Cathode followers are frequently used as the input stage for transducers with capacitive impedances; condenser microphones, crystal or ceramic microphones, and hydrophones are examples. The primary purpose of the cathode‐follower‐type connection rather than the plate‐loaded connection for the input stage is to present a higher load impedance to the transducer, thus extending its low‐frequency response. An additional advantage which is sometimes claimed for this circuit is that the signal‐to‐noise ratio is improved. This is supposed to be due to the greatly increased effective value of the input resistor, which in turn means a smaller resistive component and therefore smaller thermal noise of the input impedance. It will be shown both analytically and experimentally that this signal‐to‐noise ratio does not improve with the cathode‐loaded connection and, in fact, in most cases is poorer than with the plate‐loaded connection. A comparison of the signal‐to‐noise ratio for each of these connections with other factors held fixed will be shown. Other circuits which combine both the better noise characteristics of the plate‐loaded circuit and the high input impedance of the cathode follower will be shown.

This content is only available via PDF.