67.7. Comparative Analysis of Active and Passive Loads

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 this part, the speaker discusses the limitations of voltage gain in amplifiers when using passive loads. A passive load typically leads to a lower voltage gain in amplifiers, making it challenging to achieve the desired amplification of a signal. The discussion suggests that by modifying the load characteristic from linear to a desired non-linear characteristic, the voltage gain can be enhanced. This sets the foundation for understanding how active loads can improve amplifier performance.

Imagine trying to amplify your voice in a crowded room using a basic megaphone (passive load). It might work, but the amplification isn't great. Now, think of using a high-quality microphone and amplifier system (active load). With this setup, your voice can be significantly amplified, allowing everyone to hear you clearly, similar to how active loads improve amplification in circuits.

In the next slide we do have common source amplifier with active load. 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.

This chunk introduces the concept of an active load in a common source amplifier configuration. It describes how the circuit comprises of a transistor receiving the input signal made more effective by using another transistor as the load instead of conventional passive resistors. This arrangement helps achieve better voltage gain and reduces distortion in the output signal.

Think of this setup like using a high-performance speaker (M2) instead of just a basic speaker wire (passive load) to enhance the sound quality coming from your audio system (M1). The improved speaker allows the music to be played more clearly and at a higher volume.

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.

In this segment, the importance of ensuring that the currents through the two transistors (M1 and M2) are equal for the proper functioning of the amplifier is highlighted. This is a critical condition that helps maintain the desired operating point of the amplifier. If the currents are uneven, one transistor may enter a region of operation (like triode) that can negatively affect the circuit's performance.

Consider a well-coordinated team working on a project where each member must contribute equally (equal currents) for success. If one team member works significantly harder than the others, it can lead to confusion and chaos, similar to how unbalanced currents can destabilize an amplifier.

In other words, the (I_DS ) into its corresponding K called K′ of transistor. This condition need to be satisfied to ensure both the devices are in good condition.

This portion emphasizes the significance of maintaining uniform conditions in the active load configuration for optimal performance. It introduces the relationship between transistor currents, their respective characteristics, and the need for these quantities to be balanced to avoid adverse effects on amplifier performance. When both transistors operate effectively within their saturation regions, enhanced gain can be achieved.

Think of this as balancing the ingredients in a recipe. If you don't have the right proportions of sugar (current of M1) and flour (current of M2), the resulting dish will not turn out well, akin to how unbalanced transistors affect the amplifier's output characteristics.

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

The discussion transitions to a comparison between the voltage gains achieved with active loads versus passive loads. It specifies that the slope of the load line is crucial in determining gain and indicates that using active devices significantly increases this slope relative to passive loads, leading to better voltage gain overall.

This can be likened to comparing two hills: one steep (active load) that allows for quick descent (higher gain) compared to a gentle slope (passive load) that slows down progress. The steeper hill symbolizes the enhanced performance that active loads provide in amplifying signals.

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

This section introduces the small-signal analysis technique employed to further evaluate the performance of amplifiers using active loads. By creating equivalent circuits, the internal behavior and responses of the amplifiers can be assessed under varied small signal conditions, allowing for precise gain and bandwidth calculations.

Think of this as tuning a musical instrument where small adjustments (small signals) help achieve the perfect pitch. In circuit design, understanding how small fluctuations affect performance is critical for making accurate adjustments and enhancements, much like fine-tuning a musical instrument.

If I compare common source amplifier having passive load and active load and then if we see their gain and bandwidth particularly, what kind of comparison will you get for passive load... active load circuit.

In this segment, the trade-off between gain and bandwidth is examined. It explains that while active loads can significantly increase the gain of a circuit, they can also impact bandwidth negatively. The discussion illustrates how these two parameters are related and how adjustments to one can influence the other.

This is akin to a car's acceleration versus speed. A car that accelerates very quickly (high gain) might have a speed limit (bandwidth) that restricts it from maintaining that acceleration over long distances. Similarly, in electronics, finding the right balance between gain and bandwidth is essential for optimal performance.

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