43.3.2. Effect of Cascading on Frequency Response

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.

In this chunk, we introduce the main topic, which is the limitations of two specific amplifier types — the Common Emitter (CE) and Common Source (CS) amplifiers — particularly when they are used in a cascading configuration. Cascading refers to connecting multiple amplifiers in series to achieve a higher overall gain. This method is common in electronic circuits where performance enhancement is needed, but it can lead to certain challenges.

Think of cascading amplifiers like stacking multiple water tanks to increase water pressure. Each tank represents an amplifier, and the pressure (or gain) builds up as water flows from one tank to another. However, just as connecting tanks improperly can lead to leaks or reduced pressure, cascading amplifiers can lead to loss of performance or signal integrity.

And also if we combined, so if we; 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 stages are higher.

In a cascading setup, one might expect that the overall gain (voltage amplification) would be simply the product of the gains from each amplifier. Moreover, it's anticipated that the frequency response, particularly the lower cutoff frequency, would be determined by the stage with the higher lower cutoff frequency, leading to an overall flatter and extended frequency response.

• Chunk Title: Observations on Gain Drop-off
• Chunk Text: But in our surprise once we connect the circuit and if we if we make the observation from the primary input to primary output we will see a significant amount of change of this gain namely this gain may drop off here to some other value.
• Detailed Explanation: When engineers connect the cascaded amplifiers and measure the performance, they often find that the actual gain does not meet the expected amplification levels. This drop in gain can result from several factors including loading effects, where the output impedance of the first amplifier affects the input impedance of the second, leading to attenuation rather than the desired amplification.

Consider a relay race where the first runner passes the baton to the second. If the first runner is too slow or has gained too much weight, the second runner may not be able to run as fast as expected. The expected speed of the team could drop due to the limitations imposed by the first runner, similarly, cascaded amplifiers can experience a drop in gain due to interaction between stages.

The upper cutoff frequency may come down.

Cascading can also negatively affect the upper cutoff frequency, which is the frequency range above which the amplifier begins to lose gain. Typically, one might anticipate that cascading two amplifiers would maintain or potentially widen the upper cutoff frequency. Instead, due to the combined effects of capacitances and loading on the subsequent stages, the effective cutoff frequency can actually decrease.

This situation can be compared to a group of athletes trying to run a marathon. If the first few athletes cannot maintain speed due to fatigue, the entire group's pace is affected, slowing down the overall performance contrary to what might have been expected when considering each athlete could have individually run faster.

the input capacitance coming from the second stage; if I call say C of the second stage... may be forming a pole coming from this R and maybe the input resistance here.

This chunk discusses how the Miller effect, which describes feedback capacitance in amplifiers, plays a critical role in cascaded circuits. When two stages are linked, capacitive effects from the second stage can increase the effective input capacitance of the first stage, thereby influencing both its behavior and that of the entire cascade.

Imagine you are pouring a drink, and the glass (representing the amplifier) has a thick bottom. Each time you pour, the liquid pushes down, but the shape of the glass affects how quickly the drink will fill. In the same way, the Miller effect influences how quickly signals can be processed in cascaded amplifiers.

So, to avoid the loading effect at this node what you are looking for that input resistance of the second CS amplifier or CE amplifier.

This part provides potential solutions to the cascading issues, particularly the notable loading effect. To counteract these effects, the input resistance of the buffer stage should be increased while keeping the output capacitance small. This configuration minimizes interaction between cascading stages and helps to preserve gain and bandwidth.

Think about using a very strong signal to wake someone up. If the signal (like an alarm) is loud and clear enough, it won’t matter if there’s a little noise in the background. Similarly, if properly designed, the buffer stages will maintain signal integrity despite the loading from the cascaded elements.

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