64.5.1. Upper Cutoff Frequency Calculation

So far what we have discussed that the advantage of cascode amplifier with respect to standard CE amplifier and namely what you have seen is that in case if you are retaining this passive element for both the cases, then gain wise we do not get much advantage.

The cascode amplifier provides distinct advantages compared to a standard common-emitter (CE) amplifier. While theoretically, the gain might not show significant improvement when passive elements are simply swapped out, other characteristics, such as bandwidth and output resistance, come into play, especially when large source resistances are involved.

Think of a highway where cars can go fast. In one lane, the cars (gain) are moving well, but in another lane with heavy traffic (passive elements), the speed (gain) does not improve much even if more cars are added. However, if you switch lanes to one with fewer obstacles, you can go much faster (better performance)!

In fact, cascode amplifier it is having two types of advantages; one is extending the bandwidth as we just now we have discussed particularly in presence of significantly large value of the source resistance, the other advantage which is commonly used is that the increasing the gain drastically.

The cascode amplifier's design allows it to increase bandwidth significantly, particularly when handling high source resistances. Additionally, it can vastly improve gain, which is particularly beneficial in applications where signal amplification is crucial.

Imagine a two-stage rocket. The first stage brings the rocket high into the sky (increases gain), while the second stage, with reduced weight, allows it to soar even higher (extends bandwidth). The combination of these stages represents the cascode amplifier's dual benefits.

So, to demonstrate the capability of the cascode amplifier to increase the gain first of all let we consider a different situation instead of having this R if you put some active circuit there, probably then the advantage of the cascode amplifier particularly for enhancing the gain it will be quite prominent.

When replacing passive components with active circuits in a cascode amplifier, the potential for gain increases substantially. This change allows the circuit to overcome limitations imposed by lower resistances, leading to better amplification.

Imagine upgrading a bicycle to an electric bike. The electric bike (active circuit) provides more speed and efficiency compared to the pedal-only bike (passive component), showcasing how enhancements can improve performance.

So, yes we got the advantage, but we need to really calculate whether we made some significant amount of damage on the upper cutoff frequency defined by the input capacitance C and R.

With increased gain comes the necessity to evaluate the upper cutoff frequency. As the load capacitance grows due to the cascode configuration, it can adversely affect the frequency response of the amplifier, reducing its operational bandwidth. This trade-off must be considered in design.

Think of a water hose. If you increase the water pressure (gain) too much without adjusting the hose’s diameter (cutoff frequency), the flow may become erratic or reduced, demonstrating the need for balance in circuit design.

So, the now the alarming situation we do have here it is 5 M and C = 100 pF. So, that defines the upper cutoff frequency.

When the total resistance reaches 5 MΩ and the load capacitance is 100 pF, it defines a particular upper cutoff frequency that can limit the usefulness of the amplifier in high-frequency applications. Understanding this relationship is critical for engineers designing amplifiers for specific applications.

This is like a fitness tracker that limits your running speed to a certain limit if your heartbeat rate is too high. Beyond that threshold, you may need to slow down, just as the circuit’s frequency response is limited by its design.

So, in summary if I consider this 10 MΩ and the equivalent resistance of this part it is 10 MΩ and the ideal current here it is 2 mA.

When synthesizing necessary resistances for cascode amplifiers, there is a balance between achieving high gain and maintaining sufficient bandwidth. With an output resistance of 10 MΩ and a current of 2 mA, understanding this balance is key in providing a functional and effective amplifier.

Consider a restaurant that offers a huge menu (high gain) but has limited kitchen capacity (bandwidth). While they can serve a diverse array of dishes, their speed in delivering meals may slow down if too many orders come at once, emphasizing the need for balancing choices with capability.

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