are connected to the secondary of the center. More than 75% of the dc power applied to the circuit is lost. Class a amplifiers have a collector current waveform with. Class c amplifier the class c amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here. 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.
Pdf design and simulation for a highefficiency classb. The main advantage of the class c amplifier is, it has a lowest physical size for a given power output. Simulation results show that the classb power amplifier has the pae of 69. In class b the transistor conducts only in one half cycle of the signal. Looking at the halfwave drain current, we could also calculate.
Understanding amplifier operating classes electronic design. Principles of operation, design procedures, and experimental verification nathan o. Jan 19, 2017 the class ab amplifier is a blend of class a and class b, and strives to offer a compromise in efficiency and performance. One way to produce an amplifier with the high efficiency output of the class b. Class a power amplifier circuit theory design circuit. Class a amplifiers purely linear, with the devices always on have a theoretical maximum efficiency of 50% and some versions have efficiencies below 20%. The purpose of biasing is to establish a qpoint about which variations in current and voltage can occur in response to an ac input signal. Power amplifier classes the power efficiency of an amplifier, defined as the ratio of power output to power input, improves gets higher going from class a. Apart from the class a, b, and ab amplifier, there is another amplifier class c. Introduction to the amplifier an amplifier tutorial.
To improve the full power efficiency of the previous class a amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier circuit with two transistors in its output stage producing what is commonly termed as a class b amplifier also. We have already seen in our previous article about class b power amplifier that a pushpull circuit uses two transistors working in class b operation. Calculation of efficiency for a single supply, bjt based class b amplifier. Therefore waveform at load is not the same as waveform at device. Another type of classa amplifier uses the same circuit as figure 1, but the transistors are biased to around. In the above circuit, the biasing resistor rb is used to pull the base terminal of the q1 transistor further downwards. Thus the class c amplifier model is implemented using theactivate tool. It may be noted that maximum power is dissipated in the transistor in the zero signal.
The main objective of this project was to design a 100w power amplifier with feedbackpair complementary symmetry output power transistors. What are amplifier classes and their power efficiencies. In applications where small signal voltages must be amplified. To make the integral easy to calculate, change variables to.
In bipolar class d amplifier, the efficiency is further compromised due to. Since, the transistor is on for alternating halfcycles, it effectively acts as a halfwave rectifier. The benefit of the doherty is increased efficiency. Class a power amplifier is a type of power amplifier where the output transistor is on full time and the output current flows for the entire cycle of the input wave form. As the name implies, class ab is a combination of class a and class b type of amplifiers. They have high fidelity and are totally immune to crossover distortion. For most purposes, thats close enough to what goes on in class ab amplifiers, the difference being that well ignore.
Maximum collector circuit efficiency of class a amplifier conversion efficiency. In this topology, each of the pair of complementary active elements is biased slightly into the active region, and so there is some overlap between the two at the turnonturnoff center point, figure 3. A output linear amplifier class d amplifier fig 3 linear and class d amplifier efficiencies. 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. At an output power of 1w, efficiency is 1%, and so on with the voltages and currents as described. Efficiency of a class ab amplifier all about circuits. Theoretical maximum efficiency of class b power amplifier is 78. A complimentary amplifier, or pushpull amplifier uses. To improve the full power efficiency of the previous class a amplifier by reducing the wasted power in the form of heat, it is possible to design the power amplifier. From the above circuit it is clear that the base of the transistor q1 is not biased and the negative half cycle of the input waveform is missing in the output. Basically, a class a amplifier gives a much more linear output meaning that it has, linearity over a larger frequency response even if. Highest efficiency 100 % 70 80 90 class b amplifier power efficiency of class a, class b and class d amplifiers current flows half of the period 40 50 60 c iency % linearity compromised by crossover higher efficiency 78. Maximum efficiency for class b power amplifier electronics post.
Classa efficiency vs rf power construct efficiency in terms of minimum device output voltage and d. Abstract classe power amplifiers 16 achieve significantly higher efficiency than for conventional classb. On the design and efficiency of class a, b, ab, g, and h audio power amplifier output stages article pdf available in journal of the audio engineering society. In bipolar class d amplifier, the efficiency is further compromised due to charge storage in saturation and v ce,sat. The term class d is sometimes misunderstood as meaning a digital amplifier. It is commonly used in rf circuits where a resonant circuit must be placed at the output in order to keep the sine wave going during the nonconducting portion of the input. The schematic of a single ended class b amplifier and input, output waveforms are shown in the figure below. Classes of power amplifiers class a, b, ab, c, d amplifiers.
The q point will be fixed to below the cutoff point in. Simulation results show that the class b power amplifier has the pae of 69. Power parameters and efficiency of class b amplifier operating with resistive load and random signal. With a classa amp, the efficiency falls with reduced power, until at zero output power, efficiency is 0%. How to calculate the efficiency of class b power amplifier. Know all about classc power amplifier circuit and tutorial. We will now derive a general expression for the efficiency of. An ideal class b amplifier has a theoretical maximum efficiency of 78%.
The class c amplifier is used in the applications like rf oscillators, rf amplifier, fm transmitters, booster amplifiers, high frequency repeaters and tuned amplifiers. Class c amplifier class c amplifier operates for less than half of the input cycle. Jan 25, 2017 calculation of efficiency for a single supply, bjt based class b amplifier. A figure of merit for the power amplifier is its efficiency. The efficiency of a class a amplifier is not a number at the end of several pages of arcane math in a textbook. Vps is the total voltage across the output stage of the power amplifier. So why use a class a amplifier if its efficiency is less than 40% compared to a class b amplifier that has a higher efficiency rating of over 70%. B amplifier was developed to improve on the low efficiency rating of the classa amplifier.
The limitations due to the efficiency of class a power amplifiers. 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. 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. Mar 03, 2017 maximum efficiency for class b power amplifier. Class j amplifier characteristics fundamental impedance. For class b operation, the q point is located at cutoff on both d. Class d amplifier power efficiency with ideal switches, class d amplifier efficiency would be 100% in practice, finite switch on resistance and nonzero onoff transients limit efficiency use high f t device. Amplifier classes and efficiency electronics assignment. Amplifier classes and the classification of amplifiers.
As a consequence of the definition of efficiency equation 169, the efficiency of a classa amplifier is 0 when n signal is present. Where i dc is the average current drawn from the supply v cc. Efficiency of a class ab amplifier how can i calculate the efficiency of a class ab amplifier circuit. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. Somewhat like a class g or h amp, the class b amp sustains the output most of the time, but the class c amp cuts in on high signal peaks. The efficiency of class c amplifier is much more than the a, b, and ab. Thus for no excitation by an input signal, the dc power consumed in class b is ideally zero for a fetbased amplifier, and very low for a bipolar amplifier. This white paper covers power dissipation calculations for class b amplifiers. The output stage typically have two complementary transistors, each one carrying half a sinusoidal wave, then total current from the source is equivalent to full wave rectified. How can i determince ps in the equation efficiency pload pssupply 100% thansk. The class a amplifier has great fidelity, it stays true to the original signal.
So the class ab amplifier is a good compromise between class a and class b in terms of efficiency and linearity, with conversion efficiencies reaching about 50% to 60%. Dec 11, 2018 apart from the class a, b, and ab amplifier, there is another amplifier class c. The circuit diagram of the class c power amplifier circuit is shown above. Class c operation, especially for highefficiency, highpower radio frequency. An amplifier takes dc input power, pin, and turns it into signal power, pout, in the speaker. Electronics tutorial about the class ab amplifier circuit that is forward biased to.
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