Microwave Transistor Bias Considerations - App Note

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. Microwave transistor amplifier design requires biasing the transistor into the active region of performance and holding this bias or quiescent point constant over variations in temperature. At low frequencies, emitter resistor stabilization with negative current feedback is used for dc stability. At microwave frequencies the bypass capacitor becomes a problem since a good RF bypass at the design frequency often introduces low frequency instability and gives rise to bias oscillations. Figure 1 shows this effect.