62.2.1. Cascode Configuration Using MOSFET

So, now let us move to the cascode configuration using MOSFET. But as I said, that the cascode configuration it is combination of common source followed by common gate.

The cascode configuration of amplifiers involves stacking two amplifier stages: a common source stage on the input followed by a common gate stage. The primary purpose of this arrangement is to enhance the overall performance of the amplifier, particularly in terms of voltage gain and frequency response.

Think of the cascode configuration like a relay race where the first runner (common source) passes the baton (signal) to the second runner (common gate). Each runner plays a crucial role in ensuring the baton is passed smoothly, leading to a better overall race time (amplifier performance).

The basic purpose is to summarize and want to know what is the purpose of this configuration. Particularly, cascading common source with common gate. ... the common drain circuit is referred as the source follower.

In this configuration, the common source stage provides voltage gain, while the common gate stage helps in current transfer with minimal degradation in the signal phase. The common drain configuration, also known as the source follower, ensures that the output follows the input without additional gain, which is significant in maintaining signal integrity.

Imagine the common source stage as a speaker amplifying sound (voltage gain), while the common gate stage acts like a conduit connecting different sound systems (current transfer) without modifying the sound too much. This preserves quality while still increasing volume.

we can isolate the DC operating point of the 1st stage and 2nd stage by placing a DC decoupling capacitor there, and only feeding the signal from 1st stage to the 2nd stage.

Biasing ensures that both transistors operate in their active regions to avoid distortion. Isolating the two stages can be done using a decoupling capacitor, which blocks DC signals while allowing AC signals to pass through. This is crucial for maintaining the appropriate operating point for each transistor in the cascaded configuration.

Consider the capacitors as traffic lights at an intersection. They allow cars (AC signals) to move between intersections (transistor stages) while preventing one intersection's red light (DC bias) from affecting the other, ensuring smooth traffic flow.

the source terminal current of M2 can directly supply the required drain terminal current of M1... the connection it is direct connection...

In a cascode amplifier, the current from the common source stage (M1) can be shared to bias the common gate stage (M2) effectively. By establishing a direct connection, the required conditions for both transistors can be satisfied without requiring additional bias circuits, leading to a more efficient design.

Think of M1 as a solar panel that generates energy (current) shared with another device (M2) directly. This reduces the need for batteries (extra biasing circuits) to keep the devices powered, thereby simplifying the system.

the gate voltage here V should be more than or equal to required V to support this current Plus, V or transistor-1...

Maintaining the correct DC operating point is critical to ensure both transistors remain in saturation. The gate voltage of M2 must surpass certain thresholds to keep M1 active, preventing it from falling into the triode region where it performs poorly. This highlights the importance of voltage levels for functional stability.

Imagine setting the right temperature in an oven (gate voltage) to ensure that the cake (transistor performance) is baked perfectly. If the temperature isn’t maintained, the cake might be undercooked (transistor in triode region), leading to a poor outcome.

we need to draw the small signal equivalent circuit similar to BJT cascode amplifier...

To analyze the performance of the cascode amplifier, drawing the small signal equivalent circuit allows for calculation of the voltage gain and output impedance. The high output impedance and voltage gain are key advantages of this configuration, enabling effective amplification without significant losses.

Think of this analysis as inspecting a complex engine where understanding how each gear interacts (the small signal equivalent) allows the mechanic to determine how much power (voltage gain) is produced and how efficiently it runs (output impedance).

so that gives us the cascode configuration... voltage gain got increased by a factor of β.

The cascode configuration significantly increases the voltage gain compared to a standard amplifier configuration. However, this comes at the cost of increased output resistance, which can pose challenges in maintaining performance under varying load conditions.

Consider a multi-story parking garage where each level allows more cars (higher voltage gain) to park, but the garage itself requires a stronger foundation (increased output resistance) to support the added weight. If not properly managed, it may lead to structural failures.

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