Block diagram of push pull amplifier transistors

Effective date : Year of fee payment : 4. Year of fee payment : 8. A power amplifier circuit drives a load with a convergence correction current which is proportional to a convergence correction voltage waveform that is applied to the power amplifier circuit. A predriver stage of the power amplifier circuit comprises first and second transistors in a push-pull configuration. First and second voltage divider networks bias emitter electrodes of the first and second transistors, respectively, so that a common-mode current flow between the first and second transistors is limited.

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Content:

• Class-B Pushpull Amplifier Operation
• Module 3.4
• US20140155126A1 - Push-pull amplifier with quiescent current adjuster - Google Patents
• US5814953A - Power amplifier predriver stage - Google Patents
• Push-Pull Class A Power Amplifier
• Ask or Answer
• Push–pull output
• AES E-Library
• Push-Pull Amplifiers Circuit Diagram, Working and Applications

WATCH RELATED VIDEO: Class AB Audio Amplifier

Class-B Pushpull Amplifier Operation

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 later , or 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.

Following completion of this lab you should be able to explain the basic operation of push-pull Class B and Class AB amplifiers that use BJTs as the amplifying elements, describe crossover distortion, explain how and why it happens, and how it differs from distortion caused by other mechanisms, describe what level shifters do, and explain what can cause thermal runaway in Class AB amplifiers, why it can be dangerous, and how it can be prevented.

The clear solution to this problem was to add just enough DC bias voltage such that each transistor is slightly conducting collector current with no signal applied to the amplifier input. This solution works quite well, but has a few drawbacks, one of which is the potential for thermal runaway. Thermal runaway is a form of positive feedback, sometimes referred to as a self-reinforcing feedback loop, in which a change in a system operating condition pushes the system away from a desired stable point instead of toward a stable point.

Stable systems are designed using negative feedback in which changes in the system operating conditions push the system back toward its desired stable state. A simple example of a naturally-occurring negative feedback system is the system that regulates the amount of light that enters the eye by controlling the diameter of the iris diaphragm. When the light entering the eye increases beyond the ideal amount, the iris diaphragm decreases in diameter until the light reaches its ideal level.

Similarly, when the light entering the eye decreases below the ideal level, the iris diaphragm increases until the light reaches its ideal level. Thus, negative feedback drives the system to decrease the error between an ideal condition and the actual condition. Positive feedback systems do just the opposite of negative feedback systems. A simple example of thermal runaway is what can can occur in carbon composition resistors that have negative temperature coefficients NTC , that is, the resistances of these resistors decrease with temperature.

As a NTC resistor heats up its resistance decreases, causing it to draw more current, which further decreases its resistance, causing it to draw even more current, and so on. If not limited by something, this process can continue until the resistor overheats and destroys itself.

The worst bias arrangements for Class AB amplifiers with respect to thermal runaway are those that use bias voltages that are relatively fixed over temperature. A resistive voltage divider network provides bias voltages that are relatively fixed with temperature, and this is why they are not widely used as Class AB amplifier bias circuits.

The following schematic illustrates a simple resistive voltage divider bias circuit that could be used to bias a BJT-based Class AB amplifier. A potentiometer could also be placed between the two bases with a slightly different arrangement to provide an adjustable bias. In the schematic above the input signal would be applied between R B2 and R B3.

When properly designed, the voltage between the two bases would be equal to 2V BE on where V BE on is defined as the base-emitter voltage at which each transistor begins to conduct.

The thermal runaway problem with fixed bias can now be addressed. The collector current and base-emitter voltage have an exponential relationship, which must be obeyed. Since the base-emitter voltages are not allowed to decrease due to the fixed bias voltages provided by the voltage divider, the collector current increases instead according to the exponential dependence of I C on V BE. This arrangement presents an exponential dependence of collector current on temperature. As the collector current increases the base-emitter junction temperature increases, further increasing the collector current.

Much as was the case in the carbon composition resistor, we have an unstable self-reinforcing feedback loop in the amplifier which produces thermal runaway, and will eventually destroy the transistors. Fortunately, there are a number of better biasing schemes that can be used to avoid thermal runaway. If we replace R B2 and R B3 with forward biased diodes, or transistors configured as diodes, we can cancel out the effects of the negative V BE temperature to a great degree as long as the current vs.

This is what we did in the lab when we added the transistors connected as diodes to the Class B amplifier to change it to a Class AB amplifier. A further improvement can be made to the circuit by converting the transistors connected as diodes to emitter follower stages, providing a high input impedance to the amplifier. This type of arrangement is commonly called a diamond stage. Sometimes, however, this thermal compensation may not be enough to entirely avoid thermal runaway, since devices are not perfectly matched, and temperatures can vary between devices.

A heat sink, if available, can be used to help equalize the temperature among all of the devices if all of the devices are thermally connected to it. Another technique that is often used is to place small resistors in series with the emitters of the amplifier transistors.

This is why we placed the two 1. Crossover distortion is worse than other types of distortion since it is not reduced as a percentage of signal amplitude as the signal amplitude is reduced. Other distortion mechanisms produce harmonic distortion distortion of the shape of a sine wave from its ideal shape that scale with amplitude over a wide dynamic range, giving a rather constant percentage of harmonic distortion as the signal amplitude is varied.

Crossover distortion, on the other hand, does not decrease with signal amplitude -- the dead zones stay the same -- so the distortion percentage in a signal actually increases as its amplitude decreases. Fortunately, negative feedback is often provided around amplifiers with push-pull Class AB output stages, which significantly reduces signal distortions.

Clearly, the amplifiers themselves should be designed to produce minimal crossover distortion without the negative feedback. Return to Engineering Discovery Index.

Analog Devices Wiki. Analog Devices Wiki Resources and Tools. Quick Start Guides. Linux Software Drivers. Microcontroller Software Drivers. ACE Software. Technical Guides. Education Content. University Program Overview. Teaching and Lab Materials. Wiki Help. About Wiki. This version 03 Jan was approved by Doug Mercer. The Previously approved version 05 Dec is available. Data sheet handout for the 2N NPN transistor. Construct the following circuit on the solderless breadboard. Refer to the illustration below for one way to install the components in the solderless breadboard.

Observe the output voltage at the connection between the two emitters on Channel B and observe the large amount of crossover distortion that is present in the output signal as shown in the image below. Observe the output voltage at the connection between the two emitters on Channel B and observe how the crossover distortion has been significantly improved due to the addition of the level shifting transistors, connected as diodes.

Estimate the voltage loss factor due to the voltage divider formed between the amplifier output resistance and the load resistance. Increase the input voltage amplitude to swing between 0.

Ideal Class B amplifiers have conduction angles of degrees.

Module 3.4

So far, we have seen two types of class A power amplifiers. The main problems that should be dealt with are low power output and efficiency. It is possible to obtain greater power output and efficiency than that of the Class A amplifier by using a combinational transistor pair called as Push-Pull configuration. The construction of the class A power amplifier circuit in push-pull configuration is shown as in the figure below.

US20140155126A1 - Push-pull amplifier with quiescent current adjuster - Google Patents

We use Cookies to give you best experience on our website. By using our website and services, you expressly agree to the placement of our performance, functionality and advertising cookies. Please see our Privacy Policy for more information. The goal was W. Advanced Power Technology, Inc. It doubles. Frequency Response Diagram for Power Supply. From both performance and cost perspectives, these limitations are undesirable. Power converters based on the half-bridge topology are widely employed in the power supply industry and are , the input voltage or the ground.

US5814953A - Power amplifier predriver stage - Google Patents

Effective date : Year of fee payment : 4. A parallel push-pull amplifier using a complementary device, which basically operates for a B or AB-level amplification while having a common source configuration, thereby being capable of amplifying the full wave of an input signal without any distortion while obtaining a high gain at a radio frequency. The complementary device consists of an active element for amplifying a half wave of an input signal and a complementary active element for amplifying the other half wave of the input signal. The complementary active element has a duality with respect to the active element.

Push-Pull Class A Power Amplifier

An electronic amplifier , amplifier , or informally amp is an electronic device that increases the power of a signal. It does this by taking energy from a power supply and controlling the output to match the input signal shape but with a larger amplitude. In this sense, an amplifier modulates the output of the power supply. Numerous types of electronic amplifiers are specialized to various applications. An amplifier can refer to anything from a electrical circuit that uses a single active component, to a complete system such as a packaged audio hi-fi amplifier.

Ask or Answer

Facebook Twitter. Class-B Pushpull Amplifier Operation: Class - B operation is provided when the dc bias leaves the transistor biased just off, the transistor turning on when the ac signal is applied. This is essentially no bias, and the transistor conducts current for only one-half of the signal cycle. To obtain output for the full cycle of signal, it is necessary to use two transistors and have each conduct on opposite half-cycles, the combined operation providing a full cycle of output signal. Since one part of the circuit pushes the signal high during one half-cycle and the other part pulls the signal low during the other half-cycle, the circuit is referred to as a push-pull circuit. Figure shows a diagram for push-pull operation.

Push–pull output

The Application Activity in this is a public address system. Recall that the complete system includes the preamplifier, a power amplifier, and a dc power supply. You will focus on the power amplifier in this section and complete the total system by combining the three component parts.

AES E-Library

A push—pull amplifier is a type of electronic circuit that uses a pair of active devices that alternately supply current to, or absorb current from, a connected load. This kind of amplifier can enhance both the load capacity and switching speed. Push—pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers , and are usually realized by a complementary pair of transistors , one dissipating or sinking current from the load to ground or a negative power supply, and the other supplying or sourcing current to the load from a positive power supply. A push—pull amplifier is more efficient than a single-ended "class-A" amplifier. The output power that can be achieved is higher than the continuous dissipation rating of either transistor or tube used alone and increases the power available for a given supply voltage.

Push-Pull Amplifiers Circuit Diagram, Working and Applications

Push-Pull Amplifier is a power amplifier which is used to supply high power to the load. One transistor pushes the output on positive half cycle and other pulls on negative half cycle, this is why it is known as Push-Pull Amplifier. The advantage of Push-Pull amplifier is that there is no power dissipated in output transistor when signal is not present. Class A configuration is the most common power amplifier configuration. It consists of only one switching transistor which is set to remain ON always.

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 of distortion and performance.