59.2. Common Source Amplifier

So, in the next slide we will be talking about see common source amplifier again this numerical exercise we have seen before. So, this is prime and the main common source amplifier and sorry and then we have the information given here about the device namely which is 1 mA/V2, threshold voltage it is 1 V, supply voltage it is 12 V and so and so.

The common source amplifier (CS) is a fundamental building block in analog electronics, commonly used in amplifying signals. In this particular example, the amplifier uses a transistor characterized by several parameters. The transconductance (g_m) is 1 mA/V², meaning for every volt increase in gate-source voltage (Vgs), the current through the device increases by 1 mA. The threshold voltage indicates the minimum voltage required for the amplifier to operate, which in this case is 1 V. Furthermore, the supply voltage of 12 V provides the necessary power for the circuit to function.

Imagine the common source amplifier like a water fountain where the threshold voltage is like the height of water needed to start a flow. Once you reach that height (1 V), water (current) can flow more freely (get amplified) until a maximum potential (12 V) is crowded with water.

And, we have seen that using this information we obtain V = 3 V and then I_D = 2 mA and then corresponding small signal parameter g_m = 2 mA/V.

Using the given specifications, we can deduce that the output voltage (V) at the drain of the transistor is 3 V, and the drain current (I_D) flowing through the transistor is 2 mA. The transconductance (g_m) is recalculated and found to be 2 mA/V, representing how effectively input voltage changes can control the output current.

Think of V, I_D, and g_m as being similar to the relationship between pressure (V), flow rate (I_D), and the size of a pipe (g_m) in plumbing. Just as you need a certain pressure to push water through a pipe, you need a specific voltage to get current flowing through a circuit.

So, the voltage gain it was g_m × R_D and so that becomes 2 mA × 3 kΩ. So, the corresponding voltage gain it was only 6.

The voltage gain of the common source amplifier is calculated using the formula: Voltage Gain (A_v) = g_m × R_D. Here, g_m is 2 mA/V and the load resistance (R_D) is 3 kΩ. This results in a voltage gain of 6, which means the output voltage is 6 times the input voltage.

Imagine this as a speaker where g_m represents the input sound, and R_D represents how well the speaker amplifies that sound. A gain of 6 means if you play a sound that is 1 unit loud, it will come out of the speaker as 6 units loud.

So, the upper cut-off frequency for this case f_U was into load capacitance of 100 pF. So, it was and then 3 k × 100 pF; that means, 10^(-10) yeah. And in fact, if you calculate it this gives us 530 kHz.

The upper cut-off frequency (f_U) defines the maximum frequency at which the amplifier can effectively amplify signals. In this case, it’s calculated using the formula f_U = 1 / (2πR_DC_L), where C_L is the load capacitance. Substituting in the values leads to an upper cut-off frequency of 530 kHz, indicating the limits of the amplifier's performance in high-frequency applications.

Think of upper cut-off frequency like a music band that only plays music up to a certain speed. If the music gets too fast (higher frequency), the band can no longer keep up and the sound quality drops.

So, please recall or try to remember this information. In our next exercise where we will be cascading this CS stage by common drain stage.

This section hints at the next discussion on cascading the common source amplifier with a common drain stage (often referred to as a source follower). Cascading allows for enhancements in performance, particularly in terms of bandwidth and gain stability. This means the system can handle a wider range of frequencies without losing amplification quality.

Envision cascading like adding more speakers to a PA system that allows it to fill a larger space. Each speaker can handle a certain range of sounds, but together, they provide a fuller sound experience across more frequencies.

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