A-C and D-C Components

Author: N.H. Crowhurst

Thus far we have shown how smaller voltage or current fluctuations can, by using tubes or transistors, cause bigger voltage or current fluctuations. This is the essence of amplification, but one thing more is necessary to be able to make use of amplification.

A-C and D-C components. How do we pass on the millivolts of audio, and give the grid of the next stage the right combination of d-c bias and audio (a-c)?

In the first practical amplifier stage discussed, an input fluctuation of 5 volts at the grid produced an output fluctuation of 50 volts at the plate, We did not pay much attention to the fact that these fluctuations do not conveniently start from zero or the negative bias required by the grid of a following stage. The grid circuit fluctuation could be regarded as being 2.5 volts away from an average (d-c) bias of 7.5 volts. The plate output is a fluctuation of 25 volts each way from an average (d-c) component of 175 volts.

This would, of course, run in proportion. If we start from a microphone, the voltage fluctuations will only be measured in millivolts, or thousandths of a volt. The first stage of amplification would raise this to tens of millivolts - still a rather small signal that would require more amplification to make it useful.

The easiest way to eliminate the d-c components in the output is to use a capacitor. Current does not flow from one plate to the other of a coupling capacitor, but current can flow to the plates, producing a charge on them, accompanied by a difference in voltage between them.