Microwave Transistor Bias Considerations

By Avago Technologies

Often the least considered factor in microwave transistor circuit design is the bias network. Considerable effort is spent in measuring s-parameters, calculating gain, and optimizing bandwidth and noise figure, while the same resistor topology is used to bias the transistor. Since the cost per dB of microwave gain or noise figure is so high, the circuit designer cannot afford to sacrifice RF performance by inattention to dc bias considerations.

It has been found that the principle dependent variable in dc stability analysis is the collector current (IC). The transistor parameters that are temperature sensitive and influence IC are examined along with some passive resistive circuits that give stable dc operation and allow for trimming due to variations in transistor types.

In order to best select an optimum bias network, a method of comparison has to be developed. Analysis of transistor bias network instability involves writing a collector current equation in terms of the transistor equivalent circuit and the external bias circuitry. Partial derivatives of the collector current, with respect to the temperature dependent variables, are calculated individually and the resultant stability factors can then be considered simultaneously to predict collector current temperature behavior.