Wednesday, August 4, 2010

Experiment No. 7

Transistor as Switch

Components: 1 x Small Signal NPN transistor, 2 resistors.
Exercise: Connect the circuit as shown below and switch on the power supply.



Connect the multimeter between base and emitter ( 0.80V )
Note the voltage reading and explain what this reading is indicating.

In this switching circuit which simply states that the sum of voltages around a loop or closed path is equal to zero.

VCC = IC RC + VCE

VCC - IC RC - VCE = 0      
15 - 14.2 - 0.8V = 0



Connect the multimeter between collector and emitter ( 0.05V )
Note the voltage reading and explain what this reading is indicating.


In the plot given below what are the regions indcated by the arrows A & B?
A Region: The current flow which is saturated or fully open
B Region: The voltage drop or cut off region.


How does a transistor work n these regions? Explain in details.
The voltage drop is directly related to current flow IC, if there is a high voltage drop there will be low current flow or rise.


What is the power dissipated by the transistor at Vce of 3 Volts?
B = IC / Ib
14 / 0.5 = 28

PD = IC * Vce
14 * 3 = 42 mw

What is the Beta of this transistor at Vce 2,3 & 4 volts?
Vce 2 B = IC / Ib
20 mA / 0.8 m A = 25 mA

Vce 3 B = IC / Ib
14 mA / 0.5  mA = 28 mA

Vce 4 B = IC / Ib
5 mA / 0.2 mA = 25 mA

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