Problem 48 - Master Resource Book in JEE Main Physics

Question

Consider the transistor shown in figure, its terminals are marked as 1,2 and 3 . Using multimeter one try to identify the base of transistor, he proceed in the way as follows Experiment I He touches the common lead of the multimeter to 2 , then on touching other lead of multimeter to 1 he hasn't got any beep (indication of conduction) but when connected to 3 got the beep. Experiment II He connects the common lead of multimeter to 1 and other lead to 2 and 3 one by one then in this case he got beep for both connections. From this we conclude that (a) 1 is base (b) 2 is base (c) 3 is base (d) None of these

Step-by-Step Solution

Answer

Terminal 1 is the base of the transistor.

Step 1: Analyzing Experiment I

In Experiment I, the common lead of the multimeter is connected to terminal 2. When the other lead is connected to terminal 1, there is no conduction (no beep), but when connected to terminal 3, conduction occurs (beep is heard). This suggests that terminal 2 may or may not be the base, due to lack of complete information yet.

Step 2: Analyzing Experiment II

In Experiment II, the common lead is connected to terminal 1, while the other lead is connected to terminals 2 and 3, one at a time. Conduction is observed (beep is heard) in both cases. This indicates that terminal 1 is conducting with both terminal 2 and terminal 3, suggesting that terminal 1 acts as the base, as the base typically forms conduction paths with the other two terminals in a bipolar junction transistor (BJT).

Step 3: Conclusion from Results

Based on the experiments, terminal 1 conducts with both 2 and 3 when the common lead is connected to it, which is characteristic of a base in a BJT. Therefore, the conclusion from both experiments is that terminal 1 is most likely the base of the transistor.

Key Concepts

Multimeter Usage

A multimeter is an essential tool used in electronics for measuring various parameters such as voltage, current, and resistance. In this context, we are using it to identify the terminals of a transistor. Transistors have three terminals, and one of the most common tasks is to identify the base, collector, and emitter. This can be achieved by measuring the conductivity between the terminals.

Here's how you can use a multimeter for this purpose:

• Set the Multimeter: Switch the multimeter to diode testing mode. This mode is usually indicated by a diode symbol on the multimeter.
• Common and Testing Leads: The multimeter has two leads – red and black. The black lead is typically the common (negative) and the red is the testing lead (positive).
• Reading Conductivity: When testing a transistor, if there's conductivity between the terminals, the multimeter will beep or show a value.

By strategically connecting these leads to different terminals of the transistor, you can quickly identify which terminal is acting as the base by observing where conduction occurs uniformly with other terminals.

Base Terminal Identification

Identifying the base terminal of a bipolar junction transistor (BJT) is crucial for understanding its behavior. A BJT consists of three terminals: base, collector, and emitter.

• Base Characteristics: The base terminal is unique because it forms a conductive path with both the collector and emitter.
• Experimental Process: In Experiment I, the lack of conductivity between terminals 2 and 1 but the presence of it between 2 and 3 suggested that terminal 2 could be the base. However, this isn't conclusive.
• Further Verification: In Experiment II, when the common lead was on terminal 1, conduction was observed with both terminals 2 and 3. This uniform conductivity is a distinctive property of the base terminal in a BJT.

Thus, based on these observations, terminal 1 is identified as the base terminal, because it establishes a conduction path with both other terminals in this setup.

Bipolar Junction Transistor (BJT) Analysis

Bipolar junction transistors (BJTs) are semiconductor devices that amplify current. They come in two types: NPN and PNP, with the difference being the type of charge carriers predominantly involved (electrons and holes, respectively).

In BJTs:

• Base Role: The base is the central terminal that controls the flow of current between the other two terminals, the emitter and collector.
• Functional Differentiation: By identifying the base terminal, as we did in this exercise with multimeters, we can then distinguish the collector from the emitter. This is crucial because BJTs need proper biasing between these terminals for correct operation.
• Experiment Analysis: In our exercise, the process of determining the base involved observing the conduction pattern between terminals. When terminal 1 was shown to conduct with both 2 and 3, it confirmed its role as the base.

Understanding BJTs through this type of analysis is fundamental in electronics, paving the way for troubleshooting and designing circuits effectively.