8.common_emitter_configurations_bjt_.docx

Characteristics of Transistor (BJT) in Common Emitter configuration

Aim of the experiment:To conduct experiment on a given BJT and obtain its

(i) Input Characteristics & (ii) Output characteristics.

Apparatus:S.No 1 2 3 4

Apparatus/Component Dual DC Regulated Power Supply(RPS) Transistor Resistance DC Ammeter

5 6

Multimeter Bread Board

Range/ Number (0-30)V BC107 1KΩ (0-100) mA (0-500) µA

Quantity 1 No 1 No 1 No 1 No 1 No 1 No

Circuit Diagram:-

(0-500)µA

A 1 KΩ (0-500)µA

A

BC107

(0-30)V

IB (0-30)V DC Supply

(0-2)V

IC

V VBE

V

VCE

(0-30)V DC Supply

Fig 1(a) Theory:A transistor is a three terminal active device. The three terminals are emitter, base and collector, represented by E, B, and C respectively. The emitter and collector are the same type of semiconductor and base is of the other type semiconductor. If E and C are of p-type and B is of n-type, then the transistor is called as pnp transistor. Similarly, If E and C are of n-type and B is of p-type, then the transistor is called as npn transistor. There are two junctions. One junction is between the emitter and the base, which is called emitter junction or the emitter-base junction or input junction. The other junction is between the collector and the base, which is called collector junction or collector base junction or output junction. The direction of the arrow head in the transistor symbol indicates the direction of the emitter current (conventional electric current) when the emitter junction is forward biased. There are three regions of operation in a transistor. They are active region, cutoff region and saturation region. In active region the transistor acts as an amplifier. The output varies linearly with the input if the transistor is in active region. To make the transistor work in this region the input junction is forward biased and the output junction is reverse biased. In

cutoff region the transistor acts as an open switch. The currents flowing through the transistor are reverse saturation currents and are very small and are negligible if the transistor is in cutoff region. To make the transistor work in this region the input junction is reverse biased and the output junction is also reverse biased. In saturation the transistor acts as a closed switch. To make the transistor work in this region the input junction is forward biased and the output junction is also forward biased. The transistor can be connected in three configurations by using one of the three terminals as common to both the input port and the output port. If the common terminal is base then the transistor is said to be in common base or CB configuration. Similarly, common emitter or CE configuration has emitter as common to both input and output and common collector or CC or emitter follower has collector terminal and common terminal to both the input and output ports. The common-emitter (CE) configuration has the emitter as the common terminal, or ground, to an ac signal. CE amplifiers exhibit high voltage gain and high current gain Summary of BJT Characteristics for different configurations Configurations Characteristics Input Impedance Output Impedance

Common Base

Common Emitter

Common Collector

Low

Medium

High

Very High

High

Low

o

o

Phase Angle Voltage Gain Current Gain

0 High Low

180 Medium Medium

0o Low High

Power Gain

Low

Very High

Medium

The CB Configuration is only used in single stage amplifier circuits such as microphone pre-amplifier or RF radio amplifiers due to its very good high frequency response. The Emitter follower (CC) configuration is very useful for impedance matching applications because of the very high input impedance and it has relatively low output impedance. The CE configuration is the most widely used configuration because it gives both voltage and current gains. The input for the CE configuration is given between base and the emitter and the output is taken between collector and emitter. This type of configuration is mostly used in the applications of transistor based amplifiers. The input current and voltage are designed as IB and VBE and the output current and voltages are called IC and VCE respectively. In this configuration the emitter current is equal to the sum of small base current and the large collector current. i.e. IE = IC + IB. We know that the ratio between collector current and emitter current gives current gain and it is called alpha in Common Base configuration similarly the ratio between collector current and base current gives the current gain and it is called beta in common emitter configuration. The input characteristics are obtained between input current IB and input voltage VBE with constant output voltage VCE. The output characteristics are obtained between the output current IC and output voltage VCE with constant input current IB.

Procedure:Output Characteristics:1. 2. 3. 4. 5. 6.

Connect the circuit diagram as per the Fig.1(a). Keep VBB and VCC in zero volts before switching ON the supply. Keep IB=100µA by varying VBB smoothly by fine control. Now vary Vcc such that VCE is 0.5V and see that IB is still 100µA. Note down the collector current Ic and tabulate in table-1. Now vary Vcc such thet VCE =1V and note the resulting collector current Ic. See that IB is still 100µA. 7. Now repeat the above step for VCE in steps of 0.5V upto 7.5V and note down the resulting Ic in the tabular form. 8. Now bring back VBB and Vcc to zero. 9. Set IB = 150µA by varying VBB smoothly with fine control and repeat the above steps (1 to 7 keeping IB = 150µA constant) 10. Repeat the step 9 with IB = 200µA. 11. Draw the graph VCE Vs Ic against IB constant and find out hfe and hoe as in Fig1(b). Input Characteristics:1. Keep VBB and Vcc in zero volts. 2. Set Vcc such that VCE =1.0V and keep it constant. 3. Now vary VBB smoothly such that VBE has its first value at IB=5µA.After that vary VBE in steps of 0.02V and note down the resulting base current IB for each step in the table-2. Continue upto a base current of 250µA. 4. Repeat the above step for VCE = 1.5V and 2.0V respectively. 5. Draw graph VBE Vs IB against VCE constant. 6. From the graphs calculate hie and hre as in Fig 1(c).

Tabular form:Table-1:Output Characteristics S.No

IB=100µA VCE (V) IC (mA)

IB=150µA VCE (V) IC (mA)

IB=200µA VCE (V) IC (mA)

1 2 3 4 5 6 7

Table-2:Input Characteristics S.No 1 2 3 4 5 6 7 8 9

VCE=1.0V VBE (V) IB (µA)

VCE=1.5V VBE (V) IB (µA)

VCE=2.0V VBE (V) IB (µA)

Model Graph:Input Characteristics:-

Fig 1(b) Output Characteristics:-

Fig 1(c)

Precautions:1. 2. 3. 4.

Keep the VBB and VCC in zero position for each step. Test the continuity of the wires used before use. All the contacts must be very tight and no loose connections must exist. All input variations must be with fine control and all readings must be read accurately.

Result:Viva Questions:1. 2. 3. 4. 5.

Draw the symbol of a transistor. What does the arrow indicate on transistor symbol? What are the various regions of operation of transistor? What are the regions in which the transistor operates when it is used as a switch? What are the applications of transistors?