67.2. Multi-Transistor Amplifiers: Amplifier with Active Load (Part B)

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 section, the instructor is reintroducing the topic by discussing the limitations of voltage gain in common emitter and common source amplifiers using passive loads. They suggest that these limitations can be addressed by employing active loads. The critical insight here is that by manipulating the load line characteristics, an amplifier's gain can be significantly increased. This sets the stage for a more advanced discussion about the mechanics behind these improvements.

Imagine trying to push a swing with a friend sitting on it. If your friend sits too far back (analogous to a passive load), it becomes hard to gain momentum (or voltage gain). However, if your friend moves closer to the front of the swing (simulating an active load), it’s much easier for you to push and get the swing moving higher.

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.

For the amplifier to function correctly, the currents through the two transistors (involved in the active load configuration) must be equal. This equality is crucial because if one current is higher than the other, it can shift the operational state of one of the transistors into a different region (from saturation to triode, for example), affecting the amplifier's gain and performance. The principle of Kirchhoff's Current Law (KCL) tells us that at a junction where no additional current flows in or out, the input currents will equal the output currents.

Consider a team of runners in a relay race where each runner represents a transistor. For the team to perform well, each runner must pass the baton at the right moment without slowing down. If one runner starts running too fast (one current is larger), they may drop the baton, disrupting the entire race (the operational integrity of the amplifier). Thus, it's crucial that all runners match their speeds.

If you are increasing the slope here, instead of decreasing the slope then it may be rather the gain it may be lower.

When configuring the active load, it’s important to understand that just increasing the slope of the load line by using a stronger load can sometimes lead to lower gain. This counterintuitive idea points to the need for a balanced approach where the load's characteristics are carefully chosen to optimize performance. The load line must be correctly adjusted to ensure that the operational point is in saturation, thereby maximizing the output gain.

Think of tuning a musical instrument. If a guitar string is too tight (analogous to a high slope), it might produce a sharp, unpleasant sound (lower gain), while a perfectly tuned string resonates beautifully (optimal gain). Just like adjusting tightness helps achieve the right sound, adjusting the load helps achieve the desired amplifier output.

So, again to come back to the basic operation of the device particularly for M , we do have I versus V characteristic curve...

The operation of the amplifier involves analyzing the I-V characteristics of the transistors involved. This section reveals how to interpret the behavioral graph (I vs. V) for one of the transistors, outlining the significance of regions such as saturation and triode. It is crucial to line up these characteristics with the desired output signals to ensure the amplifier's efficiency and effectiveness.

Consider a light dimmer switch in your home. The relationship between how far the switch is turned (the voltage) and how bright the light shines (the current) can be compared to the I-V characteristics of the transistor. Just as you'd adjust the dimmer to get the perfect light setting, an engineer tweaks the voltage to achieve the right current for optimal performance.

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...

In this section, the process of constructing a small signal equivalent circuit for the active load is explained. Understanding this small signal model is essential for analyzing how small changes in input can affect output. This model simplifies calculations by representing transistors as dependent sources affected by specific input parameters. It enables engineers to predict how the circuit will respond to variations in signal strength.

Think of a small signal equivalent circuit like using a scale model of a train to understand how full-sized trains operate. Just as the model allows you to observe behavior and make predictions without the complexity of a full train, the small signal equivalent circuit allows electrical engineers to work with manageable calculations to understand larger, more complex circuits.

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