A 300W MOSFET Linear Amplifier For 50 MHz

By Richard Frey, P.E.
Microsemi Corporation

Abstract
In an earlier article, the author described a 50 MHz 125V 250W class C amplifier using the ARF448A/B high voltage MOSFET devices. This paper now describes an improved version of that amplifier which is capable of class AB linear operation. The design changes required and the procedures involved are explained and demonstrated. A complete description of the amplifier and its construction are presented as well as the measured performance.

Introduction
High voltage, high power MOSFETs have been shown to be very capable RF power amplifiers.2 The metal gate architecture of the ARF series of devices from Advanced Power Technology has raised the frequency limits for this type of device to 100 MHz. The APT448A/B is typical of the series. It has a 68000 square mil die with a breakdown voltage rating, BVdss, of 450V. The device is packaged in the inexpensive TO-247 plastic package and is available in common source symmetric pairs. Like all MOSFETs, the gate threshold voltage, Vth, has a negative temperature coefficient. This makes operation as a linear amplifier difficult to impossible.

When forward biased with a constant gate voltage, the quiescent drain current will rise as the temperature of the die increases. Operating at the typical drain voltage for these parts, about one third of the rated BVdss, the power dissipation due to the increasing Idq results in "hot spotting" and subsequent thermal runaway. This is an unstable system. The dissipation increases so rapidly that the outside surface of the case does not follow the internal junction temperature. As a result, a bias compensation scheme that uses temperature sensing cannot keep up with the Vth shift and the device is destroyed.

The power dissipation within the die is a direct function of the operating voltage. By lowering the operating voltage the thermal loop gain can be reduced to a point where the gate threshold shift can be compensated for. Thermal stability can be achieved by sensing the case temperature. Linear operation thus becomes practical at 100V and below. While this is less than 25% of the rated BVdss and results in less gain, a very rugged and useful linear amplifier results.

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