62.2. Multi-Transistor Amplifiers: Cascode Amplifier (Part B)

So, welcome back after the short break. So, now let us move to the cascode configuration using MOSFET. But as I said, that the cascode configuration it is a combination of common source followed by common gate. And, similar to BJT where we have discussed about CE followed by CG.

In this chunk, the lecturer introduces the concept of the cascode amplifier configuration. This configuration is made by stacking a common source amplifier on top of a common gate amplifier. The common source stage amplifies the input signal, while the common gate stage provides benefits such as improved frequency response and increased output impedance.

Think of the cascode configuration like a two-story building. The first floor (common source) handles the main entry and amplifies the people’s excitement (the signal), while the second floor (common gate) observes and manages the overall flow without adding itself too much weight (amplification). This structure helps maintain a balance and ensures more people can come in and enjoy the excitement.

When we talked about CE-CC amplifier we said that that circuit it is referred as emitter follower. So likewise, when you talk we refer to say common drain stage where input we give at the gate and then output we observed at the source.

This section explains the functionality of the common drain amplifier, often called a source follower because the output at the source follows the input at the gate closely. The voltage gain through this stage is approximately 1, indicating that the output signal mirrors the input signal without significant alteration.

Imagine a relay race where one runner passes the baton directly to the next without any change in speed. The baton represents the signal in an electronic circuit; the common drain stage ensures the baton is passed smoothly, preserving the integrity of the message being communicated.

For common gate circuit what we does the signal we feed at the source node. So, this is the input port and the gate it is connected to a DC voltage which is AC ground. And then, we observe the corresponding signal at the drain terminal.

In this chunk, the lecturer describes the common gate circuit where the input signal is fed into the source terminal while the gate is often held at a fixed DC voltage. The output is then taken from the drain terminal. This circuit has a current gain of approximately 1, meaning the current that flows into the circuit is essentially the same as the current that flows out.

Consider a waterway where water flows into a reservoir (the source) and is then distributed to a larger channel (the drain). The amount of water entering the reservoir (input current) is equal to the amount of water flowing out into the channel, thus maintaining a balance where no water is lost.

So, now coming to the main discussion how the common gate amplifier it can be used as a voltage gain booster along with the common source amplifier.

Here, the common source amplifier is paired with the common gate amplifier, enhancing the overall voltage gain of the system. The output of the common source can be directly coupled to the common gate stage. This direct coupling allows for efficient integration of the current from the source current to the required drain current in the next stage.

Imagine a power amplifier in a concert that takes in the music from a mic (common source) and boosts it further before sending it to the speakers (common gate). The first amplifier ensures the sound is clear, while the second one elevates the sound to ensure the entire audience can hear it.

So, what we have here in the biasing side? For M1, we need to provide a voltage here, at its gate sufficiently high. So that this transistor it will be on and that is done by this potential divider which is getting constructed by R1, R2 and the supply voltage.

This chunk focuses on the need for proper biasing of the MOSFETs to maintain them in saturation, ensuring that they operate correctly. It discusses how the gate voltages are established using potential dividers, emphasizing that these voltages need to be selected carefully to prevent the devices from entering undesirable operating regions (e.g., triode).

Consider adjusting the thermostat in your home. If the temperature is set too low or too high, the HVAC system (like a transistor) may not function efficiently or may even shut down. Just as you need the right settings to maintain temperature, the MOSFETs need the correct gate voltage to function optimally.

Once we get the proper DC operating point, the next thing is that the analysis for voltage gain and output impedance and so and so. And to do that, we need to draw the small signal equivalent circuit similar to BJT cascode amplifier.

This portion discusses the need to analyze the voltage gain and output impedance. The lecturer indicates that a small signal equivalent model is necessary to perform this analysis, which is vital for understanding how the amplifier responds to varying input signals.

Think of an engineer using a blueprint (the small signal model) to understand a building’s (the amplifier's) behavior under different weather conditions. This model allows engineers to predict how the structure will hold up and behave as conditions change, just as the small-signal model helps in analyzing amplifier performance under different input conditions.

So, if we try to directly sense this voltage through some subsequent circuit which may offer significant amount of capacitance.

In this section, the discussion shifts toward understanding how the output impedance can affect bandwidth and how capacitance in the following circuitry might limit performance. The lecturer describes the need for designing with these aspects in mind, such as by potentially adding a buffer stage.

Imagine trying to read a book (the output signal) on a windy day (the output impedance combined with other circuit effects). If the wind is too strong (high capacitance), you can’t focus on the text. Adding a wall or a shield (buffer stage) can help block the wind, letting you read clearly without obstruction.

In conclusion, what we have is that, we have continued the multi-configuration amplifiers. And today, what we have discussed, it is that common emitter amplifier cascaded with common base... voltage gain it got increased by a factor of almost β.

The lecturer summarizes the advantages of using a cascode amplifier, including increased voltage gain and decreased input capacitance, which are beneficial for overall performance. However, it is noted that the increase in output resistance may pose a challenge in practical applications, requiring careful consideration and design strategies to mitigate.

Think of a racing car that has been finely tuned for optimum speed (increased voltage gain) but now has a heavier body that reduces agility (increased output resistance). While the modifications make it faster, engineers must find a balance to ensure the car can still handle turns efficiently.

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