The Grounded-Cathode Amplifier

Author: N.H. Crowhurst

Thus far we have measured all voltages against the cathode of the tube. This assumes the cathode potential is zero, or in engineering language, the cathode is called the reference point, because everything is measured back to it. Because we also measure all potentials against the biggest reference body we can find - the earth - the part of the circuit to which everything is referred or measured, is called ground, although it may not always be literally connected to ground.

Types of tube circuit connections.

We encounter one more expression for this: because the point to which measurements are all made is common to both input and output circuits, this is called a common point. The way of working a tube we have described is called common cathode, or grounded cathode.

The grounded- or common-cathode circuit of a tube is the easiest to understand because it is the voltage between plate and cathode and between grid and cathode that controls the current between plate and cathode. Any other circuit is more complicated to understand, because we have to find out how it is related to this basic one.