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

Objective:

To study the input and output characteristics of a transistor in Common Emitter configuration.

Components:

S.No.	Name	Quantity
1	Transistor BC 107	1(One) No.
2	Resistors (1K, 100K)	1(One) No. Each
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, 0-200 A)	1(One) No. Each
3	Digital Voltmeter (0 - 20V)	2(Two) No.
4	Connecting wires (Single Strand)

Specifications:

For Transistor BC 107:

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

Circuit Diagram:

h – Parameter model of CE transistor:

Pin assignment of Transistor:

View from side of pins

View from top of casing

Operation:

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

Input characteristics are obtained between the input current and input voltage at constant output voltage. It is plotted between V_BE and I_B at constant V_CE in CE configuration.

Output characteristics are obtained between the output voltage and output current at constant input current. It is plotted between V_CE and I_C at constant I_B in CE configuration.

Procedure:

Input Characteristics:

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

Output Characteristics:

1. Connect the circuit as shown in the circuit diagram.
2. Keep emitter current I_B = 20A by varying V_BB.
3. Varying V_CC gradually in steps of 1V up to 12V and note down collector current I_C and Collector-Emitter Voltage(V_CE).
4. Repeat above procedure (step 3) for I_B = 60µA, 0µA.

Observations:

Input CharacteristicsInput Characteristics
VBB (Volts)	VCE = 0V		VCE = 5V
	VBE (Volts)	IB (µA)	VBE (Volts)	IB (µA)

 

Output Characteristics
VCC (Volts)	IB = 0 µA		IB = 20 µA		IB = 40 µA
	VCE (Volts)	IC (mA)	VCE (Volts)	IC (mA)	VCE (Volts)	IC (mA)

Graph:

Input Characteristics

Output Characteristics

1. Plot the input characteristics by taking V_BE on X-axis and I_B on Y-axis at a constant V_CE  as a constant parameter.
2. Plot the output characteristics by taking V_CE on X-axis and taking I_C on Y-axis taking I_B as a constant parameter.

Calculations from Graph:

1. Input Characteristics: To obtain input resistance find V_BE and I_B for a constant V_CE on one of the input characteristics.

Input impedance = h_ie = R_i = V_BE / I_B (V_CE is constant)

Reverse voltage gain = h_re = V_EB / V_CE (I_B = constant)

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

Output admittance 1/hoe = R_o = I_C / V_CE  (I_B is constant)

Forward current gain = hfe = I_C / I_B (V_CE = constant)

Inference:

1. Medium input and output resistances.
2. Smaller values if V_CE, lower the cut-in-voltage.
3. Increase in the value of I_E causes saturation of the transistor of an earlier voltage.

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 Emitter Configuration are studied.

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

1. The Input Resistance (h_ie)                       _______________Ohms.
2. The Reverse Voltage Gain (h_re­)               _______________.
3. The Output Conductance (h_oe)                 _______________ Mhos.
4. The Forward Current Gain (h_fe)                 _______________.

Outcomes: Students are able to

1. Analyze the characteristics of BJT in Common Emitter and configuration.
2. Calculate h-parameters from the characteristics obtained.

---

Viva Questions:

1. Can transistor be replaced by two back to back connected diodes?

Ans: No, because the doping levels of emitter(heavily doped), base(lightly doped) and collector(doping level greater than base and less than emitter) terminals are different from p and n terminals in diode.

2. For amplification CE is preferred, why?

Ans: Because amplification factor beta is usually ranges from 20-500 hence this configuration gives appreciable current gain as well as voltage gain at its output on the other hand in the Common Collector configuration has very high input resistance(~750K) & very low output resistance(~25) so the voltage gain is always less than one & its most important application is for impedance matching for driving from low impedance load to high impedance source.

3. To operate a transistor as amplifier, emitter junction is forward biased and collector junction is reverse biased. Why?

Ans: Voltage is directly proportional to Resistance. Forward bias resistance is very less compared to reverse bias. In amplifier input forward biased and output reverse biased so voltage at output increases with reverse bias resistance. 

4. Which transistor configuration provides a phase reversal between the input and output signals?

Ans: Common emitter configuration.

5. What is the range β of a BJT?

Ans: Beta is usually ranges from 20 - 500.

6. List the current components of BJT in CE configuration.

Ans:I_B = input current (from base to emitter flow), I_C = output current (from collector terminal to emiiter), I_E = is emitter current  which sum of I_B and I_C.I_c0 is reverse saturation current.

7. What is Early Effect?

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

8. Why the doping of collector is less compared to emitter?

Ans:In Transistor one terminal is intentionally heavily doped than others, to emittes the electrons. Due to diffussion process entire emitter current reaches to collector and base current is neglegibly small.

9. What do you mean by “reverse active”?

Ans: In reverse active mode, the emitter-base junction is reverse biased and the collector-base junction is forward biased. This condition is the reverse of active mode. Hence, reverse active mode.

10. What is the difference between CE and Emitter follower circuit?

Ans:In CE we will take the output at collector terminal and ,In CC or emitter follower circuithe output taken from the emitter terminal. In CE input resistance is low and output resistance is high where as in CC input resistance is high and output resistance is low.

11. What are the input and output impedances of CE configuration?

Ans:In CE input impedance is low(around 1K) and output resistance is high( around 40K).

12. Identify various regions in the output characteristics?

Ans: The output characteristics of CE configuration consists of three regions which are Active region, Saturation region and cutoff egion.

13. What is the relation between α, β and γ?

Ans:            and 

14. Define current gain in CE configuration?

Ans: Current gain in CE is ratio of output current I_C to the input current I_B, which is represented by β = I_C/I_B .

15. Why CE configuration is preferred for amplification?

Ans: The CE configuration is the only configuration which provides both voltage gain as well as current gain greater than unity.

16. What is the phase relation between input and output?

Ans: The output voltage is out of phase with input voltage.

17. Draw diagram of CE configuration for PNP transistor?

Ans: 

18. What is the power gain of CE configuration?

Ans:Power gin of CE configuration is 

19. What are the applications of CE configuration?

Ans: CE configuration is used for audio signal amplicaions.