65.1.4. Lecture – 65

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 active kind of load, where our basic motivation is to for higher gain.

In this section, the lecture resumes and highlights the focus on the Cascode Amplifier using MOSFET technology. It notes that the discussion previously covered BJTs and now shifts to identifying passive and active loads, emphasizing the goal of achieving higher gain through the use of an active load instead of a passive one (which was previously 2 kΩ).

Think of a passive load as a traditional sponge which absorbs water but doesn't enhance the flow. In contrast, an active load acts like a pump that not only moves water but can increase the flow rate, much like how an active load increases gain in amplifier circuits.

So, in the next slide we do have the formulation of the problem ok. This is the standard common source amplifier for performance comprising, I am not I am just skipping this part.

The professor indicates that the next part involves discussing the specifics of a standard common source amplifier, setting up the groundwork for understanding the configuration of the Cascode Amplifier. Although the details are noted as being skipped, it emphasizes its role in mapping out the parameters necessary for analyzing the proposed amplifier setup.

Imagine preparing for a road trip. Before you start driving, you need a map or GPS for directions. Similarly, in electronics, understanding the parameters of your circuit serves as the roadmap for proper analysis and performance assessment.

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...

This part discusses the implementation of the active load in the cascode amplifier configuration. The current I is defined as 2 mA, and the resistances are specified as 5 MΩ. The professor explains that this setup leads to a DC voltage of 6 V at a specific node in the circuit, indicating that the transistors are functioning within the saturation region, which is crucial for amplifier operation.

Consider active loads like powerful batteries compared to regular batteries. The powerful batteries can handle high load currents and maintain voltage levels effectively, just as the active load in this amplifier circuit supports its operational requirements for enhanced performance.

So, this is 5 MΩ. So, this is 5 × 106 probably you can drop this part and then divided by g , it is 2 mA multiplied m by we do have 50 kΩ. So, that gives us that is 50 kΩ...

Here, the voltage gain of the circuit is being calculated. The text details the relationship between resistor values and current gain, leading to a significant increase in voltage gain compared to previously calculated values with passive loads. It points out the result of the gain enhancement achieved through this setup, which raises the gain from an earlier low value to a much higher figure, indicating the effectiveness of using active loads.

Imagine training for a race with weights. Initially, you run with a heavy backpack (passive load), which slows you down. Once you remove it and practice with lighter weights or none, you run much faster and more efficiently, similar to how using active loads in circuits boosts performance.

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)...

The input capacitance of the amplifier is derived considering the gain from input to output. It calculates how the active load's gain affects the overall input capacitance, giving insights into how the circuit behavior shifts due to increased gains. The calculations provided illustrate the relationship between gain and input capacitance.

Think of input capacitance like the size of a funnel. The larger the funnel opening (higher gain), the more liquids it can channel effectively (current flow), but if the length of the funnel (capacitance) increases that may slow down the flow. Thus, while higher gain is beneficial, you may want to keep output manageable for efficiency.

In summary if I compared this cascode amplifier to boost to the gain and if I compare the performance of the standard common source amplifier...

The professor summarizes the findings of the cascode amplifier compared to the standard common source amplifier. It acknowledges that the cascode structure significantly boosts gain while affecting bandwidth, highlighting the trade-offs between amplification and frequency response.

Consider a car designed for speed (cascode amplifier) versus a car designed for fuel efficiency (common source amplifier). The fast car may be powerful and thrilling but can consume more fuel quickly, akin to the impact on bandwidth when you prioritize gain.

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