论文部分内容阅读
在许多小功率微波双极晶体管中,高频特性基本上是由随电流而变的发射极-基极结的时间常数所控制。使用尼尔森(Nielsen)表达式描述其性能是无效的,因为所推导的尼尔森表达式适用于描述由基区渡越时间所控制的晶体管噪声系数,而与发射极电流无关。把从渡越频率f_T推导出的与电流有关的截止频率f_a代入这个方程而得到的噪声系数值远高于测量值。Malavuiya和van der Ziel的最新近似给出了比较好的符合,但是还没有与测量完全一致。新的表达式是考虑发射极时间常数而推导出来的,在3千兆赫下和在很宽的发射极电流范围内,该表达式与测量的最小噪声系数符合得很好。
In many low-power microwave bipolar transistors, high-frequency characteristics are essentially controlled by the current-dependent emitter-base junction time constant. Using Nielsen expressions to describe their performance is inefficient because the derived Nielsen expression is suitable for describing the transistor noise figure controlled by the base transit time independent of the emitter current. Substituting the current-dependent cut-off frequency f_a derived from the crossover frequency f_T into this equation results in a noise figure much higher than the measured value. The latest approximation of Malavuiya and van der Ziel gives a good fit, but it is not exactly the same as the measurement. The new expression is derived by taking into account the emitter time constant, which is in good agreement with the measured minimum noise figure at 3 GHz and over a wide range of emitter currents.