61.6.3. Cascode Amplifier using BJTs and MOSFETs

Now the plan overall plan if you see according to our weekly plan so far we have covered CE – CC; CS - CD and CC - CC; Darlington pair etcetera both theory as well as numerical examples. And we are going to discuss about Cascode Amplifiers which are essentially I should say CE this should be CE, CE followed by CB and CS followed by CG.

In this chunk, we are introduced to the topic of cascode amplifiers. The lecture points out that previous topics included common emitter, common collector, common source, and common gate amplifiers. The cascode amplifier is essentially a combination of a common emitter amplifier followed by a common base amplifier (for BJTs) or a common source amplifier followed by a common gate amplifier (for MOSFETs). This means that the cascode configuration uses two types of amplifiers sequentially to improve certain performance characteristics.

Think of the cascode amplifier like stacking two boxes to make a taller tower. The first box (the common emitter) is a sturdy base that adds height, while the second box (the common base) enhances stability and efficiency. Together, they create a more effective structure than either one alone.

So, to start with you may recall we made a summary that a different performance matrices of CE amplifier. So this is the basic CE amplifier configuration and then we also have different performance matrices and their expressions and qualitatively we said that some of them are high, some of them are not so high or whatever it is and in fact, even though R it is high it is not good for voltage amplifier.

This chunk discusses the performance metrics of common emitter (CE) amplifiers. While CE amplifiers are known for their high voltage gain (more than 100), it’s noted that high output resistance can be problematic for voltage amplification. Therefore, combining CE with common base (CB) amplifiers can potentially improve performance metrics significantly by leveraging their strengths and compensating for each other’s weaknesses.

Consider a team working on a project. One team member might be great at generating ideas (like a common emitter's high gain), but not so good at executing them (high output resistance). A second team member (the common base) may not have as many innovative ideas but excels at turning ideas into reality, making the project succeed when they work together.

So I should say that cascode circuit, it is essentially CE-CB, but I should say it is simplified or modified version. So, here we do have the CE-CB amplifier whereas, if you see here this Q in fact, whatever the DC voltage you do have here that may be useful, I should not say DC voltage rather I should say the current.

This chunk describes how the cascode amplifier can be seen as a simplified version of the combined CE and CB configuration. In this configuration, the output of the common emitter stage is directly connected to the input of the common base stage, allowing for more efficient biasing. The setup simplifies the circuit and enhances the mutual benefits of both configurations, thus improving overall performance.

Imagine you're assembling a bicycle. The frame (common emitter) provides the structure, while the wheels (common base) allow for smooth movement. By directly connecting the frame to the wheels, you enhance the overall efficiency and performance, just like in a cascode amplifier where connections are optimized for better output.

Now we can modify this this connection without really putting this capacitor here, but then of course, we should be having a meaningful connection. And in the next slide, we will be discussing about how we can cleverly directly couple these CE and CB together and that gives us the new configuration called cascode configuration.

This section explains that the coupling between the common emitter and common base stages can be done without the need for a blocking capacitor, allowing for a more efficient connection. Directly connecting these stages helps to maintain performance while minimizing complications that arise from additional components.

Think of this as creating a seamless connection between two parts of a track. Instead of adding extra sections that may disrupt flow (like a capacitor), you design the track so that the transition is smooth, ensuring that the journey from one part to the other is efficient, just as it is in a cascode amplifier.

So, that is how we got the cascode amplifier and as I said it is a special kind of amplifier and it is having higher gain and to appreciate or to really acknowledge that let we do the detail analysis of the cascode amplifier.

The cascode amplifier is recognized for its higher gain capabilities, making it a favored choice in many high-performance applications. This section sets the stage for a detailed investigation into how exactly this amplifier achieves superior performance through its configuration and component interactions.

Imagine a high-speed train that can travel much faster than regular trains because it has a specially designed track and system. The cascode amplifier functions similarly, providing enhanced performance due to its carefully structured components working together.

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