Lab Report Bjt

Objective : To determine operational range of a BJT

Equipments and Components : 1. Resistor : 220kΩ, 1kΩ 2. BJT : BC 108 3. Capacitor : 2 x 10µF 4. Function generator 5. Multimeter 6. Oscilloscope 7. Connecting wires

Procedures :

1. The circuit as shown above is assemble. All terminal connections are ensured

correct. The transistor diagram and its symbol is drawn. 2. The 20mVp-p IkHz sinusoidal wave is applied at the AC input terminal 3. By using the multimeter, the values of IB, IC, VCE, VB, VC and VBC are measured

4. The results obtained are verified by calculation. 5. The AC output signal observed by oscilloscope is drawn.

Result : a) Transistor diagram and its symbol

e c b N P

Simple circuit using a transistor

PNP NPN

b) Experimental Data Voltages / Currents IB IC VCE VB VC VBC

Values 0.0367 mA 0.0001 mA 4.14 V 8.35 V 4.95 V 3.39 V

c) Theoretical data 1. Example calculation: VCC

=

IBRB + VBE

IB

=

VCC - VBE RB

=

12 – 0.7 270

=

41.85 µA

=

β IB

=

110 (41.85 µA)

=

4.6035 mA

VCC

=

ICRC + VCE

VCE

=

12 – (4.6035 mA) (2.2 k)

=

1.87 V

VC

=

VCE

=

1.87 V

VB

=

VBE

=

0.7 V

VBC

=

0.7 – 1.87

IC

=

- 1.17 V

d) G = Vout / Vin = 2.9 mVp-p / 20 mVp-p = 0.145

Discussion : 1. The Bipolar Junction Transistor (BJT) is an extremely common electronic device to all forms of electronic circuits. It can be used for a number of useful applications such as an amplifier, a switch, a buffer, an oscillator, a nonlinear circuit – so forth.

2. The BJT is made by P and N type semiconductor material, which should be familiar from the study of diodes. The BJT is a three terminal device Collector Base Emitter

3. There are two types of BJT transistors. They are the NPN type, and the PNP e c b N P

type.

e

c

b

b

c

e

(a) NPN

(b) PNP

Ic Ie Ib

4. T h e

arrows show the

direction of DC current flow for both the NPN and PNP

cases. In both

cases the base current (Ib)

is a very small

current in the

order of microamps while

the collector

current (Ic) and emitter current (Ie) are larger and in

the order of

milliamps. Note that for the NPN transistor, the base current flows into the transistor but for the PNP transistor, the base current flows out the transistor. Also note Ic and Ie always flow in the same direction and in the direction of the (black) arrow, the same arrow that tells us whether the transistor is PNP or NPN. Ie Ic Ib

5. Now for the voltages: The voltage at the base is normally written as Vb. The voltage at the collector is normally written as Vc. The voltage at the emitter is normally written Ve. Vc

Vb

6. For the part voltage between collector and emitter, emitter and base and base Ve and collector we use either: Vce or Vce for collector and emitter Veb or Vbe for emitter and base Vbc or Vcb for base and collector

It is written such like example below; Lets; Vc= 6V (The voltage at the collector is 6 volts) Ve = 2V (The voltage at the emitter is 2 volts)

Then Vce is 4V because the voltage at the collector is 4V higher than the voltage at the emitter. Also, Vec = -4V because the voltage at the emitter (measuring point) is 4V lower than the voltage at the collector (reference point). And so on for Veb or Vbe and Vbc or Vcb. This is the convention used for measuring voltages

between terminals of the NPN and PNP transistors. The reason for this is that in these examples the first subscript letter is usually of higher voltage than the Vbe Vcb Vce Vbc Veb Vec

second, hence all variables listed below will have positive values.

7. The

analysis or design of a transistor amplifier requires a

knowledge of both the dc and the ac response of the system. Too often it is assumed that the transistor is a magical device that can raise the level of the applied ac input without the assistance of an external energy source. In actuality, the improved output ac power level is the result of a transfer of energy from the applied dc supplies.

8. The analysis or design of any electronic amplifier therefore has two components: the dc portion and the ac portion. Fortunately, the superposition theorem is applicable and the investigation of the dc conditions can be totally separated from the ac response. However, one must keep in mind that during the design or synthesis stage the choice of parameters for the required dc levels will affect the ac response, and vice versa.

9. The dc level of operation of a transistor is controlled by a number of factors, including the range of possible operating points on the device characteristics. The range for the bipolar junction transistor (BJT) amplifier should be specified. Once the desired dc current and voltage levels have been defined, a network must be constructed that will establish the desired operating point.

Suggestions for further work in the future: 1. Prepare the electrical and electronic components which in good condition.

2. Do not use red maker while writing on the whiteboard because it is unclear. 3. Use LCD projector to show how to do the experiment. 4. Use microphone to give the explanation.

Conclusions: The operational range of a BJT was determined

References: 1. J. David Irwin, Basic Engineering Circuit Analysis, (7th Edition), John Wiley and

Sons Inc,2002 2. David E. Johnson, John L. Hilburn, Johnny R. Johnson, Peter D. Scott, Basic

Electric Circuit Analysis, (5th Edition), Prentice Hall, 1995 3. Robert J. Herrick, DC/AC Circuits and Electronics: Principles & Applications, Theorem Delwan Learning, New York, 2003 4. Thomas L. Floyd, Principles Of Electric Circuits: Electron Flow Version, (3rd

Edition), Macmillan Publishing Company, New York, 1993 5. Thomas L. Floyd, Principles Of Electric Circuits: Conventional Current Versions,

(8th Edition), Pearson Prentice Hall, New Jersey, 2007