The Grounded-Grid Amplifier

Author: N.H. Crowhurst

Let us first take the grounded- or common-grid circuit. If the input voltage is applied between cathode and grid, it is the same as placing an opposite voltage between grid and cathode. (Making the grid 5 volts negative to the cathode is the same as making the cathode 5 volts positive to the grid.) No current flows in the grid, circuit because it is negative with respect to the cathode and repels all electron flow. Current does flow in the cathode circuit, and it is the same current that flows in the plate circuit.

With the same resistance in the plate circuit as before and a supply of 250 volts, with a fluctuation between +5 an<* +10 volts at the cathode, the current will fluctuate, as in the grounded-cathode arrangement, between 5 milliamperes and 2.5 milliamperes, respectively, while the plate-to-cathode voltage fluctuates between (150 - 5) or 145 volts and (200 - 10) or 190 volts. Thus the input fluctuates between +5 volts at 5 milliamperes and +10 volts at 2.5 milliamperes, the current in each case opposing the input voltage. This means the change in input current corresponding to a 5-volt change in input voltage is 2.5 milliamperes, which represents an a-c resistance for the input circuit of 5/.0025 or 2000 ohms.

In the grounded-cathode arrangement, there is no current in the input circuit, hence the a-c input resistance is 5/0 or infinity. (Anything divided by zero is infinity.) Here, however, we have what in tube circuits is a low input resistance - in this example, 2000 ohms.

Grounded-grid amplifier