47.4.5. Upper Cutoff Frequency Reevaluation

So, we can say that the upper cutoff frequency now, so this is done. Now the upper cutoff frequency if I say that f upper cutoff frequency, it is . So that is, let me use different color should see ( ) R C .

The upper cutoff frequency (f_u) is the frequency beyond which the output voltage of the amplifier starts to significantly drop off. It's important to determine this frequency for understanding how the amplifier behaves at higher frequencies.

Think of the upper cutoff frequency as a sliding door. Once you push it too hard, it doesn't slide open anymore; instead, it starts to close. Similarly, at frequencies beyond the cutoff, the amplifier cannot amplify signals effectively.

So, we do have . So, that is 1010 alright, and so this is 10‒10. So, that comes to be 1012, 1010 divided by approximately or say to be more precise 104 × π. So, that is .

The actual formula for the upper cutoff frequency is f_u = /(R_C imes C_L), where R_C is the output resistance and C_L is the load capacitance. Here, numerical estimations are done to simplify the calculation, leading to an approximate cutoff value.

Imagine trying to fill a balloon with water (analogous to signal current). Once the water pressure exceeds its capacity (upper cutoff), the balloon starts leaking, similar to how the amplifier fails to process signals effectively past the cutoff frequency.

So, roughly we can see that approximately if I say that this part it is 3 and then this is almost 100. So, that is 10, 10 to the power it is now, this is we do have another 100. So, this is 108. So, we do have 100 divided by no.

The calculations yield an approximate upper cutoff frequency of around 30 MHz. This means that the amplifier can effectively amplify signals but starts to lose its ability to do so as the frequency approaches 30 MHz.

The 30 MHz cutoff frequency can be likened to a loudspeaker. It can handle up to a certain high pitch (30 MHz), beyond which the sound distorts or fades, much like the amplifier's performance at higher frequencies.

So, let us see in the next circuit that if we consider source resistance R which is say may be having some practical value. Then you can see what will be the corresponding impact.

The load capacitance (C_L) has a significant effect on the overall behavior of the amplifier, particularly in defining the upper cutoff frequency. This section suggests analyzing circuits under practical conditions, which can affect the frequencies at which they operate effectively.

Imagine trying to drive a car with heavy cargo. The car can move faster (signal amplification) until a certain limit (cutoff frequency). As you add more cargo (increase load capacitance), the car slows down and may struggle to reach higher speeds, much like how high load capacitance slows the amplifier's response.

So, in the next slide, it is basically the same circuit except, here we are considering the R different from different from 100 k.

When introducing a different source resistance (R_S), the effects regarding the cutoff frequency and output impedance become crucial. This change can demonstrate how sensitive the amplifier is to such variations in circuit design.

Consider a singer's performance where the microphone (representing the amplifier) picks up sound differently based on distance (source resistance). The further away the singer stands (higher source resistance), the less effective the microphone is at capturing sound, illustrating how variations in source resistance can diminish performance.

--