Amp Mosfet



A power amplifier circuit using MOSFET has been designed to produce 100W output to drive a load of about 8 Ohms. The power amplifier circuit designed here has the advantage of being more efficient with less cross over distortion and total harmonic distortion.

Rekordbox dj deezer 2

Drive your audio experience to the next level with the Planet Audio Pulse PL1600.4 Class A/B 4-Channel Full Range Amplifier. This powerful 2-Ohm stable Amplifier features 1600 Watts Max Power with a MOSFET power supply to rock your tunes. Customize the sound with Fixed High Pass Crossover, Variable Low Pass Crossover and Bass Boost. Mosfet is a 3 terminal semiconductor device used in a wide range of electronic circuits. It works like a JFET but has less current leakage owing to an oxide insulation between the conductors. Mosfet is a good choice for building linear amplifiers owing to its lesser load, and any amplifier made using it is called a Mosfet amplifier. MOSFET is an acronym for Metal Oxide Semiconductor Field Effect Transistor.It is a type of FET (Field Effect Transistor) that has an insulated metal oxide layer between its gate and channel. On the contrary, JFETs gate is connected with its channel.

Principle of Operation:

This circuit operates on the principle of multi-stage power amplification consisting of pre amplifiers, drivers and power amplification using MOSFET. The pre amplification is done using a differential amplifier, driver stage is the differential amplifier with current mirror load and power amplification is done using MOSFET class AB operation. MOSFETs have an advantage over BJT in having a simple drive circuit, being less prone to thermal stability and having high input impedance. A pre-amplifier consisting of a two stage differential amplifier circuit is used to produce a noise free amplified signal. First stage of the pre-amplifier consists of a differential mode emitter coupled amplifier using PNP transistors. The second stage consists of a differential amplifier with active load, so as to increase the voltage gain. The current mirror circuit actually ensures the output current to remain constant irrespective of the changes in input signal voltages. This amplified signal is then given to the push pull amplifier stage, which produces a high power output signal.

Also read the post: How to Design 150W Power Amplifier Circuit

100W MOSFET Power Amplifier Circuit Diagram:

Components of the Circuit:
  • R1, R4: 4k ohms
  • R2: 100 ohms
  • R3: 50k ohms
  • R5: 1k ohms
  • R6: 50k ohms
  • R7: 10k ohms
  • R8, R9: 100 ohms
  • R10, R13: 470 ohms
  • R11: 100 ohms
  • R12: 3k ohms
  • R14, R15: 0.33 ohms
  • C1: 10uF
  • C2, C3: 18pF
  • C4: 100nF
  • Q1, Q2: BC556, PNP transistors
  • Q3, Q4: MJE340, NPN transistors
  • Q5, Q6: MJE350, PNP transistors
  • Q7: n channel E-MOSFET, IRF530
  • Q8: p channel E-MOSFET, IRF9530
  • V1, V2: +/- 50 V.

Related Post – 100w Subwoofer Amplifier Circuit

MOSFET Power Amplifier Circuit Design:

1st Stage Differential Amplifier Design:
  1. Selection of Emitter Resistors: For an efficient differential amplifier, the common mode rejection ratio given by R3/R2 should be higher. This requires the value of R2 to be much lower than R3. Here we select a 100 ohm potentiometer as R2 and 50k resistor as R3.
  2. Selection of Collector Resistors: For a differential gain of around 50 and emitter resistance about 100 Ohms, the value of R1 and R4 is calculated to be about 4k.
  3. Selection of Coupling Capacitor: Here we select a capacitor of 10uF to couple the AC input signal to the input of Q1.
2nd Stage Differential Amplifier Design:
  1. Selection of R11: For a total emitter current of around 0.5A, the value of emitter resistance is chosen to be around 100 ohms.
  2. Selection of R12: The value of potentiometer R12 is determined by the Gate threshold voltage of MOSFETs and the quiescent current flowing through the collector of Q4, which is around 50mA. This gives R12 to be around 3k. Similarly value of R7 is taken to be around 10k.
  3. Selection of Load: Here the differential amplifier is connected to an active load, which is a current mirror circuit. Here we select PNP transistors MJE350 with emitter resistors 100 ohms each. The emitter resistors are selected for a approx voltage drop of 100mV across them to ensure decent matching of the transistors.
Power Amplifier Output Stage Design:

Here we select N channel MOSFET IRF530 and P channel MOSFET IRF9530 as power amplifiers. For a power of 100w and load of 8 ohms, required output voltage is about 40V and output current is about 5A. This gives the value of source resistors to be around 0.33 ohms and the current drawn by each MOSFET to be around 1.6A (output voltage/(pi multiplied by load resistance)).

100W MOSFET Power Amplifier Circuit Operation:

PNP transistors form the differential amplifier circuit where one of the transistors receives the input AC signal and the other transistor receives the output signal through feedback. The AC signal is coupled to the base of Q1 through coupling capacitor and feedback signal is fed to the base of Q2 through R5 and R6. The output of the amplifier is set by adjusting the potentiometer. The output from the first stage differential amplifier is fed to the input of the second stage differential amplifier. When input voltage is more than the feedback voltage (in case of the first differential amplifier), the voltage inputs to the transistors Q3 and Q4 of the second differential amplifier simultaneously differs from each other. The transistors Q5 and Q6 form the current mirror circuit. This current mirror circuit ensures the output current flowing to the push pull amplifier circuit to remain constant.

This is achieved because when collector current of Q3 increases, the collector current of Q4 decreases to maintain a constant current flowing through the common point of the emitter terminals of Q3 and Q4.

Also the current mirror circuit produces an output current equal to the collector current of Q3. The potentiometer R12 ensures the application of proper DC biasing to each MOSFET. Since the two MOSFETs are in complementary to each other, when a positive voltage is applied to the gate of Q7, it conducts. Similarly for a negative threshold voltage, Q8 conducts. The gate resistors are used to prevent the MOSFET output from oscillating.

The input to the circuit is given by a 1khz AC input voltage of 4Vp-p. An oscilloscope is connected such that channel A is connected to input and channel B is connected to output. The power at the load is observed by connecting a wattmeter to the load.

Applications of 100w MOSFET Power Amplifier Circuit:

  1. It can be used to drive audio loads like loudspeaker, as an audio amplifier.
  2. It can be used to drive RF loads like high power antenna.
  3. It can be used to implement a distributed speaker system
  4. This circuit can be used in electronic devices like televisions, computers, mp3 players etc.
Limitations of this Circuit:
  1. MOSFET is more prone to electrostatic discharge.
  2. The MOSFET draws quite high current from the supply, which can damage the whole circuit, unless safety fuses are used.
  3. This circuit is prone to high frequency oscillations.
  4. This circuit is a theoretical circuit and is for education purpose.

Related Posts:


RANDOM PAGE
SITE SEARCH
LOG
IN
SIGN UP
HELP


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

To gain access to revision questions, please sign up and log in.

  • AS and A2:
  • MOSFET
a

Uses

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

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.
c200

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.

Amplifier Mosfet

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.

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.

Jensen 600 Watt Amp Mosfet

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.

f

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.
g

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).
h

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.
i

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.

j

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.

Moonlight uses one man's story to offer a remarkable and brilliantly crafted look at lives too rarely seen in cinema. Moonlight (formerly Limelight) is an open source implementation of NVIDIA's GameStream protocol. We implemented the protocol used by the NVIDIA Shield and wrote a set of 3rd party clients. You can stream your collection of PC games from your GameStream-compatible PC. Noun A figure appeared in the moonlight. Verb She is a secretary who moonlights as a waitress on weekends. Recent Examples on the Web: Noun The suite written for violin, harp, cello, flute. Moonlight. “Moonlight” is a smooth love song about a girl who X seems to admire and gets jealous over, as well as a track that outlines his success despite dealing with mental health issues and his dark past.

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

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

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

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.

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.

reviseOmatic V3 Contacts, ©, Cookies, Data Protection and DisclaimersHosted at linode.com, London