43.3. Cascading of Common Emitter Amplifiers

Dear students, welcome to this NPTEL online certification course on Analog Electronic Circuits. Today’s discussion is primarily about the Limitation of Common Emitter and Common Source Amplifier particularly when these blocks are getting cascaded. We have discussed the main feature performance of common emitter and common source amplifier in our previous lectures. Let’s look into the flow and understand where we stand today.

In this introduction, the speaker welcomes students to the course and sets the context for today's lesson. The focus is on understanding the limitations of both Common Emitter (CE) and Common Source (CS) amplifiers when they are cascaded together in a circuit. This is important because combining stages can lead to unexpected behaviors and performance issues that need to be addressed.

Think of cascading amplifiers like building a multi-story building where each floor (amplifier) adds height (gain) to the structure. However, if the foundation (circuit design and limitations) isn't strong enough, the building might sway or even collapse (performance issues), stressing the importance of addressing limitations before stacking additional floors.

To appreciate the limitations or restrictions in common emitter or common source amplifier configurations, we revisit the frequency response of these amplifiers. We will see that when we cascade two common emitter amplifiers, the expected gain and bandwidth can change significantly.

When cascaded, it’s anticipated that the overall gain would be the product of the gains from each amplifier. However, the frequency response can be affected due to loading conditions introduced by the cascading. Each stage can impact the frequency cutoffs, which are boundaries that define the operational limits of the amplifier’s frequency response.

Imagine you have two speakers (amplifiers) connected to create a stereo sound. You expect the combined sound to be loud (gain) and clear across all frequencies. But if one speaker is too close to a wall (circuit loading), it might distort sound (frequency response), making your listening experience less enjoyable.

Expectedly, when two amplifiers are cascaded, if the first has a gain of A1 and the second A2, overall gain should be A = A1 × A2. However, we observe that after cascading, there's a notable reduction in gain and a shift in the cut-off frequencies.

Upon cascading, the gain observed at the output isn't simply the product of the gains from the individual stages due to loading effects. The upper and lower cutoff frequencies also shift, meaning that the bandwidth is affected. This deviation can lead to disappointing performance if not accounted for, as the cascaded system may not function as effectively as anticipated.

Consider a relay race where each runner (amplifier) is expected to complete their segment in a set time, contributing to an overall fast race time. If one runner stumbles (loading effect), not only does it slow them down (gain), but it also makes the final performance (cutoff frequency) worse than just the sum of their parts.

To address these limitations, we introduce a buffer between the cascaded amplifiers. The buffer isolates stages, reducing loading effects and helping to retain the desired frequency response.

A buffer acts as an intermediary that can prevent the loading effects of one amplifier on the next. By using a buffer with a high input resistance and low output capacitance, the integrity of the original signal can be maintained, thereby improving overall performance in a cascaded configuration.

Think of a buffer like a relay runner who keeps the baton (signal) smooth and uninterrupted as it is passed between runners (amplifiers). By ensuring that each runner doesn’t overly exert influence on the next, the race can continue smoothly without any deceleration or distortion, ensuring better overall performance.

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