If this classa buffer delivers more than 3a of current, then it is no longer operating in classa. These amplifiers are usually employed in rf radiofrequency power amplification. To have this kind of operation, q point must be located at cutoff. There are class a,b,c mainly used in transmitters and d pwm amplifiers sometimes wrongly referred to as digital once used in servo motor applications but now used in many subwoofer circuits as a speaker is simply a motor amplifiers. Please consult the sony user manual or website for the information. The usual application for classc amplifiers is in rf transmitters operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load on the amplifier.
Question 1 a common class of operation used in radiofrequency rf amplifier circuits is classc. This thesis mainly deals with class b power amplifiers. Amplifier classes and the classification of amplifiers. 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. Class ab power amplifiers are slightly inefficient than the class b configurations but far better in terms of distortion when compared to class a configurations. In order to compensate these problems, the pushpull configuration is introduced in class b amplifier. Ingram school of engineeringtexas state universitysan marcos, txdecember 5th, 2011 2. Class c bjt amplifiers discrete semiconductor devices and circuits pdf version. However, the class c amplifier is heavily biased so that the output current is zero. If you do not like background music then please go to this link. Class c power amplifier is a type of amplifier where the active element transistor conduct for less than one half cycle of the input signal. Class c has very good efficiency because whenever the. The classa window of operation only allows 18w of classa power, but 72w of classab watts might be summoned, if the power supply can sustain the higher current demand.
Its efficiency is about 75% because the active device is biased beyond cutoff. In the above circuit, a tank circuit can be formed by using a capacitor c1 and an inductor l1 which help in the removal of the necessary signal from the pulsed op of the transistor. Class ab and class c power amplifiers tutorialspoint. Class c would not be suitable for a audio amplifier, for example. Introduction amplifier types class a class b class ab class c. Class c amplifier vhf, uhf and shf frequencies generally, amplifiers can be subdivided into two distinct types depending upon their power or voltage gain.
The class of an amplifier is selected to meet the overall requirements. Class c amplifier class c amplifier operates for less than half of the input cycle. 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 hopefully we remember from our tutorials about. The limitations due to the efficiency of class a power amplifiers. Complementary symmetry class b and class ab power amplifiers. Class c amplifiers electronics assignment help and. 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 for class b operation, the q point is. The classes a, b, ab and c refer to the way the amplifiers are biased, although class c is. Small signal amplifies are designed to amplify very small.
Power amplifier circuits output stages are classified as a, b, ab and c for analog designsand class d and e for switching designs. In general, where amount of power involved is 1w or more, the amplifier is termed as power amplifier. The device used here, x1, is the motorola mrf873 npn power bjt. Classa operation is bounded by current flow, not voltage swings. It is created using two active devices which conduct half of the actual cycle, ie 180 degrees of the cycle. Class b amplifiers were invented as a solution to the efficiency and heating problems. Cw efficiency characteristic of the amplifier and the pdf of the signal, the average.
Its not what you get when a class b driver has a little gap. There is even a class c although its used for high power radio transmission, not audio. Class c operation for completeness, we will briefly introduce the class c amplifier. Class c classc amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies up to 90% are possible. Making a bladeless wooden fan scrapwood challenge ep38 duration. The class b amplifier is a bit different from the class a. Highly efficient and highly linear rf power amplifiers tu delft. The operation of class b power amplifier in terms of a. Class c exploits resonance, and therefore usually works over a very narrow frequency range. Class c bjt amplifiers worksheet discrete semiconductor. 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. As you can see, the operating point q is located at collector cut off voltage.
It can produce 15 watts of output power in the 806960 megahertz range. In class b amplifier, the negative halfcycle of the signal is cut off and hence severe distortion occurs. Clamper bias of class c amplifier altair university. 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 c amplifiers are biased so that conduction occurs for much less than class c amplifiers are more efficient than either class a or pushpull class b and class ab, which means that more output power can be obtained from class c operation. Class ab and class c power amplifiers the class a and class b amplifier so far discussed has got few limitations. Class b waveforms pt i ct v ct v cc 0v since the voltage at the load is ideally a perfect sinusoid, the voltage on the collectors is likewise sinusoidal. Here, the main function of the transistor is to generate a current pulse in series according to the ip and make it flow through the resonant circuit.
Class b amplifier and the classb transistor amplifier. Classes of amplifiers university of north carolina at. Know all about classc power amplifier circuit and tutorial. High efficiency high gain pushpull transistors power amplifier av. Explain what this means, contrasting it against the classa and classb operations common in audiofrequency amplifier circuits. Is it possible to convert a class ab amplifier to a class. Difference between class a amplifier and class b amplifier. The previous classes, a, b and ab are considered linear amplifiers, as the output signals amplitude and phase are linearly related to the input signals 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 class ab configuration is a compromise the efficiency of the class a and the linearity of the class b stage have been significantly although not perfectly improved. 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. What is class a and how does it differ from class ab. The pot you want to mess with is called the vbe multiplier. Is it possible to convert a class ab amplifier to a class a amplifier by messing with the pot inside it. Power amplifier classes are, in electronics, letter symbols applied to different power amplifier. With a class b amplifier, the audio signal is separated into a negative and positive halfwave. In the above image, an ideal class b amplifier configuration has been shown. Amplifiers are given a classification according to the way in which they are biassed and they operate. The fundamental difference between class b and class ab operation is biasing. When the collector current flows for less than half cycle of the input signal, the power amplifier is known as class c power amplifier.
To explain the traditional classes of the rfpa a, ab and c, the following ideal device is. C1, c2, and l1 are simulating class c rf amplifiers figure 1, schematic of an rf amplifier running in class c operation at. Classb and classab power amplifiers assure signal continuity by. Class t amplifier designs combine both the low distortion signal levels of class ab. Amplifiers operating in class ab mode are less powerefficient than. Classes of power amplifiers class a, b, ab, c, d amplifiers. The class c amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here. 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. The reason for this is a very low quiescent current characteristic of this type of amplifier. 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. 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. It will increase the bias to the output stage in such a way that the output tra. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current.
Classc amplifiers are used primarily in highpower, highfrequency applications, such as radiofrequency transmitters. Class c amplifier the class c amplifier design has the greatest efficiency but the poorest linearity of the classes of amplifiers mentioned here. Another problem common to class b and ab output stages is due to the unequal. Uses class a low power amplifiers where efficiency is not important class b output power amplifiers class c tuned rf amplifiers but cannot be used as audio amplifiers due to high distortion 14. There are also some speciality classes which crossover the rest commonly called e,f,g and h. The power dissipated by each transistor is thus the product of a sine and a half sine as. Your author notes that complementary symmetry amplifiers may also be used in class c operation, especially for highefficiency, highpower radio frequency rf amplifier applications. In class ab amplifiers, very high orders of harmonics occur, and the peak distortion can be as much as thirty times the average distortion. Power amplifier circuits output stages are classified as a, b, ab and c for analog designsand.
Biased to operate simply refers the various voltages on the tubes and how they relate to one another, especially the negative bias applied to the grid. These amplifiers combine classd switching output efficiency and classg. Class c power amplifier circuit diagram and theory. There are different classes of amplifiers starting from a, b, c, ab, d, e. Compared to class a, class b amplifiers use very little electricity, making them markedly more efficient. Two devices provide combined current drive for the load. Typical class a amplifiers will exhibit low order harmonics, and their peak distortion is less than twice the average distortion. Class b bjt amplifiers worksheet discrete semiconductor. Since the active devices are slightly prebiased there will be a small amount of collector current flowing and this is the reason behind the slightly reduced efficiency. One type is called the small signal amplifier which include pre amplifiers, instrumentation amplifiers etc. The image of the conduction angle derives from amplifying a sinusoidal signal. This page compares class a amplifier vs class b amplifier and mentions difference between class a amplifier and class b amplifier. Class a amplifier the main role of classa bias is to keep amplifier free from distortion by keeping signal waveform out of the nonlinear region which exists between 0v and 0.
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