Microwave Transistor Oscillators: an Analytic Approach to Simplify Computer-aided Design

Microwave oscillator design generally represents a complex problem. Depending on the technical requirements of the oscillator, it is necessary to define the configuration of the oscillation system and transistor type, evaluate and measure smalland large-signal parameters of the transistor equivalent circuit and use an appropriate nonlinear simulator to calculate the oscillator electrical and spectral characteristics. When a transistor is used, representing the circuit as a two-port network is the most suitable approach for computer-aided design of free-running microwave oscillators. In this case, the basic parameters of the equivalent circuit either can be directly measured or approximated on the basis of experimental data with sufficient accuracy over a wide frequency range.1–3 In large-signal operation, it is necessary to define the appropriate parameters of the active two-port network as well as the parameters of the oscillator circuit’s external elements. Initially, the values of the external circuit elements are unknown. In addition, it is difficult to directly choose the values for a given microwave oscillator with a required oscillation frequency without any preliminary calculation. This process can be sufficiently timeconsuming and, in a typical case, calls for much simulation. Therefore, it is convenient to use an analytic method of optimizing microwave oscillator design that incorporates explicit expressions for feedback elements and load impedance in terms of the transistor equivalent circuit elements and its static characteristics.4 A conventional analytic approach to derive explicit expressions for the optimum values of an oscillator circuit first was applied to a bipolar transistor oscillator using an ideal transformer for feedback.5 Later, an analytic approach was used with a GaAs FET oscillator using a simplified transistor model where the influence of internal feedback from the gatedrain capacitance had not been taken into account.6 Such an approach involves a two-step procedure. First, the optimum combination of feedback elements required to realize a smallsignal maximum negative resistance to permit oscillations at the largest amplitude is defined. Then, taking into account the large-signal nonlinearity of the transistor equivalent circuit elements, the realized small-signal negative resistance is characterized to determine the optimum load impedance when maximum output power is generated for a given oscillator circuit configuration.