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MOSFET Push Pull Amplifier


This is the AQA version closing after June 2019. Visit the the version for Eduqas instead.

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AS A2

 a

Uses

 b

Source Followers

 c

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.gif MOSFET Push Pull Amp XOD.gif
 d

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 Biased.gif

 e

Adjustable Bias and Quiescent Current

The diagram below is similar but has adjustable biasing. The additional voltage divider resistors, with Rv adjustable are chosen so that both MOSFETS are just on the point of turning on. Rv is adjusted to give a small quiescent current (the current flowing when there is no input signal).

Looking at the graphs, the N Channel MOSFET needs about +3.5V to just start it conducting. The P Channel MOSFET needs -3.5V. The potential difference measured by the voltmeter will be 7 Volts.

Coupling capacitors are needed to get the AC input to the MOSFET gates at the same time as blocking the DC bias voltages. This circuit can not be used to amplify DC signals.

Diodes could be included with the biasing resistors. These would improve the thermal stability of the circuit by tending to shut down an overheating circuit.

MOSFET Push Pull Amp Biased 4.gif

MOSFET Push Pull Amp XOD 2.gif The red trace is the input signal. The blue trace is the output. The distortion is reduced.

 f

Use Negative Feedback

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

MOSFET Push Pull Amp Biased 3.gif MOSFET Push Pull Amp XOD 3.gif

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

 g

Push Pull Advantages

 h

Push Pull Disadvantages

 i

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

 j

RMS Output Power

Vrms = 0.7 x Vpeak

Power = Vrms2 / R

Power = (20 x 0.7)2 / 8

Power = 24.5 Watts

This is the theoretical maximum power output.

 k

Real Life Power Output

In real life, MOSFET push pull source followers are not perfect. The output will be lower than expected because ...

  1. The driver op-amp saturates a couple of volts below the power supply voltage.
  2. 2 or 3 volts are lost across the gate source junction in the MOSFETs.
  3. 0.7 to 4 Volts get lost in the biasing diodes depending on the type of diode used.
  4. The MOSFETs have Drain to Source resistance. Energy is lost here.

Points 1 to 3 above can be fixed by running the op-amp driver and MOSFET biasing on a higher power supply voltage. As these are low power circuits, this is not too expensive to do.

 l

Falstad Simulations

 m

Simplest Circuit - Bad Crossover Distortion

Push Pull No Bias No Feedback

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with no Bias and no Negative Feedback
In options, check European Resistors and uncheck Conventional Current.

Alternatively view Push_Pull_No_Bias_No_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

 n

Circuit With Biasing - Improved Crossover Distortion

Push Pull With Bias No Feedback

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with Bias but no Negative Feedback
In options, check European Resistors and uncheck Conventional Current.

Alternatively view Push_Pull_Bias_No_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

 o

Circuit With Biasing and Negtive Feedback - Minimal Distortion

Push Pull With Bias And Feedback

For the Falstad Circuit Simulation, CTRL+Click Push Pull Source Followers with Bias and Negative Feedback
In options, check European Resistors and uncheck Conventional Current.

Alternatively view Push_Pull_Bias_Feedback.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

 p

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

For the Falstad Circuit Simulation, CTRL+Click Overloaded Push Pull Source Followers
In options, check European Resistors and uncheck Conventional Current.
Click both the switches to double the power supply voltage.

Alternatively view Saturation.txt.
Save or copy the text on the web page. Import the saved or copied text into the Falstad simulator.

Here is the new HTML5 Simulator Site.

 

 

 

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