A general analysis indicates that, in wide-band amplifiers, stable operation is possible with triodes in circuits using the cathode as a signal terminal. The amplification, however, is approximately equal only to the square root of that available with grounded-cathode amplifier, and therefore twice as many tube units are required to obtain the same amplification. In certain applications, however, the utility of such circuits outweighs the loss of gain. A simple radio-frequency amplifier was designed for television receivers, using a cathode-input circuit. By combining a cathodeoutput and a cathode-input stage using one single twin-triode tube, a circuit was devised which compares favorably with pentode stages with respect to gain, stability, and economy, while it has far superior noise characteristics. The new circuit, called the "cathode-coupled twin-triode" amplifier, provides greater flexibility than conventional amplifier circuits, and can be used for radio-frequency, intermediatefrequency, video, converter, or detector services. Since the same tube type can also be used for synchronizing and deflection circuits, the number of tube types can be materially reduced, and greater standardization with further economical advantages may be obtained. An interesting application of the new circuit is a novel bidirectional amplifier.