AMPLIFICATION

B. SOMANATHAN NAIR

     

      When we hear the word amplification, we get the feeling that a small quantity of an item is made into a much larger quantity of that item by using a specially made instrument called amplifier. For example, an audio amplifier amplifies low-volume sound signals into a high-volume sound signal. But, there is one problem for this definition: it is against the law of conservation of energy. According to this law, a small quantity of energy can not be converted into a large quantity of energy; one form of energy can be converted into another form only without any change in its magnitude. Then what is amplification?

AMPLIFICATION (IDEAL DEFINITION)

Amplification is the process of controlling the flow of energy, the controlling energy being negligibly smaller than the controlled energy.

ILLUSTRATIVE EXAMPLE 1

The ideal definition says that amplification is the control of flow of energy. In this ideal definition, we do not find any word that has any relation to the word amplification. Then how can we say that this is the definition of amplification?   

            Let us consider a procession in which there is one leader and five followers as shown in Fig. 1. The leader shouts a slogan and his followers repeat the same in the same way the leader has shouted. The combined voice of the five followers gives an amplified version of the leader’s voice by a factor of five. In other words, we say that in this operation, we get amplification by a factor of five.

 The following points are important in this context:

1.       There is one input voice.

2.       There are five output voices.

3.     The input voice is able to control the output voices. For example, if the input says, “Jai Sriram”, all the outputs will say, “Jai Sriram”. Now, if the input says, “Jai, Jai, Jai Sriram”, the outputs will also say, “Jai, Jai, Jai Sriram”. Thus the outputs follow the variations in the input exactly in the same fashion, but at a much larger volume. Hence, we say that the system consisting of one leader and five followers has produced an amplification of five.

4.  In this case, the law of conservation of energy is not violated, i.e., we are not generating a large energy from a small energy. Both small (i.e., leader) and large (five followers) energies exist; only, the small energy is controlling the large energy in such a way that the variations in the small (controlling) energy are reproduced exactly in the same fashion in the large (controlled) energy. Incidentally, we find that this is the ideal definition of amplification (i.e., small energy controlling large energy).

5.    It may be noted that the controlling and controlled energies are both DC energies (i.e., there are no variations in them).

6.     We super impose the signal to be amplified to the controlling energy and apply it to the input section of the amplifying device.

7.   The amplifying device then produces variations in the larger energy in its output section corresponding to the variations in the input energy. The larger output, which reproduces the input variations exactly as such, then gives amplification.

ILLUSTRATIVE EXAMPLE 2      

Consider a laser torch emitting 1 milliwatt of red laser light (Fig. 2). Let this light be used for communication between a man on ship and a man on the shore. First assume that the shoreman sends laser light to the shipman. A steady laser light is a DC signal and has no meaning. To send information, we must use variations in the DC light, which are known as codes.

        There several coding schemes that we use. In this case, let us assume that Morse code is used, which makes use 1s and 0s for information transmission. 1s may be represented by presence of a short light pulse while its absence may be used to represent 0 (or vice versa). The laser light can be switched on or off by a small switch on the outer cover of the laser; a slight pressure on the switch will turn on or off the laser light.

Now suppose the shoreman wants to send certain information to the ship. He will prepare the Morse code of the information first and then press the laser switch as per the codes formed. The pressing of the laser key requires negligible power. Thus the power input is very small and this creates the signal input energy. But this input energy creates corresponding variations in the large laser energy so that the shipman can read the message by decoding the variations in the light energy. In this case, we see that amplification has occurred, since the input energy (pressing of the laser switch) is small, but the output energy (laser light) is large.          

ILLUSTRATIVE EXAMPLE 3

Consider now a common-emitter electronic amplifier. In this case, input current is the base current (microampere range) and output current is the collector current (milliampere range). Since 1 mA is 1000 μA, amplification is possible in this case. This is because the small variations in the base current can create corresponding variations on a lager scale in the collector current (for further explanation on CE amplification, see the forthcoming blog on CE amplification).