65.1.2. Department of Electronics and Electrical Communication Engineering

We are talking about the Cascode Amplifier using MOSFET. BJT part we already have completed now come here so, far we are talking about the passive load namely R it was 2 k now we are going to change this load to active kind of load, where our basic motivation is to for higher gain.

The section introduces the concept of the Cascode Amplifier, particularly using MOSFET technology after having discussed BJT (Bipolar Junction Transistor) amplifiers. The focus is on improving the circuit's performance by changing the load from a simple resistive load (a passive load) to an active load. This change is aimed specifically at achieving higher voltage gain, which is a crucial parameter in amplifier design.

Think of the amplifier like a water pump. If you initially have it drawing water through a simple tube (passive load), it doesn’t push the water very far. But by switching to a more advanced system that uses a pressurized tank (active load), you can push the water further and with greater force. Similarly, replacing passive loads with active ones enables the cascode amplifier to achieve significantly higher gain.

So, the main current it is getting supported by these 2 mA of I and then we do have the R resistance which is matching with this R similar to the BJTs circuit configuration; this 12 V with this is 5 M and the equivalent small signal resistance 5 M, it is giving us a DC voltage here it is half of that. So, that is also 6 V.

Here, the 2 mA current is emphasized as the primary current driving the circuit. The resistances mentioned (5 MΩ for instance) contribute to setting the operating point of the amplifier, allowing it to function correctly in the saturation region, which is essential for the amplifier's performance. The mention of DC voltage is also critical as it indicates the stable operating condition of the amplifier.

This scenario is akin to a swimming pool's filling system where you have a specific inflow of water (2 mA current), and you are using a series of valves (resistances) to control the flow and ensure the pool reaches a desired water level (6 V DC voltage). The correct settings of these valves ensure that the pool doesn't overflow and operates efficiently.

So, this is 5 × 106 probably you can drop this part and then divided by g , it is 2 mA multiplied m by we do have 50 kΩ. So, this is 50 kΩ and this is also 50 kΩ. So, this half of this g v it is coming from r and remaining half it is coming from this circuit. So, that gives us the current flowing though R namely i = half of this g v and hence we are getting the output voltage v = R × g.

This chunk explains the method used for calculating the voltage gain of the cascode amplifier. The voltage gain is derived from the equivalent resistance and current values. The relationship between various parameters such as transconductance (g) and the resistances set in the circuit is crucial for understanding how changes in these values affect the output voltage. The increase in voltage gain, noting that it was previously 4 and is now 5000, signifies a dramatic improvement in performance.

Consider a race car. If you enhance the engine (increased transconductance) and reduce friction (optimal resistance settings), the car will accelerate faster (higher voltage gain). Similarly, in amplifiers, fine-tuning resistance and current allows for a significant boost in output performance.

The input capacitance C which is C + C (1 ‒ A ) and the gain from this point to this point it is ‒50 and hence the input capacitance with this value of this A it is C it is 10, C it is 5 and then we do have (1 + 50) here and that gives us 265 pF yes. So, it is increasing the input capacitance.

This part discusses the input capacitance of the amplifier and how it relates to the gain. The parameters indicate that the gain affects the total capacitance experienced by the input signal, leading to a resultant capacitance that might impact bandwidth. The increased capacitance can lead to potential bandwidth issues that need managing in any design.

Imagine trying to fill a large balloon (input capacitance). As you blow up the balloon quickly (high gain), the material stretches and takes more effort to fill it completely. This is like how increased capacitance can slow down the responsiveness of an amplifier, affecting how quickly it can process changes in input signals.

Here the common source amplifier may be having very low gain, but then it may be having very high bandwidth mainly because the output resistance and the C is defining that. But then by virtue of the cascode structure we can increase the gain by a big factor, but then the corresponding bandwidth it is getting affected.

This section summarizes a critical aspect of electronic design: the trade-off between gain and bandwidth. While a standard common source amplifier may offer high bandwidth with lower gain, the cascode structure significantly boosts gain but reduces bandwidth. This relationship illustrates the engineering challenge of balancing performance metrics.

Consider a concert with a powerful sound system (high gain). The system can pump up the music (amplify signals) really well, but if it's not nice and clear for everyone (bandwidth), then the experience is less enjoyable. You might either have a loud muffled sound or a clear but soft one. Similarly, in amplifiers, one must choose the desired balance between gain and the clarity of the output signal.

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