# 100W MOSFET Power Amplifier Circuit

A power amplifier circuit employing MOSFET technology has been created to deliver a 100W output for driving an 8-Ohm load. This designed power amplifier circuit offers improved efficiency, minimizing issues like cross-over distortion and total harmonic distortion.

### Outline

- 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 operates based on multi-stage power amplification, encompassing pre-amplifiers, driver stages, and power amplification utilizing MOSFETs. The pre-amplification stage employs a differential amplifier, the driver stage utilizes a differential amplifier with a current mirror load, and power amplification relies on MOSFETs operating in class AB mode. MOSFETs offer advantages such as a straightforward driving circuit, reduced susceptibility to thermal instability, and higher input impedance when compared to BJTs.

## 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.

**MOSFET Power Amplifier Circuit Design:**

### 1^{st} Stage Differential Amplifier Design:

- 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.
- 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.
- Selection of Coupling Capacitor: To link the AC input signal to the input of Q1, we use a 10uF capacitor.

### 2^{nd} Stage Differential Amplifier Design:

- The value of emitter resistance is set to be around 100 ohms for a total emitter current of around 0.5A.
- 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.
- 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:**

For our power amplifiers, we’ve chosen the N-channel MOSFET IRF530 and the P-channel MOSFET IRF9530. To achieve a power output of 100W with an 8-ohm load, we require an output voltage of approximately 40V and a current of 5A. This translates to source resistors of approximately 0.33 ohms and each MOSFET drawing around 1.6A of current (output voltage divided by (pi multiplied by load resistance)).

**100W MOSFET Power Amplifier Circuit Operation:**

The differential amplifier circuit employs PNP transistors, with one receiving the AC input signal and the other receiving the feedback signal. The AC signal is introduced to the base of Q1 through a coupling capacitor, while the feedback signal is directed to Q2’s base via R5 and R6. A potentiometer is used to regulate the amplifier’s output. The output of the first-stage differential amplifier is fed into the input of the second-stage differential amplifier. When the input voltage surpasses the feedback voltage, as in the first differential amplifier, the voltage inputs to transistors Q3 and Q4 in the second differential amplifier simultaneously vary.

The current mirror circuit consists of transistors Q5 and Q6, maintaining a constant current flowing through the common point of the emitter terminals of Q3 and Q4. This is achieved by adjusting the collector current of Q4 in response to variations in Q3’s collector current.

Additionally, the current mirror circuit produces an output current equivalent to that of Q3’s collector current. Potentiometer R12 ensures that each MOSFET receives the appropriate DC biasing. When a positive voltage is applied to the gate of Q7, it conducts due to the complementary nature of the two MOSFETs. Q8 conducts when a negative threshold voltage is present. Gate resistors are employed to prevent oscillation in the MOSFET output.

The circuit uses a 4Vp-p 1kHz AC input voltage, with Channel A of an oscilloscope connected to the input and Channel B to the output. To measure the power at the load, a wattmeter is connected to it.

## Applications of 100w MOSFET Power Amplifier Circuit:

- It can be used to drive audio loads like loudspeaker, as an audio amplifier.
- It can be used to drive RF loads like high power antenna.
- It can be used to implement a distributed speaker system
- This circuit can be used in electronic devices like televisions, computers, mp3 players etc.

**Limitations of this Circuit:**

- MOSFET is more prone to electrostatic discharge.
- The MOSFET draws quite high current from the supply, which can damage the whole circuit, unless safety fuses are used.
- This circuit is prone to high frequency oscillations.
- This circuit is a theoretical circuit and is for education purpose.