Jul 30, 2017 if you do not like background music then please go to this link. These amplifiers are usually employed in rf radiofrequency power amplification. Class a amplifier article about class a amplifier by the. What is class a and how does it differ from class ab. Its efficiency is about 75% because the active device is biased beyond cutoff. Explain what this means, contrasting it against the classa and classb operations common in audiofrequency amplifier circuits. The most commonly constructed amplifier classes are those that are used as audio amplifiers, mainly class a, b, ab and c and to keep things simple, it is these.
Class a class b class ab class c class d class a amplifier class a amplifying devices operate over the whole of the input cycle such that the output signal is an exact scaledup replica of the input with no clipping. Stage efficiency p lmax p s x 100 % where p s is dc input power. Part i, class a,ab, b and c find, read and cite all the research you need on. What distiguishes an class a amplifier from a class b amplifier. The operation of class b power amplifier in terms of a. The class ab amplifier is a variation of a class b amplifier as described above, except that both devices are allowed to conduct at the same time around the waveforms crossover point eliminating the crossover distortion problems of the previous class b amplifier. These classes are defined as class a, class b, class ab and class c.
Class a circuit but less than in a class b circuit, such operation is termed class a b. Putting this into perspective, you would need ten el34s to power a 100w amp in class a, and only two for class b. Class c amplifier class c amplifier operates for less than half of the input cycle. On the other hand an op amp is almost perfect at producing a clean output. In class b amplifier, the negative halfcycle of the signal is cut off and hence severe distortion occurs. It maintains the sound quality of a class a amplifier, but seeks the efficiency of a class b amp. In order to compensate these problems, the pushpull configuration is introduced in class b amplifier. Power amplifier classes are, in electronics, letter symbols applied to different power amplifier. In case of class a amplifier, the conduction angle is 360 degree. I know that in class a amplifiers transistors conduct during the entire cycle of signal and in a class b they conduct for half the cycle sinal. Audio amplifier classes, amplifiers, audio, amplifier classes, class b amplifier, class c. 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. 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. Pushpull class b and class ab amplifiers analog devices.
A bipolar transistor amp cannot be a sub 1 or sub 2 class because they do not have grids to have or not have grid current. Class ab and class c power amplifiers tutorialspoint. In class b amplifiers there still is a possibility of crossover distortion. The class of an amplifier is selected to meet the overall requirements. Amplifier output stage class a, class b, class c, pdf file. We are now going to look at pushpull class b and class ab amplifiers, which are comprised of devices with conduction angles less than 360 degrees. This page provides comparison between amplifier classes viz. Amplifiers are given a classification according to the way in which they are biassed and they operate. Class ab output stages, on the other hand, may be advertised as sounding as good as a singleended and class a pushpull amplifiers, but reality sadly disappoints.
For example, a classb or c power stage operating at 65% collector or drain efficiency losses 35% of input power. There are other classes such as class c, which shift the grid bias even more to the left. To have this kind of operation, q point must be located at cutoff. Pdf on jan 1, 2011, bill slade and others published amplifier alphabet soup. Class a amplifier is a high gain amplifier with high linearity. A common class of operation used in radiofrequency rf amplifier circuits is class c. Now were starting to cook with gasthe class ab amplifier is the combination of both of the former classes and it really delivers the best of both worlds. 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. Amplifier classes class a,class b,class ab,class c. The reason for this is a very low quiescent current characteristic of this type of amplifier. If you do not like background music then please go to this link. It is difficult to obtain coupling transformers and ideal inductors. Class b operation requires the use of two transistor amplifiers to produce a complete output waveform.
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. 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. If the collector current flows all at all times during the full cycle of the signal, the power amplifier is known as class a power amplifier. 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. 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. Notifier addressable class b bidirectional amplifiers bda are high gain, high power bandselective signal boostersbidirectional amplifiers that can integrate directly to a notifier fire alarm control panel with a builtin notifier addressable monitor module. With a class b amplifier, the audio signal is separated into a negative and positive halfwave. Class a amplifiers are the most common type of amplifier topology as they use just one output switching. That a wellengineered class a b pushpull circuit is capable of providing excellent sound quality is attested to by the fact that the output stages of many highfidelity power amplifiers are operated in class a b. 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. There is even a class c although its used for high power radio transmission, not audio. 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. We will be investigating each of the classifications in the remainder of this section of. 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 e amplifiers use lc tank circuits, like class c amplifiers, but in a class e amplifier, the active device becomes a switch. Power amplifier circuits output stages are classified as a, b, ab and c for analog. 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. In classa amplifier, the transistor is biased such that the qpoint is located at the center of load. The class c amplifier is a deeply biased hence the output current is zero for more than the onehalf of the input signal and the transistor idling at the cut off point. The trouble is, an opamp cant supply any serious power to a load but, in conjunction with a class b amplifier it can. Classes of amplifiers university of north carolina at charlotte. These amplifiers can be made to be more efficient than class a amplifiers, but suffer from a particularly undesirable form of distortion known as crossover distortion. 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. For example, in the pnp transistor, if the base becomes positive with respect to the emitter, holes are repelled at the emitterbase junction. My 40year love affair with a remarkable amplifiera class. 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. They are generally used in radio frequency rf applications, including circuits. Most cb amps are actually class c, and there is a reason for that.
A common class of operation used in radiofrequency rf amplifier circuits is classc. Using dc power supply, resistors, and digital multi meter. 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 non. Technical article my 40year love affair with a remarkable amplifiera class b amplifier for audiophiles may 04, 2018 by dermot herron the blomley amplifier was designed over forty years ago, is an alternative power amplifier design that suffers virtually no distortion, and provides a sound which has the presence often lacking in conventional amplifiers. Class c operation for completeness, we will briefly introduce the class c amplifier. This is similar to class b but a tuned circuit brings only the intended frequency out of the device. Class ab amplifiers are typically about 50% efficient.
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. The output amplitude is a nonlinear function of the. Sep 28, 2016 circuit description and power efficiency calculation for the class a amplifier. Each transistor is biased at a zero quiescent current i. The classes and classification of amplifiers and its applications. The output amplitude is a nonlinear function of the input, so class c amplifiers are not used for linear amplification. The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current.
Explain what this means, contrasting it against the class a and class b operations common in audiofrequency amplifier circuits. As you can see, the operating point q is located at collector cut off voltage. 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. Know all about classc power amplifier circuit and tutorial. Good and excellent explanation but kind help us convert it to pdf file. Solidstate amplifiers have bases or gates, and a bipolar transistor always has base current. In the hybrid ab amplifiers, four el34s are generally used for a 100w head such as a marshall. 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.
Introduction amplifier types class a class b class ab class c. This circuit will look very strange if you are accustomed to analyzing audio. Compared to class a, class b amplifiers use very little electricity, making them markedly more efficient. Class b amplifiers are greatly preferred over class a. Thus, this article discusses class c power amplifier tutorial which includes what is a power amplifier. Class b bjt amplifiers this worksheet and all related files. Moreover the position of the resting point is different.
Amplifier classa operation in this classa, amplifier operates in active region at all the times2. Class a, class b and class c amplifier introduction. The circuit diagram of a class a power amplifier is shown in fig. Clamper bias of class c amplifier altair university. 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. Classes of amplifiers university of north carolina at. One amplifier is used to amplify the positive half cycle of the input signal while the second is used to amplify the negative halfcycle. The image of the conduction angle derives from amplifying a sinusoidal signal. This prevents current from flowing in the collector circuit. As does the obvious but bad choice of running an extra rich idle current, so that there is more class a in the class ab blend.
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. Audio amplifier classes, class a amplifier, class b amplifier, class c. Benha university faculty of engineering at shoubra l. Classb amplifiers are also favoured in batteryoperated devices, such as transistor radios. Amplifiers, class b power amplifiers, class c power amplifiers. Classb amplifiers use a pair of complementary transistors in a pushpull configuration see figure. Class ab designs are the most widely used for audio applications. Since the active devices are slightly prebiased there will be a small amount of collector current flowing and this is. As we stated above, a 360degree conduction angle means the amplifier device remains active for the entire time and use complete input signal. Class b amplifier with opamp electrical engineering stack. What are the different types of audio amplifier classes. Power amplifier circuits output stages are classified as a, b, ab and c for analog designsand class d and e for switching designs. Class a amplifier the main role of class a bias is to keep amplifier free from distortion by keeping signal waveform out of the nonlinear region which exists between 0v and 0. Cutoff occurs when the basetoemitter bias prevents current from flowing in the emitter circuit.
To design classc tuned power amplifier and to study the classc tuned power. In the previous amplifiers, we have discussed the class a, b and ab are the linear amplifiers. A classc amplifier can have good efficiency as there are no lossy resistors anywhere. Class c bjt amplifiers worksheet discrete semiconductor. A practical circuit using classb elements is the pushpull stage, such as the very simplified complementary pair arrangement shown at right. The usual application for class c amplifiers is in rf transmitters operating at a single fixed carrier frequency, where the distortion is controlled by a tuned load on the amplifier. Without biasing, a class b output stage is going to produce cross over distortion. This page compares class a amplifier vs class b amplifier and mentions difference between class a amplifier and class b amplifier.
Class a amplifiers are the usual means of implementing smallsignal amplifiers. Oct 06, 2015 the operation of class b power amplifier in terms of a. A class a amplifier is conducting through all the period of the signal. The limitations due to the efficiency of class a power amplifiers. Class c operating for less than half of one wave cycle 100. Operational amplifier circuit i lab manual appendix iii. Amplifier classes and the classification of amplifiers.