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本文的目的是发展一种建立在低噪声 FET 放大器基础上的设计理论。它不是一个 GaAsFET 噪声机理的基本模型。或者说,是试图从物理上建立可测量的器件电容和电阻与器件噪声系数和最佳噪声源阻抗的关系。将要表明,FET 的噪声性能可以由两个无关的噪声源充分地描述。一个是在 FET 的输入端,它是栅-源回路中各种电阻产生的热噪声,这个噪声源与频率有关,并且它可由 FET 的等效电路计算。第二个噪声源在FET 的输出端,它与频率无关,并且对于任何测量参数没有公认的关系。此输出噪声是漏电流和漏电压的函数。FET 的噪声分解为两个无关的噪声源简化了宽带低噪声放大器的设计。只要器件在某一频率的等效电路及其噪声系数已知,则任一频率的最佳噪声源阻抗和噪声系数就可被计算。对于器件设计者,这一模型可以有助于均衡输入输出噪声的折衷。提供了宽带 GaAs FET 放大器的例子,以说明本文提出的简化噪声理论的应用。
The purpose of this article is to develop a design theory based on a low-noise FET amplifier. It is not a basic model of GaAsFET noise mechanism. In other words, it is an attempt to physically establish a measurable relationship between the device capacitance and resistance and the device noise figure and the best noise source impedance. It will be shown that the noise performance of a FET can be adequately described by two unrelated noise sources. One is at the FET input, which is the thermal noise generated by the various resistors in the gate-source loop, which is frequency-dependent and which can be calculated by the FET’s equivalent circuit. The second noise source is at the output of the FET, which is frequency-independent and has no generally accepted relationship to any measured parameter. This output noise is a function of the drain current and drain voltage. The decomposition of FET noise into two independent noise sources simplifies the design of wideband LNAs. As long as the equivalent circuit of the device at a certain frequency and its noise figure are known, the best noise source impedance and noise figure at any frequency can be calculated. For device designers, this model can help to balance the input-output noise trade-off. An example of a wideband GaAs FET amplifier is provided to illustrate the application of the simplified noise theory presented in this paper.