67.1. Analog Electronic Circuits

Yeah. So, welcome back after the short break. And we were discussing about the limitation of the voltage gain of the common emitter and common source amplifier particularly if it is having passive load. And intuitively we understand that, how it can be enhanced. Namely in case if we can get some characteristic load line characteristic like this, instead of having a linear characteristic. In fact, that is the center point of getting higher gain of any amplifier using active load.

In this segment, we look at the limitations of voltage gain in common emitter and common source amplifiers when using passive loads. Passive loads do not boost voltage gain adequately. However, when introduced with an active load, significant improvements in gain can be achieved. This study highlights how the load line characteristics of amplifiers change, emphasizing the importance of using active loads to move away from linear characteristics, which is key to increasing voltage gain.

Think of trying to lift a heavy box with a lever. If the lever's pivot point is not positioned correctly (similar to using a passive load), you may not lift the box effectively. However, if you can use a mechanical advantage (like an active load), it becomes much easier to lift it, illustrating how the position of the pivot impacts effectiveness.

So, let us see what kind of implementation we can think of to get this kind of non-linear load line, particularly this one which is giving us the gain. So, in the next slide we do have common source amplifier with active load.

Here, we begin discussing how to implement an active load in a common source amplifier. The active load is designed to replace a passive load to facilitate a non-linear load line, which is crucial for achieving a higher gain. This non-linear characteristic modifies the behavior of the amplifier favorably, allowing it to function at a higher level of performance.

Consider a car engine that operates more efficiently when tuned for performance (active load) rather than when merely chugging along (passive load). By fine-tuning what the engine requires, you maximize its potential, much like designing an amplifier for optimal gain.

Now naturally, then who defines this current? For proper operation, we require both the current should be equal and we need to satisfy some condition to ensure that I and I; I rather I they should be equal.

In this chunk, we discuss the importance of current equality in the active load circuit. For the amplifier to work correctly, the currents through the involved devices need to be equal. This balance ensures that both transistors are operating under optimal conditions, particularly in the saturation region. It prevents one device from shifting into a different operational region which would adversely affect amplification.

Imagine a seesaw at a playground – it only works when both kids are of equal weight. If one side is heavier, it tips dramatically, affecting play. Similarly, in an amplifier, ensuring equal currents is necessary to maintain balance and proper performance.

If you are not paying good attention and if you are simply saying that I do not have any other circuit connected and the current of the two devices they must be equal. Then what it may happen that that ah one of these two devices will be forced into entering into a triode region, the other one may be remaining in saturation region and it will be having a huge consequence on the gain.

This part explains that if the needs for current equality are ignored, one device could inadvertently fall into the triode region (in which its gain drops significantly), while another stays in the saturation region, where it operates efficiently. Such a discrepancy can seriously degrade the amplifier's performance and overall gain.

Consider a relay race where all team members must maintain pace. If one runner slows down (triode region), it affects the whole team's completion time (gain). Conversely, if everyone runs together (saturation), then the race is completed efficiently.

So, whenever we say that these two are equal what we mean is that, the devices are in saturation region and in saturation region whatever the current we do have they are equal.

Here, the focus is on defining equal currents in the context of saturation. When both devices are in saturation, it indicates they are operating efficiently and within their optimal performance regimes. This equality and saturation state ensure that the amplifier can achieve and maintain its intended gain.

When cooking, think of two pots boiling over the same heat source—if both are maintained at the right temperature (saturation), they will perfectly cook their contents (achieving desired gains). If one pot overheats (moves into triode), that process could spoil.

To get the corresponding load line first of all let me draw the and the current flow here namely I as function of V ; for a given value of the V.

This segment introduces the concept of the load line in relation to output characteristics, illustrating how current flow and voltage (V) interact under defined conditions. By drawing these characteristics, it visually depicts the relationship and effectively sets up for understanding how alterations in these parameters affect overall amplifier performance.

Think of drawing a graph of your monthly expenses against your income. The line would represent where your expenses are relative to your income (current versus voltage). Depending on your income (V), your spending (I) can increase or decrease, showcasing how well you manage finances just like understanding an amplifier's functionalities.

So, the output it is changing from here to here. So, so it is a big change versus if I consider on the other hand it is passive load for the same variation the intersection point of the device characteristic and the passive load line it is one is here, another is here.

This portion describes the difference in output voltage variations under active versus passive loads. It emphasizes the significant output change that an active load can bring compared to a passive one for the same input signal variation, showcasing the superior performance of active loads in amplifying signals.

Imagine using a loudspeaker in a noisy environment. An active speaker (active load) can adapt to amplify your voice significantly over background noise, whereas a regular speaker might struggle to increase volume adequately (passive load).

So, in the next slide we do have the we have drawn the small signal equivalent circuit for the common source amplifier having active load.

We transition into analyzing the small signal equivalent circuit, critical for understanding how to assess the gain and output resistance of the amplifier. The small signal model simplifies the analysis when observing how the circuit responds to minor variations, thereby providing deeper insights into the amplification process.

Think of a small signal equivalent circuit as a way to understand a car's acceleration at low speeds. You wouldn't test a car at top speed to gauge its performance. Similarly, small variations allow us to assess the amplifier's behavior without overwhelming complexities.

So, in summary what do you obtain here it is that, because the load line slope, it got changed compared to the earlier slope;

This final portion succinctly summarizes the key findings: switching from a passive load to an active load significantly alters how the gain is realized. It explains how comparative slopes affect gain and bandwidth, highlighting the trade-offs that come with transitioning between load types.

When studying for a test, if you switch from passive review methods (like rereading notes) to active engagement (like solving practice questions), you likely find your grasp of the material (gain) improves significantly, illustrating the principles discussed.

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