A Simple Analytic Method for Transistor Oscillator Design

Asimple analytic method for transistor oscillator design has been developed. This technique defines explicit expressions for optimum values of feedback elements and load through bipolar transistor z-parameters. Such an approach is useful for practical optimization of a series feedback microwave bipolar oscillator. Microwave oscillator design in general represents a complex problem. Depending on the technical requirements for designing an oscillator, it is necessary to define the configuration of the oscillation scheme and a transistor type, to measure the small-signal and large-signal parameters of a transistor-equivalent circuit and to calculate electrical and spectral characteristics of the oscillator. This approach is very suitable for implementing CAD tools if a transistor used in microwave oscillator circuits is represented by a two-port network. There are two ways to evaluate the basic parameters of the transistor equivalent circuit; one is by direct measurement and the other is by approximating based on experimental data with reasonable accuracy in a wide frequency range [1-3]. Furthermore, the equivalent circuit model can easily be integrated into a RF circuit simulator. In large-signal operation, it is necessary to define the appropriate parameters of the active two-port network and the parameters of external feedback elements of the oscillator circuit. Therefore, it is desirable to have an analytic method to design a single-frequency optimal microwave oscillator. This helps to formulate the explicit expressions for feedback elements, load impedance and maximum output power in terms of transistor-equivalent circuit elements and their current-voltage characteristics [4]. Such an approach can be derived based on a two-step procedure. First, the optimal combination of feedback elements for realizing a maximum small-signal negative resistance to permit oscillations at the largest amplitude is defined. Second, for a given oscillator circuit configuration with maximal output power, by taking into account the large-signal nonlinearity of the transistor equivalent circuit elements, the realized small-signal negative resistance will be characterized to determine the optimum load. Recent progress in silicon bipolar transistors has significantly improved frequency and power characteristics. In contrast to the field-effect transistors (FETs), the advantages of reduced low-frequency noise and higher transconductance make bipolar transistors more appealing for oscillator design up to 20 GHz. A simple analytic approach used to design a microwave bipolar oscillator with optimized feedback and load will speed up the calculations of the values of feedback elements and simplify the design procedure.