61.3. Department of Electronics and Electrical Communication Engineering

So, the concepts we will be covering in this lecture is the following. So, we shall start with CE followed by CB and in fact, with appropriate modification. It gives us a relatively simple circuit configuration which is commonly known as cascode amplifier so, that amplifier we will be discussing in depth.

In this lecture, we will explore cascode amplifiers, which are a combination of two common amplifier configurations: the Common Emitter (CE) and the Common Base (CB) amplifiers. By modifying these configurations, we can achieve a simpler circuit layout known as a cascode amplifier. This modification enhances the performance and is a common topic of study in electronics.

Think of a cascode amplifier like a relay team in a race, where the first runner (CE) hands off the baton to the second runner (CB), improving the overall speed and efficiency of the team. Just as teamwork enhances performance in a relay race, combining these amplifier stages enhances circuit performance.

I like to say that you might have observed that if we consider simply common emitter amplifier, its gain is typically quite high more than 100. On the other hand, if I consider common source amplifier it is gain it is not so high. So, we must be having some alternative for particularly for MOSFET version otherwise that circuit may not be really much of an use. And this cascode amplifier is one of the configuration in a MOSFET amplifiers which is essentially helping to get the higher gain.

Common Emitter (CE) amplifiers provide high gain, often exceeding 100. However, Common Source amplifiers do not achieve similarly high gains. Therefore, for MOSFET amplifiers, a new configuration, the cascode amplifier, is introduced. This configuration effectively increases gain while maintaining performance stability, making it a crucial design for applications demanding enhanced amplifier performance.

Imagine a two-stage rocket: the first stage propels it high into the sky, achieving incredible height, akin to the CE amplifier’s high gain. The second stage might not be as powerful, like the Common Source amplifier, but it effectively transports the rocket further into space when combined. The cascode is our rocket’s two stages working together for optimal reach.

So, now coming back to whatever we are about to say the application of the common base to enhance the circuit gain. So, we will be seeing that how this will be enhancing the gain.

The application of combining the Common Emitter (CE) and Common Base (CB) amplifiers serves to enhance the overall gain of the amplifier configuration. By utilizing both configurations, we tap into the strengths of each, effectively leading to an improved output gain. This combined approach lays the foundation for constructing the cascode amplifier, which optimally enhances circuit performance.

Think of a business partnership: one partner is an expert in marketing while the other excels in finance. When they combine their skills, they can achieve greater business success than working alone. Similarly, combining CE and CB amplifiers allows us to leverage the benefits of both to achieve greater amplification.

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.

In the cascode configuration, we simplify the Classic CE-CB amplifier setup and manage biasing effectively. The DC currents supplied to both transistors (Q1 and Q2) must be correctly set to ensure efficient operation. Proper biasing is crucial to maintaining the correct operating points for the transistors, preventing them from dropping into saturation, which would impair functionality.

Think of biasing like setting the stage for a play – if the actors (transistors) are not in the right position (operating points), the performance will be affected. Biasing ensures that the transistors operate smoothly, just as proper staging makes for a memorable performance.

Now, coming to the small signal analysis so, now, let us see the small signal analysis and in in the small signal analysis we do have voltage gain and then output impedance, input impedance, input capacitances, those things we can compare with what are the corresponding performance matrices coming out of simple CE amplifier.

Small signal analysis is key to understanding how the cascode amplifier will perform in real applications. We’ll evaluate key parameters such as voltage gain, output impedance, input impedance, and input capacitances, making comparisons to those of simple CE amplifiers. This analysis enables engineers to predict amplifier behavior under varying signal conditions.

Consider a small signal analysis like tuning a musical instrument. Musicians adjust their instruments' settings and tension to achieve the right sound (performance parameters). Just as a little adjustment can lead to a perfect melody, analyzing these parameters helps engineers fine-tune amplifiers for optimal performance.

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