65.1.1. Prof. Pradip Mandal

Welcome back after the short break. So, we are talking about the Cascode Amplifier using BJT sorry 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.

In this chunk, Prof. Mandal explains that the discussion is focused on the Cascode Amplifier, specifically using MOSFETs after previously covering BJTs. The transition is being made from a passive load of 2 k ohms to an active load, which is intended to achieve higher gain. Passive loads typically do not add gain to the circuit, whereas active loads can help increase the output signal strength.

Think of it like upgrading from a regular bicycle (passive load) to an electric bicycle (active load). While both can take you places, the electric bike can boost your speed significantly, much like how an active load can enhance the gain in a circuit.

So, based on the bias conditions the current I = I = 2 mA. 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. 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 section discusses the specific electrical parameters of the cascode amplifier. The bias current is set to 2 mA, and the resistance is 5 M ohms. This configuration leads to a DC voltage of 6 V at the output, which is critical for maintaining the transistor in saturation mode. Proper biasing is essential for optimal amplifier performance as it determines the operating area of the transistors.

You can think of this like setting up a hose system. If you know how much water (current) will pass through and the size of the hose (resistance), you can predict how high the water will shoot out (voltage), ensuring your system works effectively without causing clogs.

Now for that we need to know what will be the equivalent resistance coming here and this equivalent resistance it = ( ). So, this resistance if you put the value here what will be getting here it is R is 5 M. So, this is 5 MΩ.

In this segment, Prof. Mandal talks about determining equivalent resistance to calculate the gain of the amplifier. The equivalent resistance helps in analyzing how much output voltage can be achieved given the input current. Understanding gain is crucial as it determines how much the weak input signal is amplified to a larger output signal.

Imagine you’re trying to amplify your voice to fill a large room. If you have a good microphone (equivalent resistance), it can pick up your quietest words (input current) and project them loudly enough for everyone to hear (output voltage). The quality of your microphone greatly affects the effectiveness of this amplification.

So, and if we see the value of this R it is 5 M and g it is 2 mA/V. So, this gives us this is 10‒3 and here we do have 5 × 106 that gives us ‒ 5000. So, this big number big jump compare to when were we have consider passive load and then there we got the gain of only 4.

Here, the discussion revolves around the gain calculations of the cascode amplifier using active loads. The active load gives a gain of 5000, significantly higher than the previous passive load’s gain of 4. This emphasizes the effectiveness of using active loads for increasing gain in amplifier circuits.

Think about a person lifting weights. If they have someone helping them ('active load'), they could lift much heavier weights compared to when trying to lift on their own ('passive load'). The help represents the increased gain allowing them to perform much better.

So, to get the input capacitance C which is C + C (1 ‒ whatever the gain we do have from here to here which is let you call this is A ).

In this section, the focus shifts to calculating input capacitance and its relation to gain. The formula provided expresses how input capacitance can be affected by the gain of the amplifier. Understanding input capacitance is crucial as it can affect the overall bandwidth of the amplifier, which determines the range of frequencies that can be effectively amplified.

Consider input capacitance like the size of a bucket when trying to collect rainwater (bandwidth). A wider bucket can hold more rain (signal) but might adjust how quick you can fill it, just like gain affects how quickly you can amplify and process signals.

So, in summary if I compared this cascode amplifier to boost to the gain and if I compare the performance of the standard common source amplifier.

The final chunk summarises the comparison between the cascode amplifier and the standard common source amplifier. While the common source amplifier may have higher bandwidth, the cascode amplifier provides a substantial increase in gain. The gain-bandwidth product remains an important aspect to consider for both configurations.

Imagine choosing between a sprint car that can go very fast in short bursts (high gain) versus a regular car that can cruise comfortably but not as fast as the sprint car (bandwidth). Depending on the need, one option may be more suitable than the other.

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