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The series resistance of commercially available varactors provides a high-frequency limit on varactor parametric amplifiers. The values of pump frequency for which gain can be achieved and the resulting noise temperature are restricted. For a given signal frequency and a given varactor there is a minimum noise temperature that is achieved by correctly selecting the pump frequency. This minimum noise temperature can be related to measurable varactor parameters and to the power necessary for pumping the varactor. A plot of achievable noise temperature vs frequency indicates areas in which varactor amplifiers are competitive for low-noise receivers.