### Have you ever been on a train going through a tunnel or a plane and your ears pop? Why does this happen?

A: Inside your ear there is a pocket of air. This pocket is normally at the same pressure as the air outside your ear to help you hear, but if the air pressure around you changes, you feel the air pushing on your eardrum. Your ear has a small tube for equalising the pressure between the inside and outside of the ear that is opened when you swallow and when the pressure is equalised you often feel a pop.

In a plane, the high altitude means the air is thinner and although planes are pressurised, the air pressure is still much less than on the surface. This difference in air pressure can be felt by the ears, particularly on takeoff and landing when changes in altitude make the pressure difference happen more quickly.

The train in a tunnel is slightly different. When the train enters the tunnel it squeezes the air in front of the train creating high pressure in the cabin and you sense this change in pressure.

There are some ways to equalise the pressure between the inside and outside of the ear. One common way that divers use is to gently try and blow through your nose while you hold your mouth and nose shut.

### COMMON COLLECTOR CONFIGURATION OF A TRANSISTOR

COMMON COLLECTOR CONNECTION

In  this  configuration  the  input  is  applied  between the  base  and  the  collector and  the  output  is  taken  from  the  collector  and  the  emitter.  Here  the  collector  is common to both the input and the output circuits as shown in Fig.

Common Collector Transistor Circuit

In  common  collector  configuration  the  input  current  is  the  base current  IB  and  the output current is the emitter current IE. The ratio of change in emitter current to the  change in the base current is called current amplification factor.

It is represented by

COMMON COLLECTOR CIRCUIT

A test  circuit  for determining the  static characteristic  of an NPN transistor is shown in Fig. In this circuit the collector is common to both the input and the output circuits.   To   measure   the   base   and   the   emitter   currents,   milli   ammeters   are connected in series with the base and the emitter circuits. Voltmeters are connected   across the input an…