Input & Output Characteristics of CB Configuration and h-Parameter Calculations

Objective:

To study the input and output characteristics of a transistor in Common Base Configuration.

Components:

S.No.	Name	Quantity
1	Transistor BC 107	1(One) No.
2	Resistors (1K)	2(Two) No.
3	Bread board	1(One) No.

Equipment:

S.No.	Name	Quantity
1	Dual DC Regulated Power supply (0 – 30 V)	1(One) No.
2	Digital Ammeters  ( 0 – 200 mA)	2(Two) No.
3	Digital Voltmeter (0-20V)	2(Two) No.
4	Connecting wires (Single Strand)	2

Specifications:

For Transistor BC 107:

• Max Collector Current = 0.1A
• V_ceo max = 50V

Circuit Diagram:

 

h – Parameter model of CB transistor:

Pin assignment of Transistor:

View from side of pins

View from top of casing

Operation:

Bipolar Junction Transistor (BJT) is a three terminal (emitter, base, collector) semiconductor device. There are two types of BJTs, namely NPN and PNP. It consists of two PN junctions, namely emitter junction and collector junction.

The basic circuit diagram for studying input characteristics is shown in the circuit diagram. The input is applied between emitter and base, the output is taken between collector and base. Here base of the transistor is common to both input and output and hence the name is Common Base Configuration.

Input characteristics are obtained between the input current and input voltage at constant output voltage. It is plotted between V_EE and I_E at constant V_CB in CB configuration.

Output characteristics are obtained between the output voltage and output current at constant input current. It is plotted between V_CB and I_C at constant I_E in CB configuration.

Procedure:

Input Characteristics:

1. Connect the circuit as shown in the circuit diagram.
2. Keep output voltage V_CB = 0V by varying V_CC.
3. Varying V_EE gradually, note down emitter current I_E and emitter-base voltage(V_EE).
4. Step size is not fixed because of nonlinear curve. Initially vary V_EE in steps of 0.1 V. Once the current starts increasing vary V_EE in steps of 1V up to 12V.
5. Repeat above procedure (step 3) for V_CB = 4V.

Output Characteristics:

1. Connect the circuit as shown in the circuit diagram.
2. Keep emitter current I_E = 5mA by varying V_EE.
3. Varying V_CC gradually in steps of 1V up to 12V and note down collector current I_C and collector-base voltage(V_CB).
4. Repeat above procedure (step 3) for I_E = 10mA.

Repeat above procedure (step 3) for I_E = 10mA.

Observations:

Input Characteristics
VEE (Volts)	VCB = 0V		VCB = 4V
	VEB (Volts)	IE (mA)	VEB (Volts)	IE (mA)

 

Output Characteristics
VCC (Volts)	IE = 0mA		IE = 5V		IE = 10mA
	VCB (Volts)	IC (mA)	VCB (Volts)	IC (mA)	VCB (Volts)	IC (mA)

Graph:

1. Plot the input characteristics for different values of V_CB by taking V_EE on X-axis and I_E on Y-axis taking V_CB as constant parameter.
2. Plot the output characteristics by taking V_CB on X-axis and taking I_C on Y-axis taking I_E as a constant parameter.

Calculations from Graph:

The h-parameters are to be calculated from the following formulae:

1. Input Characteristics: To obtain input resistance, find V_EE and I_E for a constant V_CB on one of the input characteristics.

Input impedance  = h_ib = R_i = V_EE  / I_E (V_CB  = constant)

Reverse voltage gain = hrb = V_EB / V_CB   (I_E = constant)

2. Output Characteristics: To obtain output resistance, find I_C  and V_CB  at a constant I_E.

Output admitance = h_ob = 1/Ro = I_C / V_CB  (I_E = constant)

Forward current gain = h_fb = I_C / I_E (V_CB = constant)

Inference:

1. Input resistance is in the order of tens of ohms since Emitter-Base Junction is forward biased.
2. Output resistance is in order of hundreds of kilo-ohms since Collector-Base Junction is reverse biased.
3. Higher is the value of V_CB, smaller is the cut in voltage.
4. Increase in the value of I_B causes saturation of transistor at small voltages.

Precautions:

1. While performing the experiment do not exceed the ratings of the transistor. This may lead to damage the transistor.
2. Connect voltmeter and ammeter in correct polarities as shown in the circuit diagram.
3. Do not switch ON the power supply unless you have checked the circuit connections as per the circuit diagram.
4. Make sure while selecting the emitter, base and collector terminals of the transistor.

Result:

Input and Output characteristics of a Transistor in Common Base Configuration are studied.

The h-parameters for a transistor in CB configuration are:

1. The Input resistance (h_ib)                        __________________ Ohms.
2. The Reverse Voltage Transfer Ratio (h_rb) __________________.
3. The Output Admittance (h_ob)                   __________________ Mhos.
4. The Forward Current gain   (h_fb)               __________________.

Outcomes: Students are able to

1. analyze the characteristics of BJT in Common Base Configuration.
2. calculate h-parameters from the characteristics obtained.

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Discussion/Viva Questions:

1. What is transistor?

Ans: A transistor is a semiconductor device used to amplify and switch electronic signals and electrical power. It is composed of semiconductor material with at least three terminals for connection to an external circuit. The term transistor was coined by John R. Pierce as a portmanteau of the term "transfer resistor".

2. Write the relation between and ?

Ans: 

3. Define  (alpha)? What is the range of ?

Ans: The important parameter is the common-base current gain, . The common-base current gain is approximately the gain of current from emitter to collector in the forward-active region. This ratio usually has a value close to unity; between 0.98 and 0.998.

4. Why  is less than unity?

Ans: It is less than unity due to recombination of charge carriers as they cross the base region.

5. Input and output impedance equations for CB configuration?

Ans: h_ib = V_BE / I_E, 1 / h_oe = V_CE / I_C

6. What is carrier lifetime?

Ans: It is the average time taken by the carrier to recombine with minority carrier. which decides the ON state of transistor.

7. What is the importance of Fermi level?

Ans: Fermi level is the measure of the energy of least tightly held electrons within a solid. It is important in determining the thermal and electrical properties of solids.

8. Can the  junction less transistors be realized?

Ans:Yes,the junction less transistors be realized.

9. What is the doping level of E, B and C layers?

Ans:The doping level of Emitter(E) terminal is greater than Base(B) and collector(C) and base is lightly doped among the all.

10. List the various current components in BJT.

Ans:The current components in PNP BJT transistor are I_E-emitter current = I_PE + INE (sum of hole current and electron current),I_PC collector current due to majority charge carriers holes, I_C0 =reverse saturation current, I_CB0 =Reverse collector saturation current.

11. Draw the input and output characteristics of the transistor in CB configuration?

Ans:  

                  Input characteristics                            Output characteristics

12. Identify various regions in output characteristics?

Ans:The output characteristics has 3 basic regions Active, cut-off and saturation regions.

13. What are the applications of CB configuration?

Ans:  CB configuration generally used as buffer amplifier beacause its current gain is unity.

14. What are the input and output impedances of CB configuration?

Ans:The input impedence is V_EB/I_E and output impedence is V_CB/I_C .

15. What is EARLY effect?

Ans: In CB configuration when reverse bias voltage V_CB increases, the width of depletion region also increases , which reduces the electrical base width. This effect is called as Early effect.

16. Draw diagram of CB configuration for PNP transistor?

Ans:

17. What is the power gain of CB configuration?

Ans:Power gain of CB is =(V_EBI_E/V_CBI_C) (.i.e ratio of input power to the output power).