66.1.1. Lecture – 66

Dear students, welcome back to NPTEL online certification course on Analog Electronic Circuit. Myself Pradip Mandal from E and EC department of IIT Kharagpur. Today’s topic of discussion is Amplifier with Active Loads, focusing on common emitter and common source amplifiers.

In this lecture, the instructor introduces the topic of active load amplifiers, which make use of multiple transistors. The primary focus will be on two types of amplifiers: the common emitter and common source amplifiers. Active loads involve replacing traditional passive loads with active components such as MOSFETs or BJTs, allowing for enhanced amplifier performance.

Imagine traditional light bulbs (passive loads) versus smart LED lights (active loads). The smart LED lights can adjust brightness more efficiently and respond better to control systems, similar to how active load amplifiers improve performance over passive ones.

The motivation for using active loads lies in their unique characteristics where the passive load is replaced, allowing for better amplification. This section will cover the basic operation and circuit analysis of amplifiers with active loads.

Active loads provide distinct advantages over passive loads, particularly in amplification applications. By replacing passive resistors with active devices, the amplifier can achieve higher voltage gain and performance. The lecture emphasizes understanding why this replacement is beneficial and how it influences amplifier behavior.

Consider a bicycle with normal gears (passive load) versus one with electronic gear shifting (active load). The electronic system can optimize performance more effectively, just as active loads enhance signal amplification in electronics.

We will discuss the limitations of voltage gain in the common emitter amplifier, particularly regarding the voltage drop across the collector resistor as it divides the supply voltage.

The common emitter amplifier limits its voltage gain due to the characteristics of the collector resistor. This resistor converts current to voltage, but its passive nature restricts how much gain can be achieved. The max voltage gain is tied to the voltage drop across this resistor, which cannot exceed the supply voltage. This sets a theoretical limit on gain, indicating a need for a better alternative with enhanced amplification capability.

Think of a water pipe (the collector resistor). If the pipe is too narrow, it restricts the flow of water. To increase flow rate (gain), you need to either enlarge the pipe or change the way water is pumped. Similarly, in electronics, if you want more gain than traditional setups allow, you must find a better way to achieve that.

Changing the base voltage varies the base current, which in turn affects the collector current through its relationship defined by exponentials.

The relationship between the base current and collector current in a common emitter amplifier is critical. As the base current changes, it causes a change in the collector current, which is crucial for amplification. Understanding this relationship helps in comprehending how input signals are translated into output signals. The variations in these currents reflect how the amplifier processes signals.

This can be likened to a classroom where a teacher (base) engages students (collector current) through varying levels of interaction. Increased engagement from the teacher leads to more active participation from students, demonstrating how input affects output.

The gain is impacted by the relationship between current and voltage, described through specific equations that define performance limits.

The gain of the amplifier is calculated as the ratio of output voltage to input voltage, determined by the relationships of currents and resistances in the circuit. This relationship shows that while high gain is desirable, it is limited by the parameters of the circuit components. The theoretical maximum is illustrated through calculations involving voltage and current drops.

This is like a performance rating for a vehicle. Just because a car can theoretically reach a top speed doesn’t mean it will under all conditions. Factors like engine capacity, gear ratios, and weight affect actual performance, similar to how various circuit components impact gain.

The discussion will lead towards practical alternatives for enhancing the gain of the amplifier while maintaining limits of voltage and current.

To enhance the gain beyond standard passive load limits, alternatives such as employing active loads are dealt with. This involves adjusting inherent characteristics of amplifiers to achieve better performance without increasing voltage supply levels, thus maintaining system stability while improving gain capability.

Imagine upgrading your home heating system where a smart thermostat manages energy use more effectively than a standard one, allowing for better temperature control without increasing the energy input. Similarly, using active loads helps achieve better amplification without needing more power.

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