For future adaptive radio and radar transmission in a dynamic spectrum access environment, the radio-frequency circuitry will be need to be reconfigurable to allow adjustment of the operating frequency and the transmission spectrum in a congested environment. A prototype tunable amplifier matching network was implemented using varactors as the tunable elements. A method in which the control bias voltages of the varactors are directly tuned is compared with a previously demonstrated algorithm that tunes based on the reflection coefficient through a nonlinear characterization of the varactor matching network. The algorithms are both designed to optimize the power-added efficiency of the amplifier while keeping the adjacent-channel power ratio below a pre-specified constraint value. Lower average time per measurement is achieved for the voltage-based search, but the total search time is lower for the reflection-coefficient based search. This comparison of algorithms is useful in developing a reconfigurable transmitter amplifier that can adapt on the fly to meet changing spectral requirements.

Unconditional stability is often a desired criterion in amplifier design. The Smith Tube, a three-dimensional cylindrical extension of the Smith Chart with center frequency represented on the height axis, can be used as a tool to design an unconditionally stable amplifier. The stability circles taken over frequency are extended into a stability surface in the Smith Tube representing regions of stability and instability across a range of frequencies. This aids the designer in selecting parameters that will result in an unconditionally stable amplifier design over all frequencies. While this paper discusses stabilization using the Smith Tube, the Frequency Smith Tube can also be used to assess gain and noise performance over frequency, and to plot the reflection coefficient presented by a matching network to a device as frequency is varied. This allows visualization of a bandwidth-based design in a single view.