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OP-AMP Based Push Pull Amplifier



To design and study the push pull amplifier.
Components required:

 Function generator, CRO, Regulated Power supply, pnp and npn transistors ,resistance, connecting wires.
Vlab Specifications Taken:
Push pull amplifier circuit design has been implemented on the virtual breadboard using following specifications:
  • Power Supply: +12v and -12v
  • Function generator: Selected wave with following specifications:
Frequency =1 kHz
Amplitude: 2V
Duty cycle = 50%
A push-pull output is a type of electronic circuit that can drive either a positive or a negative current into a load. Push–pull outputs are present in TTL and CMOS digital logic circuits and in some types of amplifiers, and are usually realized as 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 special configuration of push-pull, though in fact an exception, are the outputs of TTL and related families. The upper transistor is functioning as an active pull-up, in linear mode, while the lower transistor works digitally. For this reason they aren't capable of supplying as much current as they can sink (typically 20 times less). Because of the way these circuits are drawn schematically, with two transistors stacked vertically, normally with a protection diode in between, they are called "totem pole" outputs.
Class B and AB

Class B amplifiers only amplify half of the input wave cycle, thus creating a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. Class B has a maximum theoretical efficiency of 78.5% (i.e., π/4). This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A single Class B element is rarely found in practice, though it has been used for driving the loudspeaker in the early IBM Personal Computers with beeps, and it can be used in RF power amplifier where the distortion levels are less important. However Class C is more commonly used for this.

A practical circuit using Class B elements is the push-pull stage, such as the very simplified complementary pair arrangement shown below. Here, complementary or quasi-complementary devices are each used for amplifying the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch in the cross-over region - at the "joins" between the two halves of the signal, as one output device has to take over supplying power exactly as the other finishes. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they're not in use. This approach is called Class AB operation.

In Class AB operation, each device operates the same way as in Class B over half the waveform, but also conducts a small amount on the other half. As a result, the region where both devices simultaneously are nearly off (the "dead zone") is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimised or eliminated altogether. The exact choice of quiescent current, the standing current through both devices when there is no signal, makes a large difference to the level of distortion (and to the risk of thermal runaway, that may damage the devices); often the bias voltage applied to set this quiescent current has to be adjusted with the temperature of the output transistors (for example in the circuit at the beginning of the article the diodes would be mounted physically close to the output transistors, and chosen to have a matched temperature coefficient). Another approach (often used as well as thermally-tracking bias voltages) is to include small value resistors in series with the emitters.

Class AB sacrifices some efficiency over class B in favor of linearity, thus is less efficient (below 78.5% for full-amplitude sinewaves in transistor amplifiers, typically; much less is common in Class AB vacuum tube amplifiers). It is typically much more efficient than class A.

Class B or AB push-pull circuits are the most common design type found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the "class A" region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. The crossover distortion can be reduced further by using negative feedback. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios.
The simple circuit configuration of push pull amplifier is shown in figure 1. Which uses complementary transistors, one of the transistors is a npn and the other is a pnp. The two transistors in a class-B amplifier conduct on alternating half-cycles of the input. The combined half-cycles then provide an output for a full 3600  of operation.
No Input :-
When the transistor is in its quiescent state (no input), both transistors are biased at cutoff.
Positive Input :-
During the positive half-cycle of the input signal, Q1 is biased above cutoff,  and conduction results through the transistor RL. During this time, Q2 is still biased at cutoff.
Negative Input :-
During the  negative half-cycle of the input signal,  Q1 is returned to the cutoff state, and Q2 is biased above cutoff. As a result, conduction of Q2 start to built while Q1 remains off.
The combined half-cycles then provide an output for a full 3600 of operation.
Fig1. Simple Push Pull Amplifier.
Crossover distortion
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. This results in a condition that is commonly called Crossover Distortion.
Crossover Distortion produces a zero voltage "flat spot" or "deadband" on the output wave shape as it crosses over from one half of the waveform to the other. The reason for this is that the transition period when the transistors are switching over from one to the other, does not stop or start exactly at the zero crossover point thus causing a small delay between the first transistor turning "OFF" and the second transistor turning "ON". This delay results in both transistors being switched "OFF" at the same instant in time producing an output wave shape as shown below.
Fig.2 Crossover Distortion Waveform
  1. Connect the circuit as shown in the circuit diagram.
  2. Give the input signal as specified.
  3. Switch on the power supply.
  4. Note down the outputs from the CRO.
  1. Observe the output waveform from CRO.
  2. Measure the frequency and the voltage of the output waveform in the CRO. Rectified output can be observed.
  3. Observe the cross over distortion.
VLab Observations Obtained:
1.      After Clicking on function generator icon on the left of the Vlab live environment page, set the frequency, amplitude and the type of waveform on function generator.
2.      Select sine wave on the function generator, click on the frequency button and then set frequency1KHz.
3.      Click on the amplitude button and select the amplitude of the sine wave, for example 2V.
4.               Check graph
5.      Circuit has been designed on the virtual breadboard with the help of procedure.
6.      Then on clicking on Run icon, the output waveform generated and the input can be observed on the CRO screen. CRO web page can be opened using icon oscilloscope at top left on the live experiment page.
7.         Click on measure and then click on quick measure. Then one can observe options like source, select, measure.

=>Click on source and select 4 i.e. the input wave.
=>Click on select and select the parameter to be measured, for example select frequency or amplitude.
=>Click on measure to get the frequency and amplitude of the input waveform.

It comes out to be Frequency: 1KHz, amplitude: 2.13V
8.      Now observe the amplitude of the output waveform.

=>Click on source and select 1 i.e. the input wave.
=>Click on select and select amplitude.
=>Click on measure to get the amplitude of the input waveform.

The output voltage comes out to be 1.06V same as the input wave.
9.      Observe the cross over distortion
The crossover time is approx. 180micro sec.
Push pull amplifier circuit using opamp  is designed and been studied successfully.



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