How transistor work as trans-resistor?

Question

by changing resistance how it amplifies direct current

Answer 1

The name transistor came from "transfer of resistance" that occurs in a transistor.  Current injected into the emitter by a potential between base and emitter does not flow out the base (mostly).  Instead, it flows through the collector-base junction out to the collector.  The emitter-base junction is forward-biased, and therefore has a very low resistance, so the emitter current is produced with low voltages.  The base-collector junction is reverse-biased and has a very high resistance.  However, the same current flows through both so the low resistance input has transformed to a high resistance output.  The high output resistance of the collector allows high load resistances to be used, therefore obtaining much higher voltage (changes) at the collector than applied at the emitter.  In this way, voltage gain is realized.

From another viewpoint, instead of considering the emitter voltage as input, view the base current as input, and you find that small currents from the base result in large currents out of the collector, providing current gain.

Answer 2

The best way to find ur answer is to visit www.howstuffworks.com

u will find how most of the electrical instruments works in that website.

Answer 3

Look at the chart at the top of this page:

http://hyperphysics.phy-astr.gsu.edu/hba...

Forget the load-line, or the flat part of the curves for Vcd greater than 2 volts (most of the chart).

Look at the left side of the chart where Vce is less than 1 volt and Ic goes from 0 to 8 mA.  Notice that it is a sloped line (then it branches off into flat lines).  That slope is a resistance.  In this case it is about 0.75V / 8 mA = 94 Ohms.  There is your trans-resistor.  

If you operate the transistor in this region, you will get a fairly linear resistance (94 Ohms) for voltages less than 1 volt and currents less than 10 mA.  This region is called "saturation".

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