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研究晶体管线路,特别是高速开关线路时,测量出晶体管的参数 f_α是十分重要的。然而,在许多出厂的晶体管说明书中往往只给出参数 f_T(即共射晶体管的β值减小到1时所对应的频率)。这主要是由于:(1)f_T 可以在相当低的频率下,用普通的测量设备进行测量。但是,许多高频晶体管的 f_α是很难直接测量的。因为,在高频测量时,寄生参数的影响十分严重。(2)参数 f_T 的大小是与参数 f_α同一量级的,只要通过简单的换算便可以得到 f_α值。
When studying transistor circuits, especially high-speed switching circuits, it is important to measure the transistor parameters f_α. However, in most of the factory transistor specifications, only the parameter f_T (that is, the frequency at which the beta value of the cascode transistor is reduced to one) is often given. This is mainly due to: (1) f_T can be measured at a fairly low frequency using ordinary measuring equipment. However, f_α for many high-frequency transistors is hard to measure directly. Because, in high-frequency measurements, the impact of parasitic parameters is very serious. (2) The size of the parameter f_T is the same order of magnitude as the parameter f_α, as long as the value of f_α can be obtained by a simple conversion.