WHY DO WE NEED DIRECT CURRENT IN COMMUNICATION?

B. SOMANATHAN NAIR

We require multiple-level current or voltages for signal representation. The minimum number of levels that we require for this purpose is two and we call this as the binary system. Direct current has only one level and hence it cannot be used to represent signals. But we find that DC currents and voltages find extensive application in electronics and communications. For example, most of the electronic devices require DC voltages for their operation.

            A course in electronic circuit theory usually starts with the theory of half-wave and full-wave rectifiers. But usually no descriptions are seen about the importance of DC current and rectifiers in the electronic circuit textbooks that a student generally follows. The following illustrative examples will highlight the importance of DC in communication.

ILLUSTRATIVE EXAMPLE 1

Consider the case of human conversation. Let one man talk to another man. Here the preposition to shows the direction of flow of human voice energy. Once the direction of energy flow is fixed, then we no longer use the word to show the direction of signal flow. It can also be seen that to represents one direction and hence constitutes a DC signal, which means that in a conversation, DC is used to indicate direction. Without this unidirectional energy flow we can not communicate with each other. But once we get the direction of sound energy flow, we no longer care for this DC component.

ILLUSTRATIVE EXAMPLE 2

Let us now take the case of human vision. Consider a room in which some objects are kept. We see these objects when a steady (DC) light (such as a tube light) is present. Here the light acts as the DC part and the objects act as signals. Once we start to see objects we no longer are bothered about the DC light source. However, we are bothered about the light source when it becomes off and we can not see the objects even though they are still there in the room.  Thus DC light acts as the background which help us to see objects. It may be noted that it is difficult to see objects when the steady DC light is replaced with a variable light source, whose intensity varies at every instant.  

ILLUSTRATIVE EXAMPLE 3

Consider a white board (or computer screen) for writing. A white board may be considered as a DC surface, because it carries a steady whiteness on its surface. Now let us write something on the surface. The white background now carries signal on it. Without the background surface, we can not write anything on it. Thus DC acts as the background in this case. Once the idea is written on the board, we no longer care for the background board unless it becomes too shady that we can not write anything further on it.  

ILLUSTRATIVE EXAMPLE 4

Consider the traffic through road, rail, sea etc. suppose we concentrate on road traffic using a car. Car takes a person from one destination to another. Since this is a directional movement, we can say that the car is a DC source. The person is the signal. The car (DC) carries the signal (person) from one destination to another. Once this transportation is over, we no longer need the car and neglect this DC component.

ILLUSTRATIVE EXAMPLE 5

Consider now a common-emitter amplifier. In this amplifier, we forward bias the base-emitter region with about 0.65 volt DC, and reverse bias (indirectly) the collector-emitter region with 10 volt DC. These DC voltages are required to make the base and collector currents, respectively. Now, the signal to be amplified is superimposed above the input base current. Thus the base current is a variable DC (DC bias current + ac signal current). This variable DC current produces corresponding variable and amplified DC collector current. However, once we get the amplified collector signal current, we remove the DC component part in it by using a coupling capacitor and get the amplified signal current across the collector-emitter output terminals. It can be seen that the DC currents are used in this case also to give specific directions to the flow of collector and base currents. Once this is established, we discard the DC.

CONCLUSION

We have seen that DC is essential in every communication system. It shows direction of energy flow or background needed to support the signal. Without DC there is no communication. This is the reason why it is included in the electronic circuit theory syllabus.