62.2.3. Biasing and Operating Point of MOSFET Transistors

For M1, we need to provide a voltage here, at its gate sufficiently high. So that this transistor it will be on and that is done by this potential divider which is getting constructed by R1, R2 and the supply voltage.

In order for the first transistor (M1) to operate properly, it needs a sufficient voltage at its gate. This voltage is supplied through a potential divider circuit made up of resistors R1 and R2, along with the supply voltage. A potential divider reduces the voltage from a higher level to a lower level based on the ratio of the two resistors, ensuring that the gate of M1 receives the right amount of voltage to turn on and function effectively.

Imagine filling a cup with water from a jug. The jug represents the supply voltage and the cup is the transistor. However, if you pour too much water too quickly, it spills over. The resistors R1 and R2 act like a funnel that controls the flow into the cup, ensuring that the cup is filled just right, allowing the transistor to function optimally without overflowing (malfunctioning).

Likewise, at the gate of transistor-2, we are giving a DC voltage which is coming from this potential divider constructed by R3 and R4. So depending on their ratio we are generating a voltage here.

The second transistor (M2) also requires a well-defined gate voltage for proper operation. Similar to M1, M2 uses a potential divider made from resistors R3 and R4 to create the appropriate voltage at its gate. The voltage created by this arrangement ensures that M2 can also turn on and operate efficiently, which is critical for the overall performance of the circuit.

Think of the potential divider like a bakery that makes specific-sized cake layers. R3 and R4 are like two bakers measuring the ingredients precisely. If they use the right amounts (based on their resistance values), they ensure that the 'cake layer' (the voltage for the gate of M2) is perfectly sized, allowing it to fit nicely into the circuit without issues.

Now we are feeding the signal at this point, but at this node we want this signal this node to be AC ground for proper operation. So, we are at this node we are connecting a large capacitor to ground to make it really AC ground.

In the circuit, to ensure stable operation, we need to maintain a point that is effectively AC ground (zero AC voltage). This is achieved by connecting a large capacitor to this node, which allows AC signals to pass to ground while blocking DC signals. By doing this, we ensure that AC fluctuations don’t disrupt the operation of the transistors, thus aiding their linear performance.

Consider a trampoline that has been secured with a heavy anchor on the ground (the large capacitor). While the trampoline can freely bounce (the AC signals), the anchor keeps it stable and prevents it from moving too much (keeping DC stable). This setup helps maintain balance during performance, allowing for safe and effective bouncing.

But then this current whatever the current it is defined by size of M1 and its corresponding bias it must be consistent with whatever the bias circuit we do have here.

The current flowing through M1 must be properly matched with the bias circuit to ensure consistent performance. If the biasing does not align with the current requirements, M1 might enter the wrong operational region, like the triode region, potentially leading to poor circuit performance. Thus, careful design of the current paths and biasing is crucial to prevent malfunction and maintain effective amplification.

Imagine you are in a relay race. Each runner (transistor) must pass the baton (current) seamlessly to the next. If one runner isn't running at the correct speed or isn't ready for the baton (inconsistent bias), it can lead to a mess, with runners tripping or dropping the baton, similar to how a transistor can malfunction if biases aren’t aligned.

We need to maintain this transistor into saturation region; otherwise its output the drain to source resistance it will be small, and that may create a problem.

To ensure that M1 operates effectively, it must remain in the saturation region—where it can provide adequate gain and minimal output resistance. If M1 slips out of this region, particularly into the triode region, it won't perform as expected, leading to compromised performance and potentially non-linear amplification. Thus, a careful balance of biasing to keep M1 saturated is essential.

Imagine a soda can being shaken. If opened too soon (leaving the saturation region), the soda could fizz out uncontrollably (poor performance). But if you wait until you're ready to pour (maintaining saturation), you can enjoy a smooth pour without accident, illustrating the importance of timing and control in circuit performance.

In summary what we, in summary what we like to say here that the gate voltage here Vg2 should be more than or equal to required Vs + Vd(sat) such that there is no problem.

The gate voltage Vg2 for M2 must exceed a certain threshold to ensure proper operation, factoring in the saturation voltage (Vd(sat)). This ensures that M2 maintains adequate current flow and functions correctly. Failing to meet this voltage requirement could lead to inadequate gain and overall circuit failure, making this condition critical for reliable operation.

Think of a water fountain that requires a specific water level to operate efficiently. If the water level (equivalent to Vg2) is too low (not meeting required conditions), the fountain won't function properly, resulting in a lack of water flow. For the circuit to work well, maintaining this voltage level is just as crucial.

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