65.1.3. Indian Institute of Technology, Kharagpur

Welcome back after the short break. So, we are talking about the Cascode Amplifier using BJT sorry MOSFET. BJT part we already have completed now come here so, far we are talking about the passive load namely R it was 2 k now we are going to change this load to an active kind of load, where our basic motivation is to for higher gain.

In this section, the lecturer welcomes students back and introduces the topic of cascode amplifiers, specifically focusing on their use with MOSFETs. The previous discussion covered BJTs (Bipolar Junction Transistors), and now the emphasis shifts to MOSFETs. There is a key change discussed regarding the load type; previously, a passive load was used (2 kΩ), but it will be changed to an active load that allows for increased gain in the amplifier's function.

Consider a water flow system where you initially use a simple faucet (passive load) just to let water flow. By upgrading to a more complex valve system (active load), you can control and boost the flow rate, similar to how changing the load in an amplifier can increase its output.

So, we do have the cascode amplifier here with active load namely the I here it is 2 mA current and this R it is 5 MΩ.

In this chunk, the specifics of the cascode amplifier are discussed. It mentions that the active load design involves a defined current (2 mA) and a resistance value of 5 MΩ. This choice of parameters is made in order to enhance performance, particularly focusing on achieving a higher voltage gain compared to using passive components.

Imagine adjusting the flow of a river by selectively placing gates (active loads) that can regulate the water better instead of just letting it flow naturally (passive loads). This regulation allows us to get more power or gain out of our system.

So, to get the maximum advantage what about the resistance we got from this circuit lower part, if we take the same resistance then we can get the maximum benefit for both the gain as well as bandwidth consideration.

This section discusses how selecting the appropriate resistance values in the amplifier setup can significantly influence the overall performance of the circuit—the gain and the bandwidth. By optimizing the lower resistance part of the circuit to match the selected active load, one can achieve the best operational efficiency.

Think of road traffic management where setting traffic lights (resistance) correctly allows cars (electrical signals) to flow freely without congestion (noise or loss of signal). The right settings optimize not just the flow (gain) but also how quickly cars can move through the area (bandwidth).

In fact, this is I should say it is a big jump and as I said that for most transistors this cascode structure it is frequently use to enhance the gains.

The lecturer highlights the significant increase in voltage gain achieved through the implementation of the cascode amplifier structure. From a previous gain of only 4, the new configuration results in a gain of 5000, showcasing how effective this design is for increasing amplification in circuits.

This can be likened to using a megaphone (cascode structure) in a speech to increase the volume significantly compared to just speaking normally (previous gain). It amplifies your voice much more effectively, allowing everyone to hear you clearly from further away.

So, to get the input capacitance C which is C + C (1 ‒ whatever the gain we do have from here to here which is let you call this is A ) and what is the A ? A = ‒ g v1 v1 v1 m1 multiplied by r in parallel with the equivalent resistance coming from this circuit.

Here, the calculation of the input capacitance is introduced. The chunk defines the total input capacitance as a combination of individual capacitance values adjusted for the gain of the amplifier. It signifies how the design choices influence other aspects such as capacitance as gain increases.

Imagine a sponge (input capacitance) that can hold water (energy). If you apply pressure (gain) to it, it can actually change how much water it can hold before overflowing. In electronic circuits, increasing gain impacts input capacitance in similar ways.

Here the common source amplifier may be having very low gain, but then it may be having very high bandwidth mainly because the output resistance and the C it is defining that.

This discussion explains the trade-off often present in amplifier designs: typically, if an amplifier has a high gain, it might have a low bandwidth and vice versa. The gain-bandwidth product concept encompasses this relationship, showing how they influence each other based on design choices.

Consider a speaker system that can produce very loud sounds (high gain) but struggles to operate effectively at high frequencies (limited bandwidth). Conversely, a system designed for clarity may not have the loudest output. This balance between clarity and volume reflects gain and bandwidth in amplifier contexts.

So, today we have in fact; cover only two numerical example, but I guess they are very extensive example to demonstrate the potential of cascode amplifier.

In conclusion, the lecturer summarizes that the examples discussed effectively highlighted the capabilities of cascode amplifiers in increasing voltage gain and maintaining input capacitance. This amplifier type is especially preferred in VLSI designs where higher gain is needed.

This is comparable to choosing specialized tools for a job—just as a particular tool might be more capable of handling heavy-duty tasks efficiently, cascode amplifiers are selected for their higher gain capabilities in complex electronic designs.

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