66.1.2. Multi-Transistor Amplifiers: Amplifier with Active Load (Part A)

Dear students welcome back to NPTEL online certification course on Analog Electronic Circuit. Myself Pradip Mandal from E and EC department of IIT Kharagpur. So, to continue this course today’s topic of discussion it is Amplifier with Active Loads. We may be having multiple amplifiers but, primarily we will be talking about common emitter and common source amplifier today.

In this introduction, the speaker welcomes students to the course and outlines the agenda for the lecture. The primary focus will be on amplifiers that utilize active loads, specifically the common emitter and common source amplifiers. Active loads differ from passive ones by using transistors to replace resistive loads, enhancing performance.

Think of a common emitter amplifier like a speaker system. Instead of simply using passive components like resistors (think of them as basic speakers), we are now using powerful active components (like amplifiers) that enhance sound quality and increase volume without distortion.

Now, today what we are planning to cover it is under this active load amplifiers, we do have to start with we do have motivation of using this active load. Then, from that we will we will be talking about basic operation of amplifier having active load and their corresponding circuit analysis including, small signal model and then finding the gain or maybe intuitively explaining the gain and so on.

The motivation behind using active loads in amplifiers lies in the need for improved performance. Traditional amplifiers with passive loads can limit gain and efficiency. By incorporating active loads, which involve using transistors, the overall functionality and gain of the amplifier can be enhanced significantly. The lecture will further explore how these improvements can be realized in practical applications.

Imagine upgrading a fountain with a water pump instead of relying solely on gravity to move water. Using a pump (active load) allows the fountain to provide a stronger and more consistent flow of water (better amplifier performance) compared to the passive system of just relying on a downhill slope (passive load).

This is a recapitulation or recalling whatever we know about CE amplifier and not only we will be talking about CE amplifier. But basic operation of the CE amplifier just to see that, what is its limitation of the voltage gain.

In this segment, the focus shifts to the common emitter (CE) amplifier, reviewing its basic operation. The CE amplifier not only amplifies signals but has its limitations in voltage gain due to the characteristics of passive components like resistors. The lecture will explore how these limitations can be addressed by switching to active loads.

Think of a common emitter amplifier as a microphone. While it amplifies sound well, it has a maximum volume limit based on the standard batteries (passive loads) used with it. To overcome this limit, you could use a more powerful power source (active load), allowing the microphone to amplify sound much better.

So, primarily CE amplifier it is having good gain. But in case if you want to enhance the gain further, then there is a scope of improving the gain and that may be done by replacing this passive element by its active equivalent circuit.

The CE amplifier typically provides a good gain, but there is a limitation in achieving higher voltage gain. This limitation can be addressed by replacing passive components with active ones that help increase the gain. The existing setup efficiently converts input signals to output, yet active loads enhance this process, allowing for better overall performance.

Consider a performance stage where the speakers (passive elements) can only amplify sound to a certain level. If you wanted to make the concert louder, you'd switch to more powerful sound systems (active loads) that can produce a much greater auditory experience, effectively improving sound quality and volume.

To come to the basic at the base, what we are doing is we are changing the voltage at the base or either you say base voltage or base to emitter voltage. And if you observe the base current flowing through the base terminal...

This section dives deeper into the operation of the common emitter amplifier by examining the relationship between the voltage at the base and the resulting current through the transistor. The behavior of transistors in response to varying base voltages is crucial for understanding how amplifiers operate and how signals are amplified. It highlights the dependence of collector current on base voltage and how these currents contribute to the output voltage.

Think of a faucet: turning the tap (voltage at the base) allows water (current) to flow out. The amount of water that flows out (collector current) depends on how much you open it (base voltage), controlling the flow effectively to achieve the desired pressure (output voltage).

Now, then if we consider the load line characteristic, as you have discussed load line characteristic it is given by essentially I-V characteristic of this R...

The load line characteristic is an essential tool for analyzing amplifiers. It helps visualize how output current changes with respect to the output voltage across a load. The intersection of the load line with the transistor characteristics determines the operating point, crucial for ensuring that the amplifier operates efficiently within its limits. The load line analysis also indicates how the amplifier behaves under different signal conditions.

Imagine a seesaw where one end represents current and the other represents voltage. As you adjust the position of the load (weight) on the seesaw, you can see how the balance (operating point) changes, teaching you how the amplifier will behave under different loads (signal variations).

If we want to really increase the gain, of course it is having a limitation of the gain will be talking about that also. If you numerically see what is the value here, if you put the expression of g into this equation that gives us that is I quiescent current...

This section discusses strategies to enhance gain in amplifiers by looking at the mathematics behind gain expressions. The relationship between output gain, current, and voltage drops is emphasized, showing how active loads can be implemented to overcome the limitations faced by traditional setups. The discussion hints at the maximum achievable gain and suggests that optimizing load conditions can allow for better performance in these amplifiers.

Imagine trying to bake a cake: if you only use basic ingredients (passive loads), the cake might not rise as much as you want. If you add eggs (active loads), the cake can rise higher, yielding a fluffier and taller dessert. In this context, using active loads is like adding those special ingredients to achieve better results.

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