Telephone Transmitter

Telephone Transmitter Points : telephone transmitter Definition Microphone in telephony is regarded as transmitter. It is a transducer, which converts sound energy into electrical energy. There are different types of transmitters hut carbon granules transmitter is the most widely used in the handset of the modern telephony. We will discuss the carbon granule transmitter only. It is based on the principle that the resistance of carbon granules is inversely proportional to pressure. The constructional details of the carbon transmitter, is illustrated in the figure.

It is the property of carbon that its resistance varies with pressure. The carbon transmitter does not produce any e.m.f. but only change its resistance with the changing pressure.

Carbon granules are placed between two electrodes in an insulated chamber. One electrode is fixed to the back of the chamber while the other electrode is attached with the movable diaphragm. The two electrodes .are connected with the battery. The transmitter offers an electrical resistance to the flow of current, which is the resistance of the carbon granules. When the diaphragm moves inward and outward, with sound pressure, the pressure on the carbon granules also changes. Thus the resistance of the carbon granules also varies with the changing pressure and hence the current flow between the two electrodes also varies. A current variation, corresponding to the sound pressure is generated in the circuit.

The electrical equivalent circuit of the carbon transmitter is shown in the figure.
The current flowing between the electrodes can be derived as:
R = Static resistance of the microphone (when there is no sound pressure)
Then, i = E/R

Suppose the sound pressure striking the diaphragm = p sin(ωt)
The varying sound pressure will vary the resistance accordingly about R.
Thus the varying resistance = r sin(ωt).
Where r = change in the amplitude of instantaneous resistance.
Hence, the total resistance will be = R ± r sin(ωt).
The plus, minus signs shows maximum and minimum pressure respectively.
Suppose the pressure is minimum, then:
The total resistance = R + r sin(ωt).
Therefore, i = E/(R + r sin(ωt)).
i = E/R {1/1+(r/R) sin(ωt)} = I { 1 +(r/R) sin(wt)}
let, m = r/R
i = I {1 +m sin(ωt)}^-1
As r < Expanding, {1 +m sin(ωt)}^-1 by binomial theorem, we get:
i = 1 + { 1 - m sin(ωt) + m²sin² (ωt) — m³sin³ (ωt) + -----}
m sin(ωt) = 1st harmonic (fundamental)
m²sin² (ωt) = 2nd harmonic
m³sin³ (wt) = 3rd harmonic

Hence it is clear that 2nd and 3rd harmonics current are also produced in the transmitter. As the value of in is very small, so neglecting m², m³, etc. We get: i = I { 1 +m sin(ωt)} = I + I.m sin(ωt)
This equation is similar to the equation of amplitude modulation. Hence we can say that the transmitter acts as a modulator of dc current. The carbon granule transmitter can also be considered as an ac. generator, producing an alternating current of I.m.sin(ωt). This current is the output of conversion of sound energy into electrical energy. The dc current through the transmitter is essential and is known as “energizing current”, otherwise, the transmitter appears as dead.