62.1. Analog Electronic Circuits

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 combination of common source followed by common gate. And, similar to BJT where we have discussed about CE followed by CG.

The cascode amplifier is a configuration that combines two common amplifier configurations: the common source (CS) followed by the common gate (CG) in MOSFETs. This is similar to how a common emitter (CE) amplifier can be followed by a common base (CB) amplifier in BJTs. The reason for using these cascaded configurations is to improve performance metrics like voltage gain, input impedance, and bandwidth.

Think of the cascode amplifier like a multi-layer cake, where each layer adds to the overall taste and experience of the cake. The bottom layer (common source) brings flavor, while the top layer (common gate) adds stability and height, making the entire dessert more impressive and enjoyable.

So, here also the basic purpose it is to have that configuration. We need to again summarize and want to know what is the purpose of this configuration. Particularly, cascading common source with common gate.

The main purpose of cascading a common source with a common gate amplifier is to achieve a higher voltage gain while also keeping the input and output impedances at desirable levels. The common gate stage helps to maintain a stable bias point and can serve to buffer the output from the first stage, allowing for improved signal integrity.

Imagine a relay race where the first runner (common source) passes the baton to the second runner (common gate). The first runner makes sure to run as fast as possible, while the second runner receives the baton smoothly and continues at an optimal pace, ensuring that the race progresses effectively.

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. So, here we do have the common source stage, followed by the common gate stage.

In a cascode amplifier, the common gate stage acts as a voltage gain booster. The input goes to the common source stage first, which can amplify it, and then the output is fed into the common gate stage, which further enhances the gain while providing a buffer. This arrangement optimizes the performance of the amplifier by allowing for both high gain and low output impedance.

Consider a car engine that first goes through a regular transmission (common source) to increase speed, followed by a turbocharger (common gate) that gives it an additional power boost. The combination allows the car to achieve higher speeds more efficiently.

Now, let me put few words about the biasing and operating point of the 2 MOSFET transistors. 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...

Biasing in MOSFETs is crucial to ensure that the transistors operate in the desired region (typically saturation for amplifiers). The gate voltage must be adequately set using a potential divider to keep the transistor on and functioning correctly. If the bias is improper, the MOSFET could enter undesired regions like the triode region, leading to suboptimal performance.

Think of biasing as ensuring that a light bulb receives the right amount of voltage to shine brightly. If the voltage is too low, the bulb won't light up properly, and if it's too high, it might burn out. Similarly, MOSFETs need the right voltage to operate efficiently.

So, if we look into this circuit that is the output resistance. And, we also have discussed about the R resistance for such kind of circuit. This circuit, what is the specialty of this circuit? We do have the active device, providing voltage dependent current source then r or r.

The output resistance of the cascode amplifier is notably high, which results in a significant voltage gain. This is due to the characteristics of the MOSFETs used and the configuration itself. By maintaining high output resistance, the amplifier can effectively amplify the input signal without much degradation.

Think of a strong water pipe system that increases pressure significantly. A high resistance at the output means the water (electrical signal) can flow with great pressure, leading to greater distance and impact when it exits the system.

In summary what we like to say here that...So, as long as the gate voltage here it is, this gate voltage it is higher than this limit, then we do not have any problem.

While cascode amplifiers enhance gain and reduce input capacitance, they also increase output resistance, which can affect bandwidth. Designers must consider these trade-offs when implementing cascode amplifiers to ensure overall system performance remains optimal. Proper biasing and configuration are essential for the correct functioning of the amplifier.

Imagine tuning a musical instrument. You can adjust for perfect sound (gain) but might sacrifice some range (bandwidth) if not careful. Similarly, engineers must balance gains and resistances in amplifiers to get the best performance without losing quality.

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