Experiment No.
Title: Test the performance of BJT working in CB mode
Relevant Program Outcomes (POs):
• Basic and discipline specific knowledge: Apply knowledge of basic mathematics, science,
and engineering fundamentals and specialization to solve Electronics & Telecommunication
engineering problems.
• Engineering tools, experimentation and testing: Apply modern Electronics &
Telecommunication engineering tools and appropriate techniques to demonstrate &
practice experimental measurements.
Skills / Competencies to be Developed:
This practical is expected to develop the following skills:
1. Identify different types of components used.
2. Mount components on breadboard.
3. Use digital multimeter to measure different voltages and currents.
Relevant Course Outcomes (COs):
• Use BJT in Electronics circuits.
• Use relevant passive components in Electronics circuits.
• Use the front panel to understand various controls of the instrument.
Practical Outcomes:
1. Identify terminals of a given transistor.
2. Measure input and output currents and voltages.
3. Sketch the graph for voltage versus current.
4. Calculate current gain α.
Relevant Affective Domain Outcome:
Handle components and equipment carefully.
Resources Required:
• Breadboard
• DC power supply (0–30V)
• BJT (BC107)
• Resistors
• Digital Multimeter
• Ammeters (0–10mA, 0–100μA)
• Connecting wires
Theory:
A transistor is a three-terminal device: emitter, base, and collector. In Common Base (CB)
configuration, the input is applied between emitter and base, and the output is taken
between collector and base. Therefore, the base is common to both input and output.
The input characteristics of the CB configuration resemble a forward-biased diode.
However, the emitter current changes significantly for small changes in VEB. Output
characteristics show the relationship between IC and VCB at constant IE. The transistor
operates in active, saturation, or cut-off regions.
The current gain (amplification factor) in CB configuration is given by:
α = ΔIC / ΔIE
Input Characteristics:
Shows the relationship between input current (IE) and input voltage (VEB) for a constant
collector-base voltage (VCB).
Input resistance (ri) is calculated as:
ri = ΔVEB / ΔIE at constant VCB.
Output Characteristics:
Shows the relationship between output current (IC) and output voltage (VCB) for a constant
emitter current (IE).
Output resistance (ro) is calculated as:
ro = ΔVCB / ΔIC at constant IE.
Procedure:
Part I: Input Characteristics
1. Connect the circuit as per CB input characteristics diagram.
2. Select appropriate range for voltmeter, ammeter, and power supply.
3. Set collector-base voltage VCB = 2V using VCC.
4. Vary VEB in steps of 0.1V and note the emitter current IE.
5. Repeat steps for different VCB values.
6. Tabulate the readings.
Part II: Output Characteristics
1. Connect the circuit for output characteristics measurement.
2. Set a constant IE using a microammeter (e.g., IE = 100 μA).
3. Vary VCB from 0 to 10V in steps of 0.5V and record IC.
4. Repeat for different values of IE.
5. Tabulate the reading
Circuit Diagrams:
Figure 1: Input Characteristics Circuit Diagram (CB Mode)
Figure 2: Output Characteristics Circuit Diagram (CB Mode)
Table 1: Input Characteristics
Table 2: Output Characteristics
Graph:
• Plot VEB (V) vs IE (μA) for different VCB values.
• Plot VCB (V) vs IC (mA) for different IE values.
Calculations (from Graph):
1. Input Resistance (Ri) =
2. Output Resistance (Ro) =
3. Current Gain (α) =
Result:
1. Input Resistance Ri = ......... Ω
2. Output Resistance Ro = .......... Ω
3. Current Gain α = ..........
Conclusion:
The input characteristics show the variation of emitter current IE with emitter-base
voltage VEB at constant VCB.
The output characteristics show how collector current IC varies with VCB at constant IE.
The current gain α is typically less than 1 in CB configuration.
Questions:
1. Define current gain α.
2. Draw the symbol and terminals of a BJT.
3. Compare CE, CB, and CC configurations.
4. List applications of BJT in different configurations.
