Conditions for minimum noise generation in backward-wave amplifiers

The noise figure of a backward-wave amplifier can be minimized by optimizing the standing-wave ratio and position of the standing wave of noise power on the beam in the precircuit region. These optimum conditions are presented as detailed functions of tube parameters for the common types of backward-wave amplifiers and are compared with the characteristics of conventional traveling-wave tubes. The problem of achieving maximum tuning range while maintaining a noise figure close to the theoretical minimum value is discussed and general design criteria and methods are evolved. Although the voltage-tuned characteristics of backward-wave amplitiers impose rigorous requirements on the noise space-charge-wave transducer, it is shown that wide low-noise tuning ranges are possible even with no programming of the noise reducing electrodes.