• AS and A2:
• MOSFET
• BJ Transistor
• Class A Amplifier
•  

AS A2

Uses

• drive loudspeakers
• amplify radio frequency energy before feeding to the antenna
• drive DC motors. Both speed and direction can be controlled.

Source Followers

• The N Channel FET provides power amplification for the positive part of the AC input.
• The P Channel FET provides power amplification for the negative part of the AC input.
• The voltage gain is 1
• No output coupling capacitor is needed (avoiding the use of a physically big component). Single ended (not push pull) amplifiers need a big output coupling capacitor.
• When there is no input, neither MOSFET is conducting. This saves energy. Single ended amplifiers consume power even when there is no input.
• When there is an AC input, each MOSFET is conducting for only 50% of the time.

Cross Over Distortion

This simple circuit suffers from cross over distortion.

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

• Quite a large input voltage is needed to turn on the FETs, 2 to 4 Volts.
• This has an unwanted side effect. The output is 2 to 4 volts less than the ideal case.
• A positive potential will turn on the top N Channel FET.
• A negative potential will turn on the bottom P Channel FET.
• Small potentials close to zero will turn on neither FET.
• This causes severe cross over distortion, most noticeable with quiet music.
• The amplifier works fairly well for potentials greater than +/- 2 to 4 volts but hardly works at all for lower potentials.

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.

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.

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

Use Negative Feedback

• This circuit uses both biasing and negative feedback to improve performance.
• The LEDs have two volts across them. This helps to reduce cross over distortion. This is an unusual way of biasing the MOSFETs but it works.
• The MOSFETS are included in the feedback path.
• The Op Amp voltage follower uses a higher power supply voltage. This allows the MOSFET source follower outputs to swing over a larger range of voltages.

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

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

• This circuit has a voltage gain of 1 but a much higher power gain (power_out / power_in).
• The Op amp output potential will be just right to ensure that Vout = Vin
• Negative feedback is being used to correct for errors in the output.
• The operational amplifier is wired up as a voltage follower so Vout should track Vin exactly.
• Cross over distortion is minimised.

Push Pull Advantages

• Don't need a large coupling capacitor between the output and the speaker.
• In other types of amplifier, this capacitor limits the low frequency response (high pass filter).

Push Pull Disadvantages

• Cross Over Distortion
• MOSFETs have good high frequency properties. Usually this is an advantage but it makes it easy to build an oscillator capable of high power outputs. The oscillations are likely to be outside the range of human hearing but still able to overheat and destroy speakers, usually the tweeters. Careful design is needed.

Saturation, Clipping, Limiting

• An ideal op amp could provide an infinite output voltage range.
• A very good op amp could provide outputs at least up to the power supply voltages.
• Most op amps fall short by about two volts so with a 12 volt supply, the output would be only ten volts.
• The output should be directly proportional to the input. That is perfectly linear.

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.

RMS Output Power

• The power supply is 20 Volts.
• An 8Ω speaker is being used.
• Decide whether to use 20V (ideal) or 18V (real life) in the calculation. If the exam question does not make it clear which one to use, just say whether you are doing the ideal or real life calculation. Below, the ideal calculation is shown.

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.

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.

Falstad Simulations

Simplest Circuit - Bad Crossover Distortion

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.

Circuit With Biasing - Improved Crossover Distortion

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.

Circuit With Biasing and Negtive Feedback - Minimal Distortion

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.

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.

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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