4 Pairs of VMOS output stage for the modular high end audio amplifier

Project presentation

Abstract

The amplifier output stage presented here is part of my modular audio amplifier system. It is designed to be equipped with four pairs of vertical MOSFETs in TO-247 housing, for example IRFP240 and IRFP9240. Of course, different pin compatible transistors can be used as well.

Design presentation

Design considerations

This output stage is basically like the four pairs BJT output stage, just with different transistors and some minor adaptions.

Schematic

The schematic is the same like the BJT output stage, except added series resistors prior to the protection network and quick discharge diodes parallel to the gate resistors for each MOSFET. The quick discharge diodes seem very uncommon in audio circuits. I haven't seen them so far in audio amplifiers output stages, although some designers may have added some before. The gate-drain capacitance is a function of the gate-drain voltage. The lower the voltage, the higher the capacitance. This means, the closer the output swings to the supply rails, the more charge is stored in this capacitance. In extreme cases, like clipping, this charge can be substantial. Discharging CGD becomes very slow and this can lead to cross-conduction of the output stage transistors, i.e. simultaneous conduction of both P and N device, possibly resulting in destruction of the output stage. The gate resistor hinders quick discharge and therefore needs to be bypassed by the quick discharge diodes. During normal operation, those diodes do not carry any current. Only in extreme cases, they can save the output stage from destruction.

Assembly

The mounting holes fit on the 20mm grid, the whole modular system is based on. The transistors in TO-247 housing can be mounted to the 20mm grid as well and this results in a very compact placement of the circuitry. The modules overall dimension is smaller by one grid unit (20mm) than the BJT output stage module.

Summary

The higher safe operating area of the vertical MOSFETs allows much higher output power than the 4 pair BJT output stage allows. Linearity may not be as good as the BJT stage, however this can be addressed using the error correction driver module. For even higher power, I have designed an output stage featuring six pairs of vertical MOSFETs.