Amplifier Circuit using MOSFET Output Stage

MOSFET amplifierThe circuit presented here is amplifier circuit using MOSFET output stages, as a substitute for the output stage based on bipolar transistors. This project is for those who want to experiment with the power MOSFET. Without going into details, the MOSFET has a characteristic sound of vacuum tubes and bipolar transistors. The main advantages MOSFET Amplifier : simplicity of operation, hundreds of watts with a simple parallel, negative temperature coefficient and fast switching times. The main disadvantages : a high input capacity from 500 to 1000 pF, sensitivity to electrostatic discharge.

MOSFETs are available in a very wide range in the PNP and NPN, the voltage from 100 to 200 volts or more, additional models (such as bipolar). Model commonly used in power amplifiers are 10N16, 10P16, IRFP140, IRFP9140, IRFP240, IRFP9240, 2SK135, 2SJ50, 2SK1530, 2SJ201. In this project, points A and C are connected to the output of the driver stage, point B is to counter-reaction and the point D the mass driver. Board is planned for the case TO3P transistors, it will slightly bend the foot for TO220-type packages.
PCB and component layout
Set the quiescent current (of about 100 mA per MOSFET) is an adjustable potentiometer of 250 ohms is placed between points A and C, without any other complications. It is always possible to use other conventional systems. Zener diode is provided to protect the MOSFET with a very simple system, for better system you can try more complex systems (+ 1N4148 zener zener in series + series resistance,…). Traditional Rx resistor wirewound .22 ohm 5 watt resistor is replaced with a 5 to 1.2 Ohm 1 watt carbon layers are connected in parallel to avoid the effects of self. Cx capacitors should be placed in accordance with the compensation that will be made, pcb is provided for all cases.

Reference Capacity in pF input Capacity in pF output

Compensation pF input / output

IRF530 (IR) 670 250 190 110
IRF9530 (IR) 860 340
IRF540 (IR) 1700 560 30
IRF9540 (IR) 1400 590 300
IRFP140 (IR) 1700 550 40
IRFP9140 (IR) 1400 590 300
IRFP240 (IR) 1300 400
IRFP9240 (IR) 1200 370 100 30
IRFP240 (Harris) 1275 500 125
IRFP9240 (Harris) 1400 350 150


Parts list :
R1, R2, R3, R4 = 100 to 470 Ohms 1 / 4 W metal film
R5 = 10 ohm 1 watt carbon
Rx = 1.2 Ohm 1 W carbon
C1, C2 = 100 uF 100 Volts
C3 = 220 nF 100 Volts
Cx = ceramic capacitor (value depends on the compensation to be made, see table)
D1, D2 = 10 Volt zener diode 400 mW
T1, T2 = example IRFP140 (100 V), IRFP240 (200 V)
T3, T4 = example IRFP9140 (100 V), IRFP9240 (200 V)

Note : This project has not yet been tested.

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