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A Level     Audio     Power and RMS     Pre-Amplifier     >Push-Pull<     Tone Controls    

Audio Push-Pull


Site for Eduqas/WJEC - Go to the AQA site.

  A Level    Audio  0 of 28    Question 1311    Audio Amplifiers 
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Deadline not set1 of 28    Question 1274    Audio Amplifiers 
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Deadline not set2 of 28    Question 653    Audio Power Amp 09 
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Deadline not set3 of 28    Question 654    Audio Power Amp 10 
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Deadline not set4 of 28    Question 655    Audio Power Amp 11 
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Deadline not set5 of 28    Question 656    Audio Power Amp 12 
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Deadline not set6 of 28    Question 657    Audio Power Amp 13 
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Deadline not set7 of 28    Question 658    Audio Power Amp 14 
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Deadline not set8 of 28    Question 660    Audio Power Amp 16 
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Deadline not set9 of 28    Question 661    Audio Power Amp 17 
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Deadline not set10 of 28    Question 662    Audio Power Amp 18 
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Deadline not set11 of 28    Question 663    Audio Power Amp 19 
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Deadline not set12 of 28    Question 664    Audio Power Amp 20 
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Deadline not set13 of 28    Question 918    Audio Power Amp 21 
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Deadline not set14 of 28    Question 665    Audio Power Amp 21 
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Deadline not set15 of 28    Question 666    Audio Power Amp 22 
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Deadline not set16 of 28    Question 667    Audio Power Amp 23 
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Deadline not set17 of 28    Question 355    Biasing 
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Deadline not set18 of 28    Question 944    Biasing 
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Deadline not set19 of 28    Question 945    Biasing and Signal Coupling 
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Deadline not set20 of 28    Question 1289    Oscilloscope 
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Deadline not set21 of 28    Question 881    Oscilloscope 
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Deadline not set22 of 28    Question 1273    Oscilloscope 
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Deadline not set23 of 28    Question 1440    Push Pull Amplifier 
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Deadline not set24 of 28    Question 739    Push Pull Amplifier 
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Deadline not set25 of 28    Question 740    Push Pull Amplifier 
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Deadline not set26 of 28    Question 741    Push Pull Amplifier 
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Deadline not set27 of 28    Question 742    Push Pull Amplifier 
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Deadline not set28 of 28    Question 743    Push Pull Amplifier 
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Uses

Source Followers

Power Output (Ideal Circuit)

PMAX    =    VRMS2 / RL    =    VS / (2 RL)        There is more information on the Power and RMS page.

Cross Over Distortion

This simple circuit suffers from cross over distortion.

The red trace is the input signal. The blue trace is the output.

MOSFET Push Pull Amp MOSFET Push Pull Amp Cross Over Distortion

Bias the MOSFETs

This diagram shows simple biasing using diodes and resistors. 0.7 Volts is lost across the diodes so the output will be lower than expected compared with using ideal components. It is possible to use LEDs. In this case about two Volts will be lost.

MOSFET Push Pull Amp with Bias

Use Negative Feedback

The red trace is the input signal. The blue trace is the output. The distortion has gone.

MOSFET Push Pull Amp with Bias and Feedback MOSFET Push Pull Amp Cross Over Distortion Removed

This push-pull amplifier uses a voltage follower and MOSFET biasing. It runs on + and - 12 Volts and is similar to the diagram above.

Push Pull Amplifier Without a Heatsink

Push Pull Advantages

Push Pull Disadvantages

Saturation, Clipping, Limiting

The image below shows ideal (black) and non-ideal (red and blue) behaviour including clipping when the op amp is saturated and the output voltage can go no higher.

Amplifiers of any type can not produce output voltages that are larger than the power supply voltages. If the input is too big, the amplifier output will increase until it is nearly equal to the supply voltage. After that the output voltage can not rise any more. The black line shows the amplifier input signal. The red line shows the output from the N Channel MOSFET. The blue line shows the output from the P Channel MOSFET.

Saturation, Clipping or Limiting

Circuit suffering from Clipping, Saturation or Limiting

This can be eliminated by using a higher power supply voltage as long as all the components can handle this and also the extra waste heat produced.

Clipping, Limiting or Saturation

Bipolar Transistor Push Pull

Bipolar Transistor Push Pull Amplifier

Before the invention of the MOSFET, versions of this circuit were very commonly used.
To prevent cross over distortion, the emitter followers are biased using a combination of resistors and diodes.
This circuit uses negative feedback to further reduce distortion.
All the MOSFET material above also applies to this circuit.
Since MOSFETs are superior in almost all respects, this circuit is now rarely found.

MOSFETS ...

MOSFETs lose more voltage (gate source 2 to 4 volts) than bipolar transistors (base emitter 0.7 volts).
This disadvantage can be overcome by using a slightly higher power supply voltage. 

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