67.8.1. Real-world Circuit Adjustments

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 chunk, we explore the issue of voltage gain limitations in amplifiers, particularly the common emitter and common source types. When these amplifiers use passive loads, their maximum voltage gain is limited. However, by introducing an active load, the circuit can achieve a higher gain. The concept is illustrated by the need to manipulate the load line characteristics of the amplifier. Instead of sticking to a linear characteristic, the goal is to achieve a non-linear load line that enhances the amplifier's performance.

Think of it like having a bicycle that you can only pedal on flat ground (passive load); going downhill would allow you to go faster (active load). By changing the slope of the hill (the characteristics of the load), you could potentially travel much faster than when riding on flat ground.

So, here the lower part we are retaining same and same circuit we do have the M1 which is receiving the signal at its gate along with the DC voltage. But then it is also having the load, which is instead of having passive load, but it is having a transistor M2. Note that this transistor M2, this is PMOS transistor right.

In this chunk, the concept of implementing an active load using a PMOS transistor (M2) in conjunction with the existing transistor (M1) is introduced. By retaining the original circuit design with M1, M2 is now used as an active load. The source of the PMOS transistor is connected to a DC voltage (VDD). This setup allows the circuit to benefit from the characteristics of the PMOS load, which is expected to enhance overall gain. Understanding how each transistor functions in relation to the DC voltage is crucial for analyzing the circuit's performance.

Imagine upgrading a car with a turbocharger; the original engine (M1) can perform adequately, but adding the turbo (M2) significantly improves its power and efficiency, allowing it to accelerate faster without changing the entire engine.

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_DS1 and I_DS2 should be equal.

This chunk emphasizes the critical requirement for the two transistors' currents (I_DS1 for M1 and I_DS2 for M2) to be equal for the amplifier to function correctly. This condition stems from Kirchhoff's Current Law (KCL), which states that the total current entering a node must equal the total current leaving the node. By ensuring I_DS1 equals I_DS2, both devices will maintain their operational status in the saturation region, which is essential for achieving the desired gain and stability in the circuit.

Consider balancing two scales at a farmer’s market: for the weights to be comparable on both sides (the currents must be equal), fruits on both sides must match perfectly. If one side has more than the other, it tips the scale, causing an imbalance (affects the circuit operations).

Well, at this node we do not have any other circuit connected. So, it is very natural to say that why do we require any condition for this two current to be equal; because it is KCL as we do not have any other circuit connected here.

Within this segment, it is reiterated that without other components connected to the circuit node under discussion, it seems intuitive to assume the currents should be equal due to KCL. However, this principle emphasizes the necessity of maintaining both transistors in the saturation region during operation. This focus on operational regions ensures that the devices perform correctly without issues such as one entering a triode region, which would adversely affect gain.

This is akin to a team relay race; if each runner (current) doesn’t keep pace with their teammates (transistor operations), the overall performance (amplifier gain) suffers. All must operate smoothly and in sync to improve the team's overall speed.

So, I must make you aware that we have to pay additional attention. So, that the both the devices are in saturation region and of course, their current should be equal.

This chunk discusses the importance of ensuring both transistors are operating within their saturation region for optimal performance. Adequate biasing is required to maintain this condition, thus avoiding any drastic changes in the gain parameters. Failure to do so could lead one transistor to inadvertently shift into a different operational region, heavily compromising the amplifier's efficiency and functionality.

Similar to a well-coached sports team, if players (transistors) aren't in their positions (saturation region), the performance isn't optimal. Imagine if a crucial player decided to join the bench at a pivotal moment (moving from saturation to triode); the team's chances of winning (gain stability) decrease significantly.

Now, to get the corresponding load line first of all let me draw the I versus V characteristic curve and this is beyond threshold this is square law.

This section focuses on how the load line characteristics in an amplifier are determined by plotting the relationship between current (I) and voltage (V) for the transistor. Established characteristics beyond the threshold reveal how transistors behave under varying input conditions, guided by the square law in the active region. Accurately plotting these characteristics facilitates understanding of the output response relative to the input signal.

It’s like mapping a hiker’s path on a mountain trail (I-V curve); understanding the steepness of the incline (load line) lets you predict how difficult the hike (output response) will be based on your starting point (input conditions).

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

In conclusion, the discussion reflects on the effect of adjusting the load line within the amplifier's circuit design. The shift from a passive to an active load modifies the gain and the overall circuit behavior. As modifications are made within the load line slope, the resulting changes clearly demonstrate how the amplifier can now leverage greater output for the same input conditions, effectively enhancing performance.

This can be likened to a sprinter who, by learning to lean forward at the starting gun (modifying the load line), increases his speed and efficiency, ensuring he can cover more distance in less time for each stride (output enhancements).

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