Input to the Grounded-Grid Amplifier

Author: N.H. Crowhurst

Grounded-grid amplifier - use of an input transformer.

With the grounded-cathode arrangement, the input circuit only needs to provide the right voltage. Current does not matter. In this circuit, however, we have to provide the right current and voltage fluctuations.

This is achieved by using a transformer. Assuming that the 5- and 10-volt figures are the two extremes of an alternating fluctuation, the average value will be 7.5 volts. Similarly the average current will be halfway between 2.5 and 5 milliamps, or 3.75 milliamps. Hence the d-c resistance in the path from cathode to ground needs to be 7.5/.00375 or 2000 ohms. This can be in the form of the winding resistance of the transformer secondary, or may include a separate resistor.

Assuming that the transformer uses a 4:1 step-down ratio, we can work out the conditions of the input circuit. In the secondary of the transformer, there is a current that fluctuates up to 5 milliamperes and down to 2.5 milli-amperes. The voltage drop due to this current (when no input is applied) is 7.5 volts. The change in voltage due to change in current will be from 10 volts (at 5 milliamperes) to 5 volts (at 2.5 milliamperes). For proper operation of the tube, however, the cathode-to-grid voltage must be +5 volts at 5 milliamperes and +10 volts at 2.5 milliamperes - just the reverse of what we have. These potentials must be provided by the induced voltage from the transformer.

Thus the induced voltage will have to offset the change in voltage drop and also provide the extra voltage for the cathode. It will have to fluctuate 5 volts each way from zero. Of this fluctuation, 2.5 volts in each direction will be taken up by the change in voltage across the secondary resistance due to change in current, and 2.5 volts will change the cathode-to-grid voltage. To produce this induced voltage on the secondary, the primary will need four times as much (or 20 volts) fluctuation each way from zero. It will also have to offset the change in magnetization due to the change in secondary current. Since this change requires 1.25 milliamperes each way on the secondary, only one-fourth of this (0.3125 milliampere) will be needed in the primary. Thus, the effective primary input must be a 20-volt fluctuation each way, accompanied by a 0.3125-milliampere fluctuation each way.

Voltages and currents in a grounded-grid amplifier

The basic grounded-grid circuit can be improved by using a separate resistor in the cathode connection to provide bias. Its value should be 2000 ohms, so that 3.75 milliamperes provide the correct middle potential of 7.5 volts. A large capacitor is connected across this resistor, so that when the current through the circuit changes, the capacitor will absorb some of it before the voltage can change. If the capacitor is big enough, the voltage will stay almost fixed at 7.5 volts.

Adding a bias resistor improves grounded-grid amplifier circuit.

This means the secondary has only to provide a voltage fluctuation of 2.5 volts each way and the primary has to receive only a 10-volt fluctuation in each direction. The input resistance thus is 10/.0003125 or 32,000 ohms.