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Practical implementations anchor a design approach that uses one additional shunt resistor to stabilize the transistors in low-noise-amplifier circuits.
Low-noise amplifiers (LNAs) form the input stage of the receiving part of nearly any communications system. The main task of these subcircuits is to amplify the wanted signal without deteriorating the signal-to-noise ratio. In most cases, the desired signal is very weak. The noise figure is seen as the main describing figure of merit for the transistors used in such applications. It determines the minimum amount of noise that is added in the transistor stage. The noise figure is not a hard figure, as it depends on the bias and especially the input matching conditions.
According to many microwave textbooks, nothing is easier than the design of a matching network for a specific transistor. The engineer simply needs to know the S- and noise parameters at an appropriate bias point for that transistor. Yet a design problem might arise if stability is a concern. The noise match is routinely done by setting the input noise and output gain match, which is a standard procedure in many textbooks (e.g., Ref. 1, Ch. 4.1.2). There is not much room to influence or even improve the stability factor. As a result, stability is often checked only after the design is completed.
* F is not affected by the load impedance, Γ^sub L^.
The stability of a given transistor is mostly described by the K-factor, which is computed as follows:
SEE EQ. 2 IN BOX AT RIGHT
Because many textbooks explain why K > 1 means unconditional stability, that information is not repeated here ( see Ref. 2 or Ref. 3 for further explanation). With the procedure described, it is helpful if the transistor is unconditionally stable at any frequency. Unfortunately, this is not the case for many devices. In a lot of applications, the circuit designer is choosing the transistor according to parameters presented on its datasheet, such as available gain, third-order intercept point (IP3), operating voltage, F^sub MIN^, and others. The last step is to check the transistor's S-parameters for stability. In the design stage, it may be found that the transistor tends to oscillation. This knowledge is especially damaging when gained at the production stage.
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