NATIONAL COLLEGE OF SCIENCE AND TECHNOLOGY Amafel Bldg. Aguinaldo Highway Dasmariñas City, Cavite ASSIGNMENT 1 CLASSES OF AMPLIFIER September 01, 2011Electronics 3/BSECE 41A1 Score: Engr. Grace Ramones Instructor
Classes of OperationThe curve is the one taken from theprevious section, and the bias has been setwhere the red spot is, roughly in themiddle of the curve.If we now superimpose a signal on thegrid voltage, the anode current will varyup and down in sympathy with the gridvoltage, hence the "Out" waveform.As the title suggests, this is "Class A". Thedistinguishing feature of class A is that thevalve is conducting current at all times. Note the "Out" current never drops down tothe zero line at any time.The most commonly used type of power amplifier configuration is the Class AAmplifier. The Class A amplifier is the most common and simplest form of poweramplifier that uses the switching transistor in the standard common emitter circuitconfiguration as seen previously. The transistor is always biased "ON" so that itconducts during one complete cycle of the input signal waveform producing minimumdistortion and maximum amplitude to the output. This means then that the Class AAmplifier configuration is the ideal operating mode, because there can be no crossoveror switch-off distortion to the output waveform even during the negative half of thecycle. Class A power amplifier output stages may use a single power transistor or pairsof transistors connected together to share the high load current. Consider the Class Aamplifier circuit below.Single-ended Amplifier Circuit
In the diagram on the right, we have setthe bias point to where the valve hasalmost stopped conducting.Note that the input signal is a lot largernow in order to drive the valve hardenough. Also, the output current is onlyfor half of the waveform.To make any use of this, we have to havea "push-pull" output stage which employstwo valves (or two banks of valves) so thateach side amplifies each half of thewaveform. While the first output valve provides the output current as shown on theright, the second valve fills in the gap which follows it.To improve the full power efficiency of the previous Class A amplifier by reducing thewasted power in the form of heat, it is possible to design the power amplifier circuitwith two transistors in its output stage producing what is commonly termed as a "push-pull" type amplifier configuration. Push-pull amplifiers use two "complementary" ormatching transistors, one being an NPN-type and the other being a PNP-type with bothpower transistors receiving the same input signal together that is equal in magnitude,but in opposite phase to each other. This results in one transistor only amplifying onehalf or 180o of the input waveform cycle while the other transistor amplifies the otherhalf or remaining 180o of the input waveform cycle with the resulting "two-halves"being put back together again at the output terminal. Then the conduction angle for thistype of amplifier circuit is only 180o or 50% of the input signal. This pushing andpulling effect of the alternating half cycles by the transistors gives this type of circuit itsamusing name, but these types of audio amplifier circuit are more generally known asthe Class B Amplifier as shown below.
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 connectedtogether. 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 cycle reducingthe output voltage and current to zero. The result is that both halves of the outputwaveform 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 is atthe cut-off point. If the transformer is accurately centre tapped, then the two collectorcurrents will flow in opposite directions (ideal condition) and there will be nomagnetization of the transformer core, thus minimizing the possibility of distortion.When a signal is present across the secondary of the driver transformer T1, thetransistor base inputs are in "anti-phase" to each other as shown, thus if TR1 base goespositive driving the transistor into heavy conduction, its collector current will increasebut at the same time the base current of TR2 will go negative further into cut-off and thecollector current of this transistor decreases by an equal amount and vice versa. Hencenegative halves are amplified by one transistor and positive halves by the othertransistor giving this push-pull effect. Unlike the DC condition, these AC currents areADDITIVE resulting in the two output half-cycles being combined to reform the sine-wave in the output transformers 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 Curves The Class B Amplifier has the big advantage over their Class A amplifier cousins in that no current flows through the transistors when they are in their quiescent state (ie, with no input signal), therefore no power is dissipated in the output transistors or transformer when there is no signal present unlike Class A amplifier stages that require significant base bias thereby dissipating lots of heat - even with no input signal present. So the overall conversion efficiency ( η ) of the amplifier is greater than that of the equivalent Class A with efficiencies reaching as high as 70% possible resulting in nearly all modern types of push-pull amplifiers operated in this Class B mode.Transformerless Class B Push-Pull AmplifierOne of the main disadvantages of the Class B amplifier circuit above is that it usesbalanced centre-tapped transformers in its design, making it expensive to construct.However, there is another type of Class B amplifier called a Complementary-SymmetryClass B Amplifier that does not use transformers in its design therefore, it istransformerless using instead complementary or matching pairs of power transistors.As transformers are not needed this makes the amplifier circuit much smaller for thesame amount of output, also there are no stray magnetic effects or transformerdistortion to effect the quality of the output signal. An example of a "transformerless"Class B amplifier circuit is given below.
Class B Transformerless Output StageThe Class B amplifier circuit above uses complimentary transistors for each half of thewaveform and while Class B amplifiers have a much high gain than the Class A types,one of the main disadvantages of class B type push-pull amplifiers is that they sufferfrom an effect known commonly as Crossover Distortion.We remember from our tutorials about Transistors that it takes approximately 0.7 volts(measured from base to emitter) to get a bipolar transistor to start conducting. In a pureclass B amplifier, the output transistors are not "pre-biased" to an "ON" state ofoperation. This means that the part of the output waveform which falls below this 0.7volt window will not be reproduced accurately as the transition between the twotransistors (when they are switching over from one to the other), the transistors do notstop or start conducting exactly at the zero crossover point even if they are speciallymatched pairs. The output transistors for each half of the waveform (positive andnegative) will each have a 0.7 volt area in which they will not be conducting resulting inboth transistors being "OFF" at the same time.A simple way to eliminate crossover distortion in a Class B amplifier is to add two smallvoltage sources to the circuit to bias both the transistors at a point slightly above theircut-off point. This then would give us what is commonly called an Class ABAmplifier circuit. However, it is impractical to add additional voltage sources to theamplifier circuit so pn-junctions are used to provide the additional bias in the form ofsilicon diodes.
In AB diagram, a small amount of biascurrent is flowing through the valve. Forthe output valves in a typical class ABguitar amplifier, this would amount toaround 30-40mA, with peaks ofapproximately 250-300mA.In the push-pull output stage, there is alittle overlap as each valve assists itsneighbour during a short transition, orcrossover period.Many larger guitar amplifiers are classAB, and well find out why a little later on.Crossover DistortionClass B suffers from a fundamental problem in that thepush-pull amplifier does not, in practice, move smoothlyfrom one half of the waveform to the other half.When the current outputs from the two valves are addedtogether by the output transformer, a kink can be seen onclass B amplifiers (diagram on right). Class AB amplifiers can suffer from this also, ifthe bias current is too low.In summary: Class B amplifiers generally introduce some crossover distortion. Class AB amplifiers may introduce some crossover distortion. Class A amplifiers introduce no crossover distortion, as both valves conduct simultaneously.
PUSH-PULL AMPLIFIERSOne use of phase splitters is to provide input signals to a single-stage amplifier thatuses twotransistors. These transistors are configured in such a way that the twooutputs, 180º out of phase witheach other, combine. This allows more gain than onetransistor could supply by itself. This "push-pull"amplifier is used where high poweroutput and good fidelity are needed: receiver output stages, publicaddress amplifiers,and AM modulators, for example.The circuit shown in figure 1-29 is a class A transistorpush-pull amplifier, but class AB or class Boperations can be used. Class operationswere discussed in an earlier topic. The phase splitter for thisamplifier is the transformerT1, although one of the phase splitters shown earlier in this topic could beused. R1provides the proper bias for Q1 and Q2. The tapped secondary of T1 develops the twoinputsignals for the bases of Q1 and Q2. Half of the original input signal will beamplified by Q-1, the otherhalf by Q-2. T2 combines (couples) the amplified outputsignal to the speaker and provides impedancematching.Figure 1-29.—Class A transistor push-pull amplifier.