44.3.2. Effect of Cascading on Signal

What we have seen here, it is suppose this is the small signal equivalent circuit and then small signal equivalent circuit of the second stage, and then if you directly connect it what we have observed that the input resistance and then output resistance of the previous stage, they were dividing the signal. As a result the signal arriving to the input of the second amplifier it is not same as whatever the signal we obtained there in unloaded condition.

And also what we have seen that the input capacitance at this the second stage, it is affecting the previous stage, namely output resistance of the previous stage and input capacitance of the second stage they were forming one pole and it was affecting the upper cutoff frequency. In fact, this is true for common source cascaded with common source amplifier also.

In electronic circuits, especially when cascading amplifiers, we observe limitations in performance. When connecting a common emitter (CE) amplifier to another CE amplifier, the input and output resistances of both stages interact negatively. This interaction can cause a voltage drop, meaning the signal fed into the second stage may be weaker than the original signal. Additionally, the capacitance at the input of the second stage can create a frequency cutoff, limiting the frequencies that are amplified and distorting the signal further.

Consider a water pipeline system. If you connect two pipes (the first stage and second stage) using junctions that restrict water flow (like input and output resistances), the water pressure (the signal) entering the second pipe will be less than what was flowing in the first. Moreover, if one of the pipes has a blockage (the capacitance effect), it will further limit how much water can pass through, akin to how high-frequency signals might get cut off.

So, what is the solution for that? It is we can use a buffer in between these two circuits and if you have some specific buffer protecting the previous stage of the first stage from the loading effect coming from the second stage, then we can say that the overall gain of the system or overall the amplifier performance it remains intact even if you are cascading it.

To overcome the limitations identified, we can introduce a 'buffer' stage between two amplifiers. A buffer acts as an intermediary that isolates the first amplifier from the loading effects of the second amplifier. By doing this, the signal strength and quality of the first stage are preserved, and the cascading of amplifiers can occur without degrading performance. Buffers typically have high input resistance and low output resistance, making them ideal for this purpose.

Imagine a relay in a marathon race where runners pass the baton. If one runner (the first amplifier) is strong but the next runner (the second amplifier) is weak, the baton pass may be inefficient. A relay zone (the buffer) helps maintain speed and efficiency during the pass, ensuring the next runner gets the baton without losing strength or momentum. The buffer ensures smooth transitions, keeping the overall race time optimal.

So, we can say that this is working as a buffer for cascading to amplifier so, whether it is CE-CE or CS-CS. And this buffer it is to get this buffer what we are looking for it is as we said that the input resistance to be high, output resistance should be small, then input capacitance should be small, that is getting obtained from different configuration.

The characteristics of an effective buffer in amplifier configurations are crucial. The buffer should have a high input resistance, which means it draws minimal current from the previous stage. It should also have a low output resistance, ensuring it can drive the next stage effectively without signal loss. Furthermore, a small input capacitance is essential to prevent affecting the frequency response of the cascaded amplifiers. Typically, these characteristics are tailored based on the type of buffer used, such as in BJT (Bipolar Junction Transistor) or MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) configurations.

Think of a professional bridge between two highways. The bridge has to handle heavy traffic from one highway (high input resistance) and allow smooth flow to another highway (low output resistance) without causing traffic jams (small input capacitance). If this bridge is designed correctly, it allows for seamless traffic flow, akin to how a good buffer allows signals to pass without degradation.

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