100M-500M低噪放设计方案
Besides providing information regarding gain, P-1dB, noise figure, and input and output return loss, the simulation provides very important information regarding circuit stability. Unless a circuit is actually oscillating on the bench, it may be difficult to predict instabilities without actually presenting various VSWR loads at various phase angles to the amplifier. Calculating the Rollett stability factor (K) and generating stability circles are two methods made considerably easier with computer simulations. Simulated and measured results show the stability factor, K>1 (see Figure 2), at the cost of reduced third-order intercept point To meet the goals for noise figure, intercept point, and gain, the drain source current (Ids) was chosen to be 60 mA. The characterization data in the device data sheet shows that 60 mA gives the best IP3, combined with a very low mini-mum noise figure (Fmin). Also, as shown in the data sheet, a 3 V drain-to-source voltage (Vds) gives a slightly higher gain and easily allows the use of a regulated 5 V supply. The use of a controlled amount of source inductance, usu-ally only a few tenths of a nanoHenry, can often be used to enhance LNA performance. This is effectively equivalent and output power, through the use of a series resistor on the output.
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K
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FREQUENCY (GHz)
Figure 2. Simulated and measured stability factor K.
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to increasing the source leads by approximately .025 inch. The effect can be easily modeled using an RF simulation tool such as ADS. The usual side effect of excessive source inductance is gain peaking at a high frequency and resul-tant oscillations.
Figure 3. Suggested RF layout to minimize inductance in feedback network.