Dynamic Gain Measurements

Author: N.H. Crowhurst

In the setup we used, the plate ¥oltage was kept constant, and a change in grid voltage caused only a change in the current in the plate circuit. This is not the kind of amplification needed. To be useful, a small input voltage fluctuation must give a larger output voltage fluctuation.

Setup used to study introduction of resistor in plate current

We can make the fluctuating plate current produce a voltage fluctuation by passing it through a resistor. The higher the value of the resistor, the greater the voltage drop that a given current will cause, and hence the greater the voltage fluctuation a given current fluctuation will produce.

The plate current, however, is dependent on plate voltage as well as on grid voltage. When the grid swings more positive (actually, less negative), the plate current tends to increase; but when a resistor is in series with the plate, the increased current causes the plate voltage to drop. A drop in plate voltage by itself causes a drop in plate current, hence the drop in voltage due to the resistor means that the current will not rise as much as it did for the same grid-voltage change, when the resistor was not in the circuit.

Dynamic gain measurement

We could take a lot of time making adjustments of this kind, but there is a quicker way to get the answers we need. This is the dynamic measuring method. It enables us to measure the voltage fluctuation continuously, instead of having to take a whole series of individual measurements and then combine them. We apply a fixed d-c voltage, or Was at the grid together with a fluctuating voltage from an oscillator. (These voltages can be measured separately.) This produces a fluctuating voltage in the plate circuit that can be broken into a fixed component and an alternating component.

Plate circuit voltages

A capacitor placed between the plate and the a-c voltmeter takes on a charge corresponding to the steady or d-c component of the voltage, but does not have time to alter that charge with the alternating component. The voltmeter, therefore, measures only this alternating component.

Using a larger value of coupling resistor increases the output fluctuations for a given input fluctuation (the gain of the tube) up to a point. Eventually, the increase flattens off, and further increase in the value of the resistor causes the output voltage to decline somewhat. This decline occurs because the larger resistor values cause the average (steady d-c) voltage on the plate to drop, so that the tube gets "strangled." This can be overcome by adjusting the supply voltage so that the average or operating voltage, measured at the plate, is the same for each resistor used.

Step by step procedure used in dynamic gain measurements