A stabilized neon-tube direct-coupled amplifier

IN recent years, the application of vacuum-tube amplifiers in many fields has greatly increased. Amplifiers capable of handling frequencies in the range of from 40 to 15,000 cycles per second are common, and in the many applications where it is necessary to amplify signals of frequencies much less than 40 cycles per second, the low-frequency response has been obtained by rather elaborate compensating devices. However, even with compensation, there exists a still lower limit of frequency, below which satisfactory amplification with allowable phase distortion cannot be obtained. One solution of the problem has been the employment of some method of direct-coupling between the amplifier stages. In addition to direct coupling, modern design dictates that the amplifier should be capable of operating from a single a-c power supply. Figure 1 is a diagram of the circuits that have been used in the development of direct-coupled amplifiers particularly suited for use with the cathode-ray oscilloscope. This method of obtaining direct-coupled amplification uses neon tubes as the coupling elements. The necessity for an amplifying circuit which is inherently stable leads to the use of the degenerative cathode resistor. This circuit is particularly applicable where several amplifying stages must be operated from the same power source. The stabilized amplifier and its power supply are shown in figure 2.