Common-Emitter Configuration of a Transistor

Summary

In this lecture on the common emitter configuration of a transistor, the focus is on how the transistor amplifies current. The common emitter setup is explained with an NPN transistor, where the emitter is common to both input and output sides. The key relationships between input and output currents are outlined, including the role of current gain (β). The transistor operates in active mode, with forward and reverse biasing applied. The lecture emphasizes the sensitivity of β to variations in α and compares the leakage current's effect in common emitter vs. common base configurations. The common emitter is shown to be an effective current amplifier.

Takeaways

• 😀 The common emitter configuration of a transistor is used to amplify current, with the emitter shared between the input and output sides.
• 😀 In the common emitter configuration, the transistor operates in active mode: the collector-base junction is reverse biased, and the emitter-base junction is forward biased.
• 😀 The input current (I_B) is the base current, and the output current (I_C) is the collector current in the common emitter configuration.
• 😀 The relationship between input current (I_B) and output current (I_C) is described by the current amplification factor, denoted as β (beta).
• 😀 β (beta) is calculated as the ratio of I_C to I_B, and typically ranges from 50 to 400 depending on the transistor.
• 😀 Small changes in the value of α (alpha) can cause significant changes in β, making β sensitive to variations in α.
• 😀 β is referred to as the current amplification factor, and its value depends on the transistor's characteristics and operating conditions.
• 😀 In the common emitter configuration, the transistor amplifies current, which distinguishes it from voltage amplifiers typically used in other configurations.
• 😀 The leakage current (I_CBO) in the common emitter configuration has a larger contribution to the output current compared to the common base configuration due to the multiplication by β + 1.
• 😀 In the common base configuration, the leakage current (I_CBO) has a minimal effect on the output current, as it is not amplified by a large factor like in the common emitter configuration.
• 😀 To calculate the output current (I_C), use the formula I_C = β * I_B, with β being the current amplification factor, which can significantly amplify the input current.

Q & A

What is the main configuration discussed in the lecture?

The lecture focuses on the common emitter configuration of a transistor.

What is the significance of the transistor being in active mode for amplification?

For the transistor to act as an amplifier, it must operate in active mode, where the collector-base junction is reverse biased, and the emitter-base junction is forward biased.

How is the collector-base junction biased in the common emitter configuration?

The collector-base junction is reverse biased by applying a potential VCC, where the positive terminal is connected to the N-type material, and the negative terminal to the P-type material.

What happens to the base-emitter junction in the common emitter configuration?

The base-emitter junction is forward biased, with the P-type material connected to the positive terminal and the N-type material connected to the negative terminal.

What is the relationship between the emitter, collector, and base currents in a transistor?

The emitter current (IE) is the sum of the base current (IB) and collector current (IC), i.e., IE = IC + IB.

What is the formula for the collector current (IC) in terms of the emitter current (IE) and leakage current (ICBO)?

The collector current can be expressed as IC = α * IE + ICBO, where α is the current gain factor.

What is the meaning of Beta (β) in the context of transistor amplification?

Beta (β) is the current amplification factor, defined as the ratio of the collector current (IC) to the base current (IB), i.e., β = IC / IB.

How does the value of alpha (α) influence beta (β)?

Beta (β) is highly sensitive to variations in alpha (α). A small change in α can result in a significant change in β. For example, with α = 0.98, β is 49, and with α = 0.95, β is 19.

How does the common emitter configuration behave as an amplifier?

The common emitter configuration primarily functions as a current amplifier, meaning it amplifies the base current (IB) into a larger collector current (IC).

How does the leakage current affect the common emitter configuration compared to the common base configuration?

In the common emitter configuration, the leakage current (ICBO) has a larger contribution to the output current due to multiplication by β + 1, whereas in the common base configuration, its contribution is smaller because it is not multiplied by such a large factor.