In a Common Emitter (CE) BJT amplifier, which of the following options is correct?

Explanation:

Correct Option Analysis

In a Common Emitter (CE) BJT amplifier, the correct option is:

Option 4: If the magnitude of the output voltage increases, the input current decreases.

To understand why this option is correct, we need to delve into the operating principles of a CE BJT amplifier and the relationship between its input and output parameters.

Common Emitter (CE) Amplifier:

A Common Emitter amplifier is one of the most widely used configurations in transistor amplifiers. In this configuration, the emitter terminal of the Bipolar Junction Transistor (BJT) is common to both the input and output circuits, hence the name "Common Emitter". It is known for providing significant voltage amplification.

Working Principle:

In a CE amplifier, the input signal is applied to the base-emitter junction, and the amplified output signal is taken from the collector-emitter circuit. The transistor operates in the active region where the base-emitter junction is forward-biased and the collector-base junction is reverse-biased.

The relationship between the input and output parameters can be described as follows:

• The input current (IB) is the current flowing into the base of the transistor.
• The output current (IC) is the current flowing through the collector of the transistor.
• The output voltage (VCE) is the voltage across the collector-emitter terminals.

In the active region, the collector current (IC) is primarily controlled by the base current (IB), following the relation:

IC = β × IB

Where β is the current gain of the transistor.

Explanation of Option 4:

Option 4 states that "If the magnitude of the output voltage increases, the input current decreases." This statement is correct and can be explained by the following points:

• In a CE amplifier, when the output voltage (VCE) increases, it implies that the collector-emitter voltage is increasing.
• This increase in VCE means that the collector current (IC) is increasing, as VCE is proportional to the product of IC and the load resistance (RC).
• Since IC is proportional to the base current (IB), an increase in IC would typically result in a decrease in IB because the transistor is operating in the active region where the relationship between IC and IB is governed by the current gain (β).
• Therefore, if the output voltage (VCE) increases, the input current (IB) decreases as the transistor adjusts to maintain the balance between the input and output currents.

Important Information:

Now let's analyze the other options to understand why they are incorrect:

Option 1: The CE amplifier has the lowest input impedance among all the configurations.

This statement is incorrect. The CE amplifier does not have the lowest input impedance among all configurations. In fact, the Common Base (CB) configuration has the lowest input impedance, while the Common Collector (CC) configuration has the highest input impedance. The CE configuration has an intermediate input impedance, which is lower than the CC configuration but higher than the CB configuration.

Option 2: Its input impedance is lower than CB configuration but higher than CC configuration.

This statement is incorrect. As mentioned earlier, the input impedance of the CE configuration is higher than the CB configuration but lower than the CC configuration. Therefore, the statement should be reversed to be accurate.

Option 3: The input characteristics are plotted between the input current (IB) and input voltage (VBE) at a constant IC.

This statement is incorrect. The input characteristics of a CE amplifier are plotted between the input current (IB) and the input voltage (VBE) at a constant collector-emitter voltage (VCE), not at a constant collector current (IC). The input characteristics show the relationship between IB and VBE, reflecting the behavior of the base-emitter junction.

Conclusion:

In summary, the correct option in the context of a Common Emitter (CE) BJT amplifier is Option 4: "If the magnitude of the output voltage increases, the input current decreases." This is due to the relationship between the input and output currents governed by the current gain (β) of the transistor in the active region. The other options are incorrect based on the analysis of input impedance and input characteristics in different transistor configurations.