34.1.10. Guidelines for Lower Cutoff Frequency

If the lower cutoff frequency is given to us typically we may target say 10 maybe 10 Hz. So, we can convert this Hz into radian and from that you can find what will be the corresponding C, its value may be coming in the range of µF.

The primary goal of the guidelines is to ensure that if a lower cutoff frequency for the amplifier is specified, such as 10 Hz, we should convert this frequency into radians to comply with the mathematical framework of electronic circuit analysis. The conversion is typically done using the formula: $f = \frac{1}{2\pi RC}$ which allows us to find the capacitance (C) required to achieve that specific frequency. The calculated capacitance will usually fall within the microfarad (µF) range, which is a common value for capacitors used in filtering applications.

Think of the lower cutoff frequency as the age limit for a movie. If a movie is rated for ages 10 and up, it can be viewed by anyone aged 10 years or older. Similarly, if you specify a lower cutoff frequency of 10 Hz, it means that signals below this frequency will be filtered out, just like how children younger than 10 cannot watch the movie. In both cases, you set a threshold (or cutoff point) that defines what is allowed and what isn’t.

Once we find the resistances particularly input resistance from that you can find what will be the corresponding C, because if I combine these two namely the first coupling capacitor; so, that gives us the lower cutoff frequency.

After determining the input resistance of the designed circuit, the next step is to calculate the required capacitance (C) using the relationship established for the lower cutoff frequency. In amplifier design, the coupling capacitor plays a critical role in determining how low frequencies will affect the amplifier's performance. By knowing both the resistance and the targeted cutoff frequency, we can rearrange the frequency formula to solve for capacitance, allowing us to select an appropriate capacitor that meets the design specifications.

Imagine you are waiting to enter a concert. There is a bouncer at the door checking for tickets, much like the input capacitor filtering out unwanted low frequencies. If the concert starts at a certain time (the cutoff frequency), only those who have tickets (the correct frequencies) can enter. The size of the concert (the capacitance value) needs to be big enough to allow enough people (the right frequencies) to pass through without any unnecessary delays.

So, we have discussed these guidelines through this numerical exercises. Yeah, I think that is all to cover now and in the next session we will be moving for the frequency response.

The section ends by emphasizing the practical application of the discussed guidelines through numerical examples. These examples serve to illustrate the process of calculating component values based on frequency response, showcasing how theoretical knowledge translates into actionable design practices in real-world scenarios. The upcoming lectures will further explore the frequency response of the amplifiers, which is a critical aspect when evaluating circuit performance.

Consider building a bridge over a river. Before construction, you would go through the basic engineering guidelines and calculations to ensure the bridge can handle various loads (like frequency response in amplifiers). After reviewing these computations through example scenarios, actual construction will begin, similar to how the upcoming sessions will delve deeper into frequency response and implications for circuit design.

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