Push pull output Push pull Amplifier

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Push-pull amplifier realized with NPN and PNP BJT is given in figure bellow.
Simulate push-pull output.

push-pull-output

Push-pull output is combination of NPN and PNP transistors, also called push pull amplifier. Each of the transistors operates as class B amplifier. NPN is conducting during positive swing of the input voltage, and PNP is conducting during negative swing of the input voltage. It should be emphasized that push pull amplifier amplifies the output current, not the input voltage. Class B amplifier means that amplifier amplifies signal exclusively during positive or negative input signal polarity. For that reason, class B amplifiers have better efficiency then class A amplifiers, since they consume power during half duration of the input signal and have no DC bias current when input voltage is zero. On the other hand, they have cross over distortion problem. Actually, output is 0.6V – 0.7V lower then the input signal (for positive periods of the input signal), and also 0.6V – 0.7V higher then the input signal (for negative periods of the input signal). So output voltage is a little bit attenuated, but current is significantly amplified. That’s why push-pull output is choice to be op-amp output. And also, input signal into push-pull amplifier needs no further voltage gain, but only current gain. Cross over distortion comes when input signal is in range from -0.7V to 0.7V. This range is “dead zone” for amplifying input signal, since then both NPN and PNP BJT are blocked and output is zero. By adding small bias with two diodes and resistors

cross-over-distortion

cross over distortion problem can be significantly reduced.

Go to push-pull amplifier simulation.

External Links:

Push-pull Amplifier on Ecircuitcenter
Push-pull Amplifier on Wikipedia
Push-pull Amplifier on Electronics-Tutorials
Push-pull Amplifier on Circuitstoday

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