A push pull amplifier is an amplifier which has an output stage that can drive a current in either direction through through the load. The output stage of a typical push pull amplifier consists of of two identical BJTs or MOSFETs one sourcing current through the load while the other one sinking the current from the load. Push pull amplifiers are superior over single ended amplifiers using a single transistor at the output for driving the load in terms github token distortion and performance.
A single ended amplifier, how well it may be designed will surely introduce some distortion due to the non linearity of its dynamic transfer characteristics. Push pull amplifiers are commonly used in situations where low distortion, high efficiency and high output power are required. Generally this splitting is done using an input coupling transformer. The circuit diagram of a typical Class A push pull amplifier is shown above.
Q1 and Q2 are two identical transistor and their emitter terminals are connected together.
R1 and R2 are meant for biasing the transistors. Power supply is connected between the center tap of the T2 primary and the emitter junction of the Q1 and Q2. Base terminal of each transistor is connected to the respective ends of the secondary of the input coupling transformer T1. Input signal is applied to the primary of T1 and output load RL is connected across the secondary of T2. From the figure you can see the phase splited signals being applied to the base of each transistors.
From the figure you can understand that the collector currents of Q1 and Q2 ie; I1 and I2 flows in the same direction trough the corresponding halves of the T2 primary. It is clear that the current through the T2 secondary is the difference between the two collector currents.
Harmonics will be much less in the output due to cancellation and this is results in low distortion. The Class B push pull amplifier is almost similar to the Class A push pull amplifier and the only difference is that there is no biasing resistors for a Class B push pull amplifier.
This means that the two transistors are biased at the cut off point. The Class B configuration can provide better power output and has higher efficiency up to Since the transistor are biased at the cutoff point, they consumes no power during idle condition and this adds to the efficiency.
The advantages of Class B push pull amplifiers are, ability to work in limited power supply conditions due to the higher efficiencyabsence of even harmonics in the output, simple circuitry when compared to the Class Why are my notifications not showing up on iphone configuration etc. The disadvantages are higher percentage of harmonic distortion when compared to the Class A, cancellation of power supply ripples is not as efficient as in Class A push pull amplifier and which results in the need of a well regulated power supply.
The circuit diagram of a classic Class B push pull amplifier is shown in the diagram below.Power amplifier classes are, in electronicsletter symbols applied to different power amplifier types. The class gives a broad indication of an amplifer's characteristics and performance.
Push pull amplifier
The classes are related to the time period that the active amplifier device is passing current, expressed as a fraction of the period of a signal waveform applied to the input. A class A amplifier is conducting through all the period of the signal; Class B only for one-half the input period, class C for much less than half the input period.
A Class D amplifier operates its output device in a switching manner; the fraction of the time that the device is conducting is adjusted so a pulse width modulation output is obtained from the stage.
Additional letter classes are defined for special purpose amplifiers, with additional active elements or particular power supply improvements; sometimes a new letter symbol is used by a manufacturer to promote its proprietary design.
The classes are based on the proportion of each input cycle conduction angle during which an amplifying device passes current. The angle of flow is closely related to the amplifier power efficiency. In the illustrations below, a bipolar junction transistor is shown as the amplifying device. The active element remains conducting  all of the time. Amplifying devices operating in class A conduct over the entire range of the input cycle.
A class-A amplifier is distinguished by the output stage devices being biased for class A operation. Subclass A2 is sometimes used to refer to vacuum-tube class-A stages that drive the grid slightly positive on signal peaks for slightly more power than normal class A A1; where the grid is always negative  . This, however, incurs higher signal distortion [ citation needed ]. Class-A power amplifier designs have largely been superseded by more efficient designs, though their simplicity makes them popular with some hobbyists.
There is a market for expensive high fidelity class-A amps considered a "cult item" among audiophiles  mainly for their absence of crossover distortion and reduced odd-harmonic and high-order harmonic distortion.
Power amplifier classes
Class A power amps are also used in some "boutique" guitar amplifiers due to their unique tonal quality and for reproducing vintage tones. Some hobbyists who prefer class-A amplifiers also prefer the use of thermionic valve tube designs instead of transistors, for several reasons:.
Transistors are much less expensive than tubes so more elaborate designs that use more parts are still less expensive to manufacture than tube designs. A classic application for a pair of class-A devices is the long-tailed pairwhich is exceptionally linear, and forms the basis of many more complex circuits, including many audio amplifiers and almost all op-amps.
Class-A amplifiers may be used in output stages of op-amps  although the accuracy of the bias in low cost op-amps such as the may result in class A or class AB or class B performance, varying from device to device or with temperature. They are sometimes used as medium-power, low-efficiency, and high-cost audio power amplifiers. The power consumption is unrelated to the output power. At idle no inputthe power consumption is essentially the same as at high output volume.
The result is low efficiency and high heat dissipation.In class B push pull amplifier, output current collector current flows for only half the cycle of the input signal. Hence distortion is excessive. Single ended operation is, therefore, not possible in class B audio amplifier.
Class B audio un-tuned amplifier must necessarily use push pull operation to reduce distortion. In effect, in class B push pull amplifier, one transistor say Q 1 conduct during one half cycle while the other transistor namely Q 2 conducts during the other half cycle.
Class A power amplifiers may use either single ended or push pull operations. However, in most class A power amplifierpush pull operation is preferred because of the various merits mentioned above. The circuit of class B push pull amplifier is the same as that of class A push pull amplifier. The difference lies in the order of biasing. In class B circuit, the transistor are biased approximately to cutoff. Hence the circuit of figure 1 represents class B push pull amplifier also if resistor R 1 is made zero because a silicon transistor is basically at cutoff if its base is short circuited to emitter.
Because of zero power drainage with zero signal, in amplifiers using solar cell or simple batteries, class B operation is preferred. We assume that the transistor characteristics are linear, parallel and equi-spaced for equal increments of excitation so that the dynamic transfer characteristics is a straight line as shown in figure 2. We also assume that the minimum current is zero. Figure 2 demonstrates the graphical method of determination of collector current and collector voltage wave shaped for a single transistor say transistor Q 1 operating under class B condition.
With sinusoidal excitation base current i Bthe collector current i C and the collector voltage v C are also sinusoidal during one-half of the a. The effective load resistance R L1 for one transistor is where R L is the load resistance across the secondary of the output transformer TR 2N 1 is the number of turns on one-half of the primary and N 2 is the number of turns on the secondary as shown in figure 1.
Waveform of transistor Q 2 are similar to those shown in figure 2 but 0 out of phase. Hence the load current in the secondary being proportional to the difference of the two collector currents, forms a perfect sine wave.
Hence for this half sine wave. Equation 5 shows that the maximum possible value of collector circuit efficiency for class B amplifier is i. Basically, this high value of collector circuit efficiency in class B amplifier becomes possible because of the fact that in class B amplifier, current is zero with zero excitation whereas in class A amplifierthere is drain from V CC source even at zero signal.
It is worth nothing that in a class B amplifier, collector dissipation is zero signal and increases with increase of signal magnitude while in a class A amplifiercollector dissipation is maximum with zero signal and decreases with increases of signal magnitude.
In class B amplifier, the d. Hence power supply regulation must be good. Total collector dissipation P D in both transistors is equal to the dc power input from collector supply voltage minus the power delivered to the load.
Hence, as per equation 8, collector dissipation is zero with zero excitation. Using equation 8, it may be shown that the collector dissipation increases with increase of excitation and passes through a maximum at. Hence maximum collector dissipation P Dmax is obtained from Equation 8 on putting. For a class AB amplifier designed to deliver maximum power output of say 50 watts, P Dmax — 20 watts i. Hence, we conclude that class B pushpull amplifier is capable of providing output power 5 times the power dissipation capability of each transistor.
This complete power of watts must be dissipated in the two transistors are zero signal.We learned that the conduction angle of a Class A amplifier is degrees, meaning that the amplifying element is conducting current throughout the entire cycle of a sine wave that is being amplified. We saw that Class A amplifiers offer reasonable linearity, but have poor performance with respect to efficiency. We are now going to look at push-pull Class B and Class AB amplifiers, which are comprised of devices with conduction angles less than degrees.
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. We will investigate crossover distortion and some commonly-used methods to ameliorate it as we progress through the lab.
An ideal Class B amplifier has a conduction angle of degrees, or one half-cycle of a sine wave. This type of amplifier therefore requires two amplifying elements to produce a full sine wave at its output. One of the elements conducts for the positive portion of the signal being amplified and the other conducts for the negative portion of the signal. For a sine wave, the positive portion of the signal is the positive degree half-cycle of the waveform and the negative portion of the signal is the negative degree half-cycle of the waveform.
This is where the definition of the degree conduction angle originates. An amplifier that uses two amplifying elements in this type of arrangement is often referred to as a push-pull Class B amplifier because one device pushes current into the load and the other pulls current from the load.
V BE characteristics. If we are designing the positive half-cycle section of a complementary push-pull Class B amplifier with a NPN BJT, we could connect the emitter to ground and drive the base directly. In this case the input voltage would have to rise to at least 0. When the input was substantially less than this, the transistor would be off and no significant emitter or collector current would flow.
It is not completely correct to describe this type of amplifier as Class B, therefore, since no current flows in the devices in their dead zones, and this situation produces a conduction angle of less than degrees. This undesirable type of distortion happens when the sine wave is crossing over from positive to negative, or negative to positive, and is appropriately called crossover distortion.
Crossover distortion in a BJT-based push-pull Class B amplifier is generally unacceptable because it is so large, so we need to find ways to minimize it. The most common way is to provide a small amount of bias to each transistor such that they are just barely on when the signal crosses over from negative to positive or positive to negative. If we were to bias each transistor such that it is perfectly on the verge of conducting with no input signal applied, we would have something as close to a perfect push-pull Class B amplifier as we could achieve since the conduction angle for each device would be very close to degrees.
In other words, an extremely small input voltage would cause one of the transistors to go from the non-conducting state to the conducting state. Real transistors do not have a perfectly abrupt transition between non-conducting and conducting states, however, so we cannot achieve perfect Class B operation even with the bias added.
The best solution is to bias each transistor such that it is conducting a small amount with no input signal applied. Because we are adding a small amount of bias in addition to what is required to overcome the base-emitter drop, the resulting amplifier is called a Class AB amplifier because it has a small amount of Class A bias in it to reduce the crossover distortion.
We can lower the crossover distortion if we bias the input voltage up at least 0. By doing this to each section we can cancel out most of the ill effects incurred due to the V BE voltage drop.
This can be accomplished are by using another transistor or diode to provide the V BE voltage drop, using a resistive voltage divider not usually done for reasons we will see lateror some other equivalent means.
Diodes and transistors that are used to shift the DC levels of signals are called level shifters. Class AB amplifiers are often used as amplifier output stages in emitter-follower and common-emitter configurations. The common-emitter Class AB stage is used in rail-to-rail operational amplifier op-amp stages in order to allow the output voltage to swing very close to the power supply voltages.
In this lab we will first build a push-pull class B common-emitter amplifier comprised of one NPN and one PNP transistor and use this to illustrate the large crossover distortion that is produced without transistor biasing. We then add biasing and observe how doing this significantly reduces the crossover distortion. Providing a thermally stable, well-positioned bias point can be the most challenging part of a Class AB amplifier design. To study and understand push-pull Class B amplifiers constructed with BJTs as the amplifying elements and view the characteristic crossover distortion associated with them.
To see how using level shifters to add a small amount of bias to the Class B amplifier sections to produce a push-pull Class AB amplifier can significantly reduce crossover distortion.
To understand how the crossover distortion mechanism differs from other distortion mechanisms, and gets worse as a percentage of signal amplitude as the output signal gets smaller. To understand how thermal runaway can occur in Class AB amplifiers and investigate techniques to prevent it.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.
The reduced conduction angle improves the efficiency to a great extend but causes a lot of distortion. Due to the huge amounts of distortion, the Class C configurations are not used in audio applications. The most common application of the Class C amplifier is the RF radio frequency circuits like RF oscillator, RF amplifier etc where there are additional tuned circuits for retrieving the original input signal from the pulsed output of the Class C amplifier and so the distortion caused by the amplifier has little effect on the final output.
Input and output waveforms of a typical Class C power amplifier is shown in the figure below. From the above figure it is clear that more than half of the input signal is missing in the output and the output is in the form of some sort of a pulse.
In the above figure you can see that the operating point is placed some way below the cut-off point in the DC load-line and so only a fraction of the input waveform is available at the output. Biasing resistor Rb pulls the base of Q1 further downwards and the Q-point will be set some way below the cut-off point in the DC load line. That is the reason why the major portion of the input signal is absent in the output signal. Inductor L1 and capacitor C1 forms a tank circuit which aids in the extraction of the required signal from the pulsed output of the transistor.
Actual job of the active element transistor here is to produce a series of current pulses according to the input and make it flow through the resonant circuit. Values of L1 and C1 are so selected that the resonant circuit oscillates in the frequency of the input signal.
Since the resonant circuit oscillates in one frequency generally the carrier frequency all other frequencies are attenuated and the required frequency can be squeezed out using a suitably tuned load. Harmonics or noise present in the output signal can be eliminated using additional filters.
A coupling transformer can be used for transferring the power to the load. RF oscillators. RF amplifier. FM transmitters. Booster amplifiers. High frequency repeaters. Tuned amplifiers etc. Author jojo. Do you know how RFID wallets work and how to make one yourself?
December 21, Dan 2 years ago. Nicely done explanation, concise, and to the point. Isaac Asante 8 years ago. Tittu thanks 4 more details.EEVblog #600 - OpAmps Tutorial - What is an Operational Amplifier?
Tittu 8 years ago. Submit Type above and press Enter to search. Press Esc to cancel.In our previous article, we have introduced you to amplifiers and its types. Here in this article, we are going to explore about Push-Pull Amplifierits circuit, operation, advantages, and disadvantages in detail. A push-pull amplifier is a type of amplifier that can drive current in either direction through the load. In this arrangement, one transistor amplifies the positive half cycle whereas another transistor amplifies the negative half cycle of the signal i.
All audio power amplifiers used in the record player, transistor radio receivers, tape recorders etc make use of push-pull arrangement because these systems are usually operated by batteries where efficiency is the primary factor.
Both the transistors are operated in class B operation i. Through a driver transformer, the input signal is given to the circuit from the driver stage. The secondary of driver transformer is center tapped which supplies equal and opposite voltages to the base circuits of the two transistors.
Cente tap primary of the output transformer is connected to the collector circuit and at the secondary side, the load is connected. To get maximum output from the load, a suitable impedance matching is obtained by designing proper turn ratio of the output transformer. In a push-pull ampthe input signal appears across the secondary AB of the driver transformer.
By this, the base-emitter junction of the transistor T1 becomes forward biased and T2 becomes reverse biased. This further allows the current conduction across the transistor T1 and makes the transistor T2 in the cut-off state. Therefore, the first half cycle of the signal is amplified by the transistor T1 and appears in the upper half of the primary of the output transformer.
So now, the base-emitter junction of the transistor T2 is forward biased and T1 is reverse biased which allow the current conduction across the transistor T2 and make the transistor T1 in the cut-off state.
Therefore, the first half cycle of the signal is amplified by the transistor T2 and appears in the lower half of the primary of the output transformer. Complete sine wave output in the secondary is obtained when the center tapped primary of the output transformer combines the two halves of the cycle. By proper impedance matching, maximum power can be transferred to the load.
As we know, due to the nonlinearity of the transistor, even if the input is sinusoidal, the output current contains harmonics. The output i. In this amplifier, the voltage induced in the secondary of the output transformer is proportional to the difference between the two collector currents i. Note: In push-pull amplifiers, all even harmonics get canceled. Push Pull Amplifier Advantages:. Push Pull Amplifier Disadvantages:. Hope you all like this article.
For any suggestions please comment below. We always appreciate your suggestions. Rate the article below. Thanks Sahil, If you want to know about any topic related to this blog, shoot your question in comments. Plz help Is operation means working of push pull Also is there any difference between class B amplifier and push pull amplifier. Hi Shah Burhan, Thanks for commenting.
Yes, operation means working of a push-pull amplifier. There can be a class A push-pull too. Not reserved for class B only. The plain Class B is able to amplify either positive or negative half cycle of the full signal waveform.
So, obviously, we need two class B stages connected as push pull to have the full cycle of the input signal amplified. In one half, one of the amplifiers seems to push the amplitude up, while on the other half, the other takes over and seems to pull the amplitude down. The combined effect is to give a large amplification of the waveform suitable for the last stage of power amplification.
Feel free to ask any question. We are glad that you find our website useful for you.The class A and class B amplifier so far discussed has got few limitations. Let us now try to combine these two to get a new circuit which would have all the advantages of both class A and class B amplifier without their inefficiencies.
Before that, let us also go through another important problem, called as Cross over distortionthe output of class B encounters with. In the push-pull configuration, the two identical transistors get into conduction, one after the other and the output produced will be the combination of both. When the signal changes or crosses over from one transistor to the other at the zero voltage point, it produces an amount of distortion to the output wave shape. For a transistor in order to conduct, the base emitter junction should cross 0.
At the zero voltage point, the transition period of switching over the transistors from one to the other, has its effect which leads to the instances where both the transistors are OFF at a time. Such instances can be called as Flat spot or Dead band on the output wave shape. The above figure clearly shows the cross over distortion which is prominent in the output waveform.
Class AB and Class C Power Amplifiers
This is the main disadvantage. This cross over distortion effect also reduces the overall peak to peak value of the output waveform which in turn reduces the maximum power output. This can be more clearly understood through the non-linear characteristic of the waveform as shown below.
It is understood that this cross-over distortion is less pronounced for large input signals, where as it causes severe disturbance for small input signals. This idea leads to the invention of class AB amplifier, which is the combination of both class A and class B amplifiers, as discussed below. As the name implies, class AB is a combination of class A and class B type of amplifiers. 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.
The cross over distortion is the problem that occurs when both the transistors are OFF at the same instant, during the transition period. In order to eliminate this, the condition has to be chosen for more than one half cycle. Hence, the other transistor gets into conduction, before the operating transistor switches to cut off state.
This is achieved only by using class AB configuration, as shown in the following circuit diagram.