Coupling Capacitors

Author: N.H. Crowhurst

In the absence of fluctuations (audio), the plate or foil of the capacitor connected to the grid resistor takes the grid bias voltage. If there is any charge that would produce a different voltage, it flows away through the resistor, so that the voltage is again equalized. The same thing happens at the foil connected to the plate of the previous tube - it is at the same voltage as the tube's plate.

Use of a coupling capacitor: the coupling capacitor prevents the DC voltage from reaching the grid of the next stage while passing on the audio fluctuations.

When audio signals come along, the voltage at the plate goes up and down from its steady resting point. Because the grid resistor is so large, the charge on the capacitor does not have time to change, hence the voltage across the capacitor does not change. This means the voltage at the grid fluctuates up and down from its steady voltage in exactly the same way as the previous tube plate. (The capacitor acts as a short-circuit for the fluctuations, while isolating the d-c components at plate and grid.) If the voltage at the previous tube plate changed permanently, the charge on the capacitor would also change, so that the potential on the grid side would be the same as ground potential, while that at the plate side assumed the new plate potential. This process (as we have shown) takes a time dependent on the size (value) of the capacitor.