A high-efficiency power amplifier that operates in the S band (frequencies of the order of a few gigahertz) utilizes transistors operating under class-D bias and excitation conditions. Class-D operation has been utilized at lower frequencies, but, until now, has not been exploited in the S band.

The Output Waveform of the amplifier is of an intermediate form achieved in an effort to obtain a square-wave output from a sinusoidal input.

Nominally, in class D operation, a transistor is switched rapidly between “on” and “off” states so that at any given instant, it sustains either high current or high voltage, but not both at the same time. In the ideal case of zero “on” resistance, infinite “off” resistance, zero inductance and capacitance, and perfect switching, the output signal would be a perfect square wave. Relative to the traditional classes A, B, and C of amplifier operation, class D offers the potential to achieve greater power efficiency. In addition, relative to class-A amplifiers, class-D amplifiers are less likely to go into oscillation.

In order to design this amplifier, it was necessary to derive mathematical models of microwave power transistors for incorporation into a larger mathematical model for computational simulation of the operation of a class-D microwave amplifier. The design incorporates state-of-the-art switch-ing techniques applicable only in the microwave frequency range. Another major novel feature is a transmission-line power splitter/combiner designed with the help of phasing techniques to enable an approximation of a square-wave signal (which is inherently a wideband signal) to propagate through what would, if designed in a more traditional manner, behave as a more severely band-limited device (see figure).

Several Measurements were made on each amplifier stage to characterize its performance.

The amplifier includes an input, a driver, and a final stage. Each stage contains a pair of GaAs-based field-effect transistors biased in class D. The input signal can range from –10 to +10 dBm into a 50-ohm load. The table summarizes the performances of the three stages.

This work was done by William H. Sims of Marshall Space Flight Center. This invention has been patented by NASA (U.S. Patent No.6,388,512). Inquiries concerning nonexclusive or exclusive license for its commercial development should be addressed to

Sammy Nabors, MSFC Commercialization Assistance Lead, at (256) 544-5226 or This email address is being protected from spambots. You need JavaScript enabled to view it..

Refer to MFS-31455.