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Any noisy amplifier can be represented by an equivalent noiseless amplifier plus two noise generators either at the input or the output of the amplifier. The choice of two particular noise generators (the equivalent short-circuit noise voltage and the open-circuit noise current) to characterize a noisy amplifier has a number of advantages over the concept of noise figure. The noise generators can easily be measured separately from the source noise, and the optimum source impedance and the noise figure at any source impedance can then be calculated. Since the amplifier noise is measured separately from the source noise, low noise figures can be easily measured. The optimum source impedance equals the quotient of the two noise generators, and the noise figure depends upon their product. Neither feedback nor input impedance is a consideration in determining noise figure and optimum source impedance. Several transistor noise diagrams show how the two noise generators are affected by emitter current, collector voltage, and frequency. Noise diagrams can be used to select the most suitable amplifying devices and optimum operating conditions for various applications.