59.2.3. Upper Cut Off Frequency

The common source amplifier has a gain of 6, calculated as the product of the transconductance (g) and the output resistance (R).

In a common source amplifier, the voltage gain (A) is determined by multiplying the small signal transconductance (g) and the load resistance (R). In this context, g is given as 2 mA/V, which means for every volt increase at the input, the output current changes by 2 mA. The output resistance in this case is given as 3 kΩ. Therefore, the voltage gain (A) can be calculated as A = g * R = 2 mA/V * 3 kΩ = 6.

Think of the common source amplifier as a loudspeaker (the output) that becomes more powerful with a specific amount of input sound (voltage). If you shout (increase the input voltage), the speaker emits sound even louder than your voice, represented by the gain of 6.

The upper cut off frequency (f_U) for the amplifier was calculated as 530 kHz, using the load capacitance and output resistance.

The upper cut off frequency (f_U) is defined as the frequency at which the output power falls to half of its maximum value. In this case, f_U is calculated using the formula f_U = 1 / (2πRC), where R represents the output resistance, and C represents the load capacitance. Here, R is 3 kΩ and C is 100 pF, which results in an upper cut off frequency of approximately 530 kHz.

Imagine a water faucet representing the amplifier. The water pressure (input voltage) allows water (signal) to flow out at a certain rate (frequency). If we increase the number of water pipes (bandwidth or capacitance), water can flow even faster until the faucet can no longer provide it, marking the cut-off point at 530 kHz.

By cascading the common drain stage with the common source amplifier, the upper cut-off frequency was increased to 4.24 MHz without significantly altering the gain.

The integration of a common drain stage with the common source amplifier improves overall performance. While the common source amplifier alone has an upper cut-off frequency of 530 kHz, adding a common drain stage allows for a higher upper cut-off frequency of 4.24 MHz. The gain remains consistent at around 6, indicating that the amplifier can handle a broader frequency range without sacrificing output strength.

Consider the amplifier as a two-lane highway that can handle a certain amount of traffic (signal). The common source stage is like a toll booth that checks cars before letting them through. By adding a shared lane (the common drain), even with the same number of cars, the overall highway can now accommodate much more traffic without congestion (increased frequency response).

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