64.1.5. Multi-Transistor Amplifiers: Cascode Amplifier (Contd.)-Numerical Examples

So far what we have discussed that the advantage of cascode amplifier with respect to standard CE amplifier and namely what you have seen is that in case if you are retaining this passive element for both the cases, then gain wise we do not get much advantage. But then what we have seen that in case if we have the input resistance and the source resistance together forming a significant significantly low frequency pole due to this large value of the C , in then we have seen that the cascode amplifier it is giving some advantage.

The cascode amplifier offers a certain advantage compared to standard common-emitter (CE) amplifiers, particularly when it comes to managing impedance and bandwidth. In scenarios where passive elements remain constant, the gain may not improve substantially. However, in situations where input resistance and source resistance result in a low-frequency pole affected by large capacitance, the cascode amplifier shows notable benefits.

Imagine trying to listen to music through a small speaker in a crowded room; the sound gets drowned out. Now, if you amplify that sound through a high-quality speaker, it becomes clearer and more defined amidst the noise. Similarly, the cascode amplifier enhances signals, managing noise and improving the clarity of the desired output.

So, to demonstrate the capability of the cascode amplifier to increase the gain first of all let we consider a different situation instead of having this R if you put some active circuit there, probably then the advantage of the cascode amplifier particularly for enhancing the gain it will be quite prominent.

Next, a scenario is introduced where an active circuit is used instead of a passive resistor to further illustrate the gain enhancement capabilities of the cascode amplifier. By doing so, the inherent advantages of the cascode configuration become more pronounced, especially in terms of achieving higher output voltage gains.

Think of using a microphone to amplify your voice in a large hall. Without the microphone (passive setup), your voice might get lost. But with the microphone (active circuit), your voice carries effectively, making it easy for the audience to hear you. This is similar to how the cascode amplifier increases signal strength using active components.

On the other hand if I consider practical circuit and let me use the analysis slight analysis here… drop across this one it will be 0. So, naturally the output voltage it will be 12 V.

In a practical analysis of the circuit, the voltage across specific resistances and the associated current flow are examined. If no current flows through a particular resistor, then the voltage across it remains zero, leading to the output voltage being equal to the supply voltage (12V in this case). This emphasizes how the circuit's design directly impacts its output characteristics.

Consider a faucet that has a blockage preventing water from flowing through the pipe. If you measure the water before and after the blockage, you'll find the pressure remains high before the blockage but is lost after. Here, if no current flows through a resistor, the full voltage is maintained, similar to how water pressure builds up before a blockage.

So, in summary if I consider this 10 MΩ and the equivalent resistance of this part it is 10 MΩ and the ideal current here it is 2 mA… it is maybe in the near vicinity of 6 V keeping transistor-2 as well as transistor-1 in active region of operation.

Summary calculations suggest that when considering the equivalent resistance of 10 MΩ, the output voltage settles around 6V. The presence of sufficient current (2mA) and appropriate resistor configurations ensures both transistors remain in their active operational regions, which is crucial for maintaining desired amplifying conditions.

Imagine trying to maintain a steady speed while cycling uphill. If you pedal consistently, you can keep moving (like holding the transistor in the active region). If you slow down (like an insufficient current), you may stall and have to start over. Maintaining conditions ensures a steady cruise.

Now in before we go into the small signal equivalent circuit, I like to recall that value of g and g both are then r and m1 m2 o1… G = g (1 + g) / r.

Before analyzing the small signal equivalent circuit, it is important to recall the transconductance values for the two transistors and their relevant resistances. Understanding these parameters is key to calculating the overall voltage gain of the amplifier configuration under small signal conditions.

Imagine trying to assess how sound travels through a room. You would consider the type of speakers (g), the nature of the walls (resistances), and how they interact with each other to amplify sound. Similarly, in circuits, careful consideration of components helps to predict overall behavior under varying conditions.

So, to get the C we need to know what will be the gain from the base terminal here to the collector terminal of transistor-1… So, we can see that the difference here it is , but in this case cascode circuit it is having worse performance than the CE amplifier.

Calculating input capacitance involves understanding the gain from one transistor stage to another and factoring in the capacitive effects of the circuit. The input capacitance influences the overall behavior of the amplifier, including bandwidth limitations, and it reflects the trade-offs involved in using cascode structures.

Think of how a sponge absorbs water. The more you push water into it (gain), the more water it can hold. However, if the sponge gets too full or heavy, it stops working effectively (bandwidth issues). Balancing amplification and capacitance is akin to managing how much water you can effectively pour into a sponge.

So, depending on our application if you are looking for very high gain, but then we can if in case if we are we can tolerate with lower bandwidth, then we will be going for cascode amplifier.

The final discussion wraps up the cascode amplifier's essence in enhancing gain while highlighting the trade-off with bandwidth. If applications prioritize high gain and can accept lower bandwidth, the cascode configuration stands as an optimal choice, as it leverages these behaviors to deliver performance in specific scenarios.

Consider selecting a car for a long-distance journey. If speed (gain) is your priority despite sacrificing some fuel efficiency (bandwidth), you would choose a high-performance vehicle. Making such adaptations in circuits similarly allows us to tailor performance to our specific needs.

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