Transistor oscillators with impedance noise matching

A theory is derived which makes it possible to predict the carrier-to-noise ratio (CNR) of a transistor oscillator with real components depending on parameters of the active element and the oscillator circuit, which can be easily measured. The theory leads to certain new ideas for low-noise oscillator design which include the use of a multiple-stage active element and an impedance condition for noise matching. In this context the conditions for the use of GaAs FETs in low-noise oscillators are investigated. The resulting possibility of predicting the CNR depending only on transistor and circuit parameters is verified experimentally with seven different single-bipolar-transistor oscillators in the frequency range around 150 MHz. The measured values of the CNR show very good agreement with the values derived from theory. The differences between measured and calculated values are smaller than the measurement uncertainty of 3 dB