York College, CUNYMATH 250exam 01 (853).ALL ANSWERS CORRECT

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A three-stage amplifier is to have an overall noise temperature no greater than 70 K. The overall gain of the amplifier is to be at least 45 dB. The amplifier is to be built by adding a low-noise fi... rst stage to an existing two-stage amplifier that has the gains and noise figures shown below. (The stage numbers refer to locations in the new amplifier.) (a) What is the minimum gain (in decibels) that the first stage can have? (b) Using the gain you calculated in part (a), calculate the maximum noise figure (in decibels) that the first stage can have. (c) Suppose the gain of the first stage could be increased by 3 dB without affecting its noise figure. What would be the effect on the noise temperature of the complete amplifier? Answer : To convert the gain and noise figure into ratios from decibels: From the above formulas the 3-stage amplifier has power gain and noise figure in ratios as follows: Stage Power Gain Noise Figure 2 100 1.99 3 31.6228 3.98 (a) The overall gain is the product of the individual gains and is 45 dB for this amplifier which is 31622.8 in ratio. The power gain is: To convert the gain from ratios into decibels: Hence, the minimum gain of first stage in decibels is . (b) [Show More]

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