43.2. Flow of Discussion

Dear students, welcome to this NPTEL online certification course on Analog Electronic Circuits, myself Pradip Mandal associated with E and EC department of IIT Kharagpur. So, today’s discussion is primarily the Limitation of Common Emitter and Common Source Amplifier particularly when it is when those blocks are getting cascaded. We have discussed about the main feature performance of common emitter and common source amplifier in our previous lectures.

In this chunk, the instructor welcomes the students to the course and introduces the topic. The focus is on discussing the limitations of two specific types of amplifiers: the Common Emitter (CE) and the Common Source (CS) amplifiers, especially when they are cascaded together. Previously, students have learned about the key features of these amplifiers, which sets the stage for exploring their limitations in cascading scenarios.

Think of a CE or CS amplifier as a person talking loudly to another person across a busy street. The first person can be heard well, but if they try to pass their voice to another person over a long distance without proper channels, the message might get distorted or lost, similar to how cascading amplifiers can lead to loss of signal or gain.

And hence, we establish the need of some other circuit configurations which are referred as buffer. So, we will be discussing those things in detail.

The instructor explains that due to the limitations encountered in cascading CE and CS amplifiers, there is a need for additional circuit elements called buffers. Buffers can help mitigate some of the issues experienced during cascading by isolating the stages and preventing losses associated with loading effects.

Consider a relay race where runners hand off a baton. If the first runner doesn’t effectively pass the baton (signal) to the next, the race time (signal quality) suffers. Buffers act like smooth handoff techniques that ensure the baton is passed without loss of speed or form.

So, what are the concepts it will be covered in this today’s class. It is to appreciate what are the limitations or restrictions are there for common emitter or common source amplifier configuration will revisit frequency response of common emitter and common source amplifier, but then the basic difference is that we will cascade to common emitter amplifier.

This chunk introduces the educational goals of the class, particularly focusing on the frequency response of CE and CS amplifiers. The discussion will highlight the restrictions faced when these amplifiers are cascaded and the anticipated effects on their frequency responses.

Imagine tuning a radio. If you tune in to a station that is not strong enough and then try to use it to reach another weak station, the sound quality drops, just like how cascading amplifiers may introduce frequency response issues when trying to amplify a weak signal.

And also in case this A the first stage circuit, in case if it is having some certain bandwidth and the second stage may be having similar kind of profile; maybe it is having certain bandwidth. And then if we combined, so this is the second stage maybe second stage frequency response. And if we combine these two stages through this cascading we may be expecting that the overall frequency response it may be having very high gain, and then the lower cutoff frequency it is may be defined by whichever the lower cutoff frequency of the two stay out of these two stages are higher.

Here, the discussion centers on the expectation that when cascading two amplifiers, the overall gain should be the product of the two individual gains, and the overall frequency response should reflect the characteristics of both stages. However, practical observations often show a different result due to loading effects and bandwidth limitations in cascading setups.

Think of stacking two amplifiers like stacking two bowls to catch rainwater. You expect the final amount of water (gain) to be the sum of what each bowl collects, but if the lower bowl has holes (loading effects), the total amount is less than expected, illustrating the cascading challenges in amplifiers.

In other words, it introduces one attenuation factor which is defined by R and this output resistance here. Now, depending on the relative value of this R and R it may be having different value, but typically these two resistances may be having the same order of magnitude.

This chunk focuses on how cascading amplifiers introduces attenuation factors that can significantly affect overall gain. The interaction between the output resistance of the first stage and the input resistance of the second stage may form a voltage divider that decreases the expected voltage gain.

Imagine trying to fill a relaxed sponge (the first amplifier) with water. If the second sponge you use to catch the overflow has holes (representing loading effects), you won't collect as much water. The interaction between two sponge sizes (resistances) changes the total amount collected, highlighting how cascaded amplifiers influence each other's performance.

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