Transistor Characteristics in Common-Base (CB) Configuration
1. Aim
To study and determine the input and output characteristics of a bipolar junction transistor (BJT) in Common-Base (CB) configuration and to find the current gain (α).
2. Apparatus Used
- NPN Transistor (2N3904 or equivalent)
- DC Power Supply (0-30V, variable)
- Ammeter (0-100mA) - 2 Nos.
- Voltmeter (0-20V) - 2 Nos.
- Rheostat/Potentiometer (1kΩ, 5kΩ) - 2 Nos.
- Connecting Wires
- Breadboard
- Digital Multimeter
3. Circuit Diagram
Fig 1. Common-Base Transistor Configuration Circuit
4. Theory
In the Common-Base (CB) configuration of a Bipolar Junction Transistor (BJT), the base terminal is common to both the input and output circuits. The input signal is applied between the emitter and base, while the output is taken from the collector and base.
Key characteristics of CB configuration:
- The input impedance is low (typically 20-50 ohms)
- The output impedance is high (typically 0.5-1 MΩ)
- The current gain (α) is less than 1 (typically 0.95 to 0.99)
- There is no phase shift between input and output signals
- It provides excellent high-frequency response
There are two important characteristics for a CB configuration:
1. Input Characteristics: The input characteristics show the relationship between the emitter current (IE) and emitter-base voltage (VEB) at constant collector-base voltage (VCB).
2. Output Characteristics: The output characteristics show the relationship between the collector current (IC) and collector-base voltage (VCB) at constant emitter current (IE).
Fig 2. Typical Input and Output Characteristics of CB Configuration
From these characteristics, we can determine the current gain (α) of the transistor in CB configuration.
5. Formula
The current gain (α) in Common-Base configuration is defined as:
$$\alpha = \frac{I_C}{I_E}$$
Where:
- IC = Collector current
- IE = Emitter current
The relationship between currents in a transistor:
$$I_E = I_C + I_B$$
Where IB is the base current.
The value of α is always less than unity (typically 0.95 to 0.99) because:
$$\alpha = \frac{I_C}{I_E} = \frac{I_C}{I_C + I_B} < 1$$
The relationship between α (CB current gain) and β (CE current gain):
$$\alpha = \frac{\beta}{1 + \beta}$$
$$\beta = \frac{\alpha}{1 - \alpha}$$
6. Procedure
A. For Input Characteristics
- Make the connections as shown in the circuit diagram for CB configuration.
- Keep the collector-base voltage (VCB) constant at 0V initially.
- Vary the emitter-base voltage (VEB) from 0 to 0.7V in small steps and note down the corresponding emitter current (IE).
- Repeat steps 2 and 3 for different values of VCB (e.g., 5V, 10V).
- Plot the graph between VEB (X-axis) and IE (Y-axis) for different values of VCB.
B. For Output Characteristics
- Set the emitter current (IE) at a constant value (e.g., 1mA) using the potentiometer in the emitter circuit.
- Vary the collector-base voltage (VCB) from 0 to 10V in steps and note down the corresponding collector current (IC).
- Repeat steps 1 and 2 for different values of IE (e.g., 2mA, 3mA, 4mA, 5mA).
- Plot the graph between VCB (X-axis) and IC (Y-axis) for different values of IE.
C. For Determining Current Gain (α)
- From the output characteristics, select a suitable operating point in the active region.
- For a fixed value of VCB (e.g., 5V), note the values of IC corresponding to different values of IE.
- Calculate the current gain α = IC/IE for each set of values.
- Find the average value of α.
7. Observation Tables
A. Input Characteristics
| VCB = 0V | VCB = 5V | VCB = 10V | |||
|---|---|---|---|---|---|
| VEB (V) | IE (mA) | VEB (V) | IE (mA) | VEB (V) | IE (mA) |
| 0.1 | 0.1 | 0.1 | |||
| 0.2 | 0.2 | 0.2 | |||
| 0.3 | 0.3 | 0.3 | |||
| 0.4 | 0.4 | 0.4 | |||
| 0.5 | 0.5 | 0.5 | |||
| 0.6 | 0.6 | 0.6 | |||
| 0.7 | 0.7 | 0.7 | |||
B. Output Characteristics
| VCB (V) | IC (mA) for IE = 1mA | IC (mA) for IE = 2mA | IC (mA) for IE = 3mA | IC (mA) for IE = 4mA | IC (mA) for IE = 5mA |
|---|---|---|---|---|---|
| 0 | |||||
| 1 | |||||
| 2 | |||||
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| 10 |
C. Current Gain Calculation
| IE (mA) | IC (mA) at VCB = 5V | α = IC/IE |
|---|---|---|
| 1 | ||
| 2 | ||
| 3 | ||
| 4 | ||
| 5 |
8. Calculations
From the data collected in the observation tables, we can calculate:
1. Current Gain (α):
For each value of IE, calculate α using:
$$\alpha = \frac{I_C}{I_E}$$
For example, if at IE = 3mA and VCB = 5V, the measured IC = 2.94mA, then:
$$\alpha = \frac{2.94\text{ mA}}{3\text{ mA}} = 0.98$$
2. Average Current Gain:
Calculate the average value of α from all the measurements:
$$\alpha_{avg} = \frac{\sum \alpha_i}{n}$$
Where n is the number of measurements.
3. Input Resistance (ri):
From the input characteristics, at a specified operating point:
$$r_i = \frac{\Delta V_{EB}}{\Delta I_E}$$
This is the slope of the input characteristic curve at the operating point.
4. Output Resistance (ro):
From the output characteristics, at a specified operating point:
$$r_o = \frac{\Delta V_{CB}}{\Delta I_C}$$
This is the reciprocal of the slope of the output characteristic curve at the operating point.
9. Result
- The input and output characteristics of the BJT in Common-Base configuration have been plotted.
- The current gain (α) of the transistor is found to be ____________.
- The input resistance (ri) at the operating point is ____________ ohms.
- The output resistance (ro) at the operating point is ____________ ohms.
-
Observations:
- The input characteristics show that the emitter-base junction behaves like a forward-biased diode.
- The output characteristics demonstrate that the collector current IC is nearly independent of VCB in the active region.
- The current gain α is less than 1, as expected in a CB configuration.
10. Precautions
- Always connect the ammeter and voltmeter with proper polarity.
- Do not exceed the maximum ratings of the transistor used in the experiment.
- Ensure that the base terminal is properly grounded.
- Keep the power supply voltage within safe limits to prevent damage to the transistor.
- Make sure all connections are tight and secure.
- Handle the transistor carefully to avoid static damage.
- Note the readings carefully and accurately.
- Never short-circuit the power supply.
- Turn off the power supply when making changes to the circuit.
- Avoid touching the circuit components when the power is on.
- Keep the wattage of the circuit within permissible limits to avoid overheating.
- Use proper value resistors in the circuit as specified.
11. Viva Voice Questions
Q1: What is the Common-Base configuration of a transistor?
Ans: In a Common-Base configuration, the base terminal is common to both input and output circuits. The input signal is applied between emitter and base while output is taken from collector and base.
Q2: What are the main characteristics of the Common-Base configuration?
Ans: The main characteristics include: low input impedance (20-50 Ω), high output impedance (0.5-1 MΩ), current gain less than unity (0.95-0.99), no phase shift between input and output signals, and excellent high-frequency response.
Q3: Why is the current gain (α) always less than 1 in CB configuration?
Ans: The current gain α = IC/IE. Since IE = IC + IB, and IB is always positive, IC is always less than IE, making α less than 1.
Q4: Compare Common-Base, Common-Emitter, and Common-Collector configurations.
Ans:
- Common-Base: α < 1, low input impedance, high output impedance, no phase inversion, good for high frequencies.
- Common-Emitter: β > 1, medium input impedance, medium output impedance, 180° phase inversion, commonly used for amplification.
- Common-Collector: gain ≈ 1, high input impedance, low output impedance, no phase inversion, used as buffer/impedance matcher.
Q5: What is the relationship between α and β?
Ans: The relationship between α (CB current gain) and β (CE current gain) is:
α = β/(1+β) and β = α/(1-α)
Q6: Why does the collector current remain almost constant with changes in VCB in the active region?
Ans: In the active region, the collector current is primarily controlled by the emitter current and is relatively independent of collector voltage. This is because the width of the depletion region at the collector-base junction doesn't significantly affect the flow of majority carriers from emitter to collector.
Q7: What are the practical applications of the Common-Base configuration?
Ans: Common-Base configurations are used in:
- High-frequency amplifiers and oscillators
- VHF and UHF amplifiers
- RF amplifiers in radio receivers
- Impedance matching applications
- Voltage amplifiers where current gain is not required
Q8: What are the regions of operation of a BJT?
Ans: The regions of operation are:
- Cut-off region: Both junctions are reverse biased, no current flows
- Active region: Emitter-base junction is forward biased, collector-base junction is reverse biased
- Saturation region: Both junctions are forward biased
- Inverse active region: Emitter-base junction is reverse biased, collector-base junction is forward biased
Q9: How does temperature affect the transistor characteristics in CB mode?
Ans: Increase in temperature:
- Increases leakage current (ICBO)
- Decreases the forward voltage drop across emitter-base junction
- Slightly decreases the current gain (α)
- Can lead to thermal runaway if not properly biased
Q10: Why is CB configuration preferred for high-frequency applications?
Ans: CB configuration is preferred for high-frequency applications because:
- It has lower input capacitance compared to other configurations
- The grounded base acts as a shield between input and output, reducing feedback capacitance
- It has better high-frequency response due to reduced Miller effect
- It provides good isolation between input and output circuits
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Note: This lab manual is designed for educational purposes. Students should follow all safety procedures and guidelines provided by their instructor or laboratory supervisor.