67.6.1. Output Characteristics and Voltage Gain Calculation

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.

In this part, we are introduced to the topic of voltage gain limitations in amplifiers. Both common emitter and common source amplifiers face constraints on how much they can amplify a signal when using a passive load. Essentially, a passive load does not provide the necessary feedback to significantly increase the gain. The lecturer hints that there are methods to enhance voltage gain, particularly through different load configurations, which we will explore in detail.

Think of a common emitter or source amplifier like a human voice trying to echo in a room. If the room has soft walls (passive load), the voice will not carry far. However, if the walls are reflective (active load), the voice amplifies, much like how different configurations can enhance the voltage gain of the circuit.

In fact, that is the center point of getting higher gain of any amplifier using active load. I must also say in this context that in case if you are putting some arbitrary active load thinking that that may be improving the gain, but then it may not.

Active loads are crucial for enhancing the gain of amplifiers. They provide better control and characteristics that can lead to higher gains. However, it’s essential to choose the right type of active load, as arbitrarily selected loads may in fact reduce gain instead of increasing it. This insight sets the stage for deeper analysis and specific configurations we will explore.

Choosing the right equipment is similar to picking the right microphone for a concert. Just as a poorly chosen microphone can mute a singer's voice instead of amplifying it, a poorly chosen active load can diminish an amplifier's gain.

(Refer Slide Time: 02:30) 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.

The segment introduces an active load configuration involving two transistors, M1 and M2. M1 receives the input signal while M2, being a PMOS transistor, is configured as the load. This configuration is crucial for enhancing the voltage gain. The relationship between these transistors and how they interact with the DC voltage ensures that the amplifier operates efficiently, tapping into the benefits that active loads provide.

Imagine a power amplifier in a sound system where one speaker (M1) is receiving the audio signal, and another device (M2) powers the speaker. If the second device is set up correctly (like an active load), it ensures that the sound system can project audio at high volume levels without distorting the sound, leading to a more pleasant listening experience.

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 section focuses on the need for both transistors in the amplifier to handle equal currents for stable operation. The condition is emphasized with respect to saturation, which means both transistors must operate within their optimal ranges to ensure that the overall circuit functions correctly. If one transistor operates outside of saturation, it could lead to reduced gain and operational issues.

This can be likened to two gears in a well-oiled machine. If one gear (transistor) runs faster (out of saturation), it can impact the efficiency of the entire mechanism, causing slowdowns or jams elsewhere in the system.

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.

This part delves into Kirchhoff's Current Law (KCL), which states that the total current entering a junction must equal the total current leaving it. It affirms that under the right conditions, the currents through both transistors must be equal since they are the only pathways at this node. This establishes a foundational understanding of how these currents interrelate within the circuit and impact overall functionality.

Visualize water flowing through two connecting pipes. If both pipes are designed to carry the same amount of water, they must work together in perfect harmony. If one pipes starts carrying less water (current), the entire system fails to function efficiently.

So, if I consider the slope of this line, it is slope of this line it is change in I_DS or I with respect to V_DS. In fact, this is nothing, but the slope of transistor-2.

In this section, the focus is on how the characteristics of the transistors, particularly their I-V curves, relate to the overall gain of the amplifier. The slope of the line on this characteristic curve indicates how changes in the output voltage affect the output current, which in turn influences gain. Understanding this slope is critical, as it may reflect the behavior of the active load and passive load.

Think of this slope as the gradient of a hill. A gentle slope allows for a smoother ascent (higher gain), while a steep slope represents a sharp climb, which may be more difficult to navigate (lower gain) depending on the context.

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.

This part summarizes the impact of different load configurations on overall gain. By comparing passive and active load slopes, it becomes evident that active loading enhances the gain considerably. The segment indicates that despite changes in output resistance and characteristics, the proper calculation methods can allow us to quantify and realize the advantages of using active loads.

When comparing two vehicles – one with a gas engine (passive) and another with an electric motor (active) – you may note that the electrical vehicle accelerates faster due to improved torque distribution (similar to how active loads enhance gain in circuits).

And both of the changes are getting created by the same entity called R_out. So, as a result this bandwidth here it got decreased by the same factor as the gain got increased.

Here, we explore how changes in load configuration, particularly the shift to active loads, lead to modifications in both gain and bandwidth. The key takeaway is that as the output gain increases, the available bandwidth tends to reduce, which is a crucial consideration when designing amplifier circuits to ensure they meet required specifications for frequency response.

Think about a musical concert. As the sound increases in volume (gain), it can sometimes lead to distortion if the sound system isn't balanced correctly (bandwidth reduction). So, while it's great to have more volume, it's equally important to manage clarity and quality.

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