65.2. Cascode Amplifier Overview

So, we are talking about the Cascode Amplifier using 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.

The cascode amplifier is a configuration that enhances the performance of traditional amplifiers. Initially, the speaker discusses that they have previously covered BJT (Bipolar Junction Transistor) amplifiers and are now focusing on MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) amplifiers. They highlight a change from a passive load (2kΩ resistor) to an active load, which is intended to achieve a higher gain. The motivation behind this transition is to improve the circuit's amplification capabilities.

Think of an amplifier like a microphone system in a large auditorium. Initially, using a basic microphone (passive load) may not capture enough sound to reach the back rows. Upgrading to a better microphone with an amplifier (active load) ensures the sound is loud enough to be heard clearly by everyone, improving the overall experience.

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 is also 5 MΩ.

In this chunk, the speaker introduces key parameters of the cascode amplifier configuration. They mention a steady current of 2 mA, which is critical for sustaining the amplifier's operation. They also discuss the use of a specific equivalent resistance of 5 MΩ in the configuration. The configuration is structured to achieve the desired gain through the careful selection of load resistances, allowing for efficient small signal parameter calculations.

Imagine a multi-tiered water fountain where each tier represents a stage in the amplifier. The water flow (current) is constant, and each level (resistor) is intentionally designed to maintain proper water pressure and enhance the aesthetic (gain) of the fountain's flow. Each part must work together efficiently to create a stunning visual effect.

Now, with this we need to find what will be the voltage gain...to increase the gain and it is also affecting the input capacitance...

Here, the speaker talks about the essential calculation for voltage gain in the cascode amplifier setup. They highlight how the equivalent resistance and the small signal parameters contribute to the voltage gain calculation. They calculate the voltage gain, which has shown significant improvement from a small value to about 5000. This demonstrates how effective the cascode configuration is for amplifying signals and the implications it has on the input capacitance which is also influenced by higher resistance values.

It's like adjusting the level of a megaphone to amplify a speaker’s voice. Initial settings may project the voice only moderately, but by adjusting the amplifier’s settings (resistances), the voice can be made much louder, allowing the entire crowd to hear clearly even from a great distance.

The input capacitance...it is increasing the input capacitance, but probably still it is not so, alarmingly high...

In this part of the lecture, the speaker explains the relationship between resistance and input capacitance in the cascode amplifier. They indicate that while the input capacitance has increased due to higher resistance, it remains manageable. This balance between gain and input capacitance is crucial for ensuring optimal performance in circuits where bandwidth is a consideration.

Consider a garden hose. If you increase the diameter of the hose (resistance), more water (signal) can flow through, improving the garden's irrigation system (gain). However, with a larger hose, it may take a bit longer to fill a small bucket (input capacitance) but does not significantly hinder the overall efficiency.

To get the maximum advantage what about the resistance we got from this circuit lower part...this gain got increased from 4 to 5000...

The speaker summarizes the advantages of using a cascode amplifier over a standard common source amplifier. They emphasize that the cascode structure significantly enhances the gain while potentially affecting the circuit's bandwidth. Importantly, the gain-bandwidth product remains comparable for both amplifier types, making cascode amplifiers a preferred choice in many high-gain applications.

Think of this as the difference between a regular bicycle and a high-performance racing bicycle. The racing bicycle can go significantly faster (higher gain) but may require more skill to handle at high speeds (potential impact on bandwidth). Both are effective in their own right, but the choice depends on the rider's needs and conditions.

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