R-C Coupling

Author: N.H. Crowhurst

At higher frequencies, the charge on the capacitor hardly changes at all during an audio-frequency fluctuation. At low frequencies, however, there is time for the charge to change, which changes the voltage across the capacitor. If the frequency is low enough, the voltage across the capacitor changes as much as the plate potential, and the potential of the grid of the second tube hardly changes at all. At intermediate frequencies, the voltage at the second-stage grid fluctuates by an intermediate amount.

At the higher frequencies, the current through the grid resistor, due to the audio fluctuations, is the same as if the resistor were connected directly to the plate, without the steady d-c voltage difference being there. Where does this audio current come from? The plate circuit of the tube has to supply it. When the plate fluctuates negative, due to momentarily greater current through the coupling resistor, the grid of the following stage goes negative as the result of current flow through the grid resistor from grid to ground, adding to the momentary plate current. We use the term coupling resistor for the component that feeds B plus to the plate. Some call it the plate resistor, which must be carefully distinguished from plat© resistance. Many call it the load resistor, which can be misleading. As we have just seen, current fluctuations related to stage amplification divide between this resistor and the grid resistor coupled to it by the coupling capacitor. So, at most frequencies, the load for the tube's plate is these two resistors effectively in parallel. We will call this the load resistance.

Characteristics of RC-coupling.

Because this increase in plate current is controlled by the voltage applied to the grid, this extra current means that the rise in current through the coupling resistor will not be so great as before the capacitor and grid resistor were connected. The effect is the same as if the grid resistor were connected in parallel with the load resistor.

Taking audio fluctuations from the plate circuit

Whether the resistor to which the output side of the capacitor is connected goes to ground or to B+ will only make a difference to the charge on the capacitor and the steady voltage across it. The fluctuations across the capacitor and the audio currents will be the same either way because B-f is always a fixed voltage difference from the ground.