Amplifier Circuit Diagrams

100W MOSFET Power Amplifier Circuit

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.


  • Principle of Operation:
  • 100W MOSFET Power Amplifier Circuit Diagram:
    • Components of the Circuit:
    • 1st Stage Differential Amplifier Design:
  • 2nd Stage Differential Amplifier Design:
  • Power Amplifier Output Stage Design:
  • 100W MOSFET Power Amplifier Circuit Operation:
  • Applications of 100w MOSFET Power Amplifier Circuit:Limitations of this Circuit

Principle of Operation:

This circuit works on the premise of multi-stage power amplification, which includes pre amplifiers, drivers, and MOSFET-based power amplification. A differential amplifier is used for pre-amplification, a differential amplifier with current mirror load is used for the driver stage, and MOSFET class AB operation is used for power amplification. MOSFETs feature a simple driving circuit, are less prone to thermal instability, and have a high input impedance compared to BJTs.

100W MOSFET Power Amplifier Circuit Diagram:

Power Amplifier
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.

MOSFET Power Amplifier Circuit Design:

1st Stage Differential Amplifier Design:

  1. Emitter Resistors: The common mode rejection ratio indicated by R3/R2 should be higher for an efficient differential amplifier. This necessitates a significantly lower R2 value than R3. R2 is a 100 ohm potentiometer, while R3 is a 50 kilohertz resistor.
  2. Collector Resistors: The value of R1 and R4 is calculated to be around 4k for a differential gain of around 50 and emitter resistance of around 100 Ohms.
  3. Selection of Coupling Capacitor: To link the AC input signal to the input of Q1, we use a 10uF capacitor.

2nd Stage Differential Amplifier Design:

  1. The value of emitter resistance is set to be around 100 ohms for a total emitter current of around 0.5A.
  2. The Gate threshold voltage of MOSFETs and the quiescent current flowing through the collector of Q4, which is roughly 50mA, are used to estimate the value of potentiometer R12. R12 is estimated to be roughly 3k as a result of this. R7 is estimated to be worth roughly ten thousand dollars.
  3. The differential amplifier is coupled to an active load in this case, which is a current mirror circuit. We’ll use MJE350 PNP transistors with 100 ohm emitter resistors. To guarantee good matching, the emitter resistors are chosen to have a voltage drop of about 100mV between them.

Power Amplifier Output Stage Design:

As power amplifiers, we use N channel MOSFET IRF530 and P channel MOSFET IRF9530. The required output voltage and current for a power of 100w and an 8-ohm load are around 40V and 5A, respectively. This results in source resistors of roughly 0.33 ohms and a current of around 1.6A drawn by each MOSFET (output voltage/(pi multiplied by load resistance)).

100W MOSFET Power Amplifier Circuit Operation:

The differential amplifier circuit is made up of PNP transistors, with one receiving the input AC signal and the other receiving the output signal through feedback. The AC signal is coupled to Q1’s base via a coupling capacitor, while the feedback signal is sent to Q2’s base via R5 and R6. The potentiometer is used to control the amplifier’s output. The first stage differential amplifier’s output is fed into the second stage differential amplifier’s input.When the input voltage exceeds the feedback voltage (as in the first differential amplifier), the voltage inputs to transistors Q3 and Q4 of the second differential amplifier differ simultaneously. The current mirror circuit is made up of transistors Q5 and Q6. The output current flowing to the push pull amplifier circuit is kept constant by this current mirror circuit.

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.

In addition, the current mirror circuit generates an output current that is equivalent to Q3’s collector current. The potentiometer R12 guarantees that each MOSFET receives correct DC biassing. When a positive voltage is given to the gate of Q7, it conducts since the two MOSFETs are complementary. Q8 conducts when the threshold voltage is negative. To keep the MOSFET output from oscillating, gate resistors are utilised.

A 4Vp-p 1kHz AC input voltage is used as the circuit’s input. Channel A of an oscilloscope is connected to the input, whereas channel B is connected to the output. By connecting a wattmeter to the load, the power at the load can be measured.

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.

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