NATIONAL COLLEGE OF SCIENCE & TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite Assignment # 1 Classes of AmplifierValladolid, Charles Edison September 01, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
CLASS-AClass-A amplifiers are the simplest in design, and can be the most distortion-free of allamplifier classes. In class-A, the output devices are biased on all the time with a currentlarge enough to produce the largest output signal. Some class-A amplifiers may employboth a positive and negative device in a push-pull arrangement to increase outputpower, but both devices still are biased on and conduct all the time. Class-A amplifiersare generally considered to be the most accurate of all classes in low to moderatepower ranges and are useful for applications such as preamp stages; however, theycreate tremendous amounts of heat due to their very low efficiency, making themimpractical for high-power amplification. Other amplifier classes have been developedover time to overcome the class-A efficiency problemMany class A amplifiers use the same transistor(s) to reproduce both the top andbottom halves of the audio waveform. In this configuration, the output transistor(s)always has current flowing through it, even if it has no audio signal (the output transistorsnever turn off). The current flowing through it is D.C. A pure class A amplifier is veryinefficient and generally runs very hot even when there is no audio output. The currentflowing through the output transistor(s) (with no audio signal) may be as much as thecurrent which will be driven through the speaker load at FULL audio output power.Many people believe class A amps to sound better than other configurations (and thismay have been true at some point in time) but a well designed amplifier wont haveany sound and even the most critical ear would be hard-pressed to tell one designfrom another.NOTE: Some class A amplifiers use complimentary (separate transistors for positive andnegative halves of the waveform) transistors for their output stage.Class A AmplifierThe most commonly used type of power amplifier configuration is the Class A Amplifier.The Class A amplifier is the most common and simplest form of power amplifier that usesthe switching transistor in the standard common emitter circuit configuration as seenpreviously. The transistor is always biased "ON" so that it conducts during one completecycle of the input signal waveform producing minimum distortion and maximumamplitude to the output. This means then that the Class A Amplifier configuration is theideal operating mode, because there can be no crossover or switch-off distortion to theoutput waveform even during the negative half of the cycle. Class A power amplifieroutput stages may use a single power transistor or pairs of transistors connectedtogether to share the high load current. Consider the Class A amplifier circuit below.
Single-ended Amplifier CircuitThis is the simplest type of Class A power amplifier circuit. It uses a single-ended transistorfor its output stage with the resistive load connected directly to the Collector terminal.When the transistor switches "ON" it sinks the output current through the Collectorresulting in an inevitable voltage drop across the Emitter resistance thereby limiting thenegative output capability. The efficiency of this type of circuit is very low (less than30%) and delivers small power outputs for a large drain on the DC power supply. A ClassA amplifier stage passes the same load current even when no input signal is applied solarge heatsinks are needed for the output transistors.However, another simple way to increase the current handling capacity of the circuitwhile at the same time obtain a greater power gain is to replace the single outputtransistor with a Darlington Transistor. These types of devices are basically two transistorswithin a single package, one small "pilot" transistor and another larger "switching"transistor. The big advantage of these devices are that the input impedance is suitablylarge while the output impedance is relatively low, thereby reducing the power loss andtherefore the heat within the switching device.
CLASS-BA class B amplifier uses two transistors (or two groups of transistors). One transistor (orgroup of transistors) is used to reproduce the top half of the waveform. A secondtransistor (or group of transistors) is used to reproduce the bottom half of the waveform.In a class B amplifier, there is typically no idle/bias current flowing through the outputtransistors when there is no audio. In most cases, if the amplifier has no biaspotentiometers and its not a class D amplifier, its a class B amplifier.Class B Push-pull Transformer Amplifier CircuitThe circuit above shows a standard Class B Amplifier circuit that uses a balancedcentre-tapped input transformer, which splits the incoming waveform signal into twoequal halves and which are 180o out of phase with each other. Another centre-tappedtransformer on the output is used to recombined the two signals providing theincreased power to the load. The transistors used for this type of transformer push-pullamplifier circuit are both NPN transistors with their emitter terminals connected together.Here, the load current is shared between the two power transistor devices as itdecreases in one device and increases in the other throughout the signal cyclereducing the output voltage and current to zero. The result is that both halves of theoutput waveform now swings from zero to twice the quiescent current thereby reducingdissipation. This has the effect of almost doubling the efficiency of the amplifier toaround 70%.Assuming that no input signal is present, then each transistor carries the normalquiescent collector current, the value of which is determined by the base bias which isat the cut-off point. If the transformer is accurately centre tapped, then the twocollector currents will flow in opposite directions (ideal condition) and there will be nomagnetization of the transformer core, thus minimizing the possibility of distortion. Whena signal is present across the secondary of the driver transformer T1, the transistor baseinputs are in "anti-phase" to each other as shown, thus if TR1 base goes positive drivingthe transistor into heavy conduction, its collector current will increase but at the sametime the base current of TR2 will go negative further into cut-off and the collectorcurrent of this transistor decreases by an equal amount and vice versa. Hence negativehalves are amplified by one transistor and positive halves by the other transistor giving
this push-pull effect. Unlike the DC condition, these AC currents areADDITIVE resulting inthe two output half-cycles being combined to reform the sine-wave in the outputtransformers primary winding which then appears across the load.Class B Amplifier operation has zero DC bias as the transistors are biased at the cut-off,so each transistor only conducts when the input signal is greater than the base-emitter voltage. Therefore, at zero input there is zero output and no power is beingconsumed. This then means that the actual Q-point of a Class B amplifier is onthe Vce part of the load line as shown below.Class B Output Characteristics CurvesThe Class B Amplifier has the big advantage over their Class A amplifier cousins in thatno current flows through the transistors when they are in their quiescent state (ie, with noinput signal), therefore no power is dissipated in the output transistors or transformerwhen there is no signal present unlike Class A amplifier stages that require significantbase bias thereby dissipating lots of heat - even with no input signal present. So theoverall conversion efficiency ( η ) of the amplifier is greater than that of the equivalentClass A with efficiencies reaching as high as 70% possible resulting in nearly all moderntypes of push-pull amplifiers operated in this Class B mode.
CLASS-ABClass AB amplifiers use two groups of transistors like class B amplifiers. In most respects,class AB and class B amplifiers are very similar. As we said earlier, a class A amplifier isvery inefficient. This is not good for a car audio amplifier. Some people believe thatclass B amplifier can never produce clean audio because their output transistors arentbiased on. A class AB amplifier is generally considered to be the best compromise. Aclass AB amplifier is a class B amplifier which has a small amount of bias currentflowing through the output transistors at all times. This eliminates virtually all of thecrossover distortion thats possible with class B amplifiers. The bias current is flowingbecause the output transistors are always conducting current (even without an audiosignal). This differs from a pure class A amplifier in the amount of current flow. A pureclass A amplifier typically has an enormous amount of current flowing through itsoutput transistors with NO audio signal. A pure class B amplifier has NO current flowingthrough its outputs with no audio signal. A class AB amplifier is much more efficientthan the class A but without the possible distortion of the class B. MANY of the caraudio amplifiers which claim to be a class A amplifier are just a high bias class ABdesign. These amplifiers are only class A at very low power output levels. At higherpower levels, one of the output transistors will switch off while the other output transistoris conducting. I dont want you to think that I am telling you that there are no class Aamplifiers. There are a few high quality mobile amplifiers which are a true class Adesign.Class AB AmplifierThe Class AB Amplifier circuit is a compromise between the Class A and the Class Bconfigurations. This very small diode biasing voltage causes both transistors to slightlyconduct even when no input signal is present. An input signal waveform will cause thetransistors to operate as normal in their active region thereby eliminating any crossoverdistortion present in pure Class B amplifier designs. A small collector current will flowwhen there is no input signal but it is much less than that for the Class A amplifierconfiguration. This means then that the transistor will be "ON" for more than half a cycleof the waveform but much less than a full cycle giving a conduction angle of between180 to 360o or 50 to 100% of the input signal depending upon the amount of additionalbiasing used. The amount of diode biasing voltage present at the base terminal of thetransistor can be increased in multiples by adding additional diodes in series.
Push-pull amplifiersOne use of phase splitters is to provide input signals to a single-stage amplifier that usestwo transistors. These transistors are configured in such a way that the two outputs, 180ºout of phase witheach other, combine. This allows more gain than one transistor couldsupply by itself. This "push-pull"amplifier is used where high power output and goodfidelity are needed: receiver output stages, public caddress amplifiers, and AMmodulators, for example.Transformerless Class B Push-Pull Amplifier