43.5. Cascading in Common Source Amplifiers

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 introduction, the instructor reiterates the focus of the current lecture, which is about the limitations encountered when cascading common emitter and common source amplifiers. Previous knowledge about these amplifier configurations is assumed, as students are expected to already be familiar with their fundamental features and performances. The purpose of today’s lecture is to dive deeper into the challenges that arise specifically in cascading setups.

Think about stacking layers of sponge cake to make a taller cake. Each layer (or amplifier) is delicious on its own, but sometimes when you stack them up, the lower layers can become soggy and lose their texture due to the weight of the layers above. Similarly, cascading amplifiers can complicate their performance, leading to unexpected changes in gain and frequency response.

So, how we do that output of the first CE amplifier will be connecting to the input of the second CE amplifier. But of course, with the use of C we can isolate the DC operating point of the first stage and the second stage. … we may be expecting that the overall gain say A = A × A.

The instructor describes how to connect two common emitter amplifiers in a cascading configuration. A coupling capacitor (C) is used to link these amplifiers while preventing DC signals from affecting their operation points. The expected overall gain from both stages is the product of their individual gains (A1 and A2). However, the reality can differ once the two amplifiers are connected due to loading effects.

Imagine two water hoses connected in series to improve both pressure and flow. If each hose can push a certain amount of water (like each amplifier's gain), we expect the combined flow to be greater. However, if one hose is narrower (representing loading), it limits the overall flow, just like how cascading amplifiers may not deliver the expected gain due to their interactions.

However, once we connect the circuit and 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 and also may be the upper cutoff frequency may come down.

When the second common emitter amplifier is connected to the first, the loading effect occurs because the output resistance of the first amplifier interacts with the input resistance of the second one. This interaction often leads to a lower actual gain and a shift in the cutoff frequency compared to expectations, meaning the output performance does not align with the theoretical calculation of A = A1 × A2.

Consider a train that is pulling additional carriages. Initially, the train can carry a certain number of passengers (higher gain of the first amplifier). If the additional carriages are too heavy for the engine, the train might not move as fast as expected, reducing the overall capacity for passengers (gain). The loading effect is like the additional weight that reduces performance.

The second effect; second effect is on the cutoff frequency. So, let us see the cutoff frequency. So, what we can say let me clear here yeah, the cutoff frequency particularly the upper cutoff frequency it is having two candidates one is the cutoff frequency coming due to this R and then C and second one it is of course, this resistance loaded with R and then whatever the input capacitance.

The upper cutoff frequency is important in determining the bandwidth of the cascaded amplifiers. The overall cutoff frequency will be defined by the lowest value among two candidates: one from the original setup (based on R and C) and the other influenced by loading effects when cascading occurs. This change means that the frequency response might be limited or shifted due to the loading resistances.

Imagine you are using a microphone connected to a speaker through multiple cables. If one cable is low quality or too long, it might distort the sound and limit the range of frequencies you can hear clearly. Just as with sound, cascading amplifiers can hinder the frequency response, causing some frequencies to get lost along the way.

If we put a buffer; so if we put a buffer say or say some intermediate circuit having some important feature we will be discussing that such that, the input resistance of this stage; call R . If it is quite high then whatever the attenuation it will be getting introduced by R and R will be minimized.

The use of buffer amplifiers can help address the limitations experienced in cascading configurations. A buffer can isolate the stages, ensuring that the next amplifier does not influence the previous one. High input resistance and low output capacitance in the buffer will minimize voltage loss and maintain frequency response, allowing for more effective amplification.

Think of a buffer as a middle person in a conversation. If you have a friend who often interrupts or changes the tone of your discussion (the loading effect), having a neutral person to communicate between you helps keep the original message intact. In electronics, the buffer allows each amplifier to work more effectively without negatively impacting the other.

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