67.5. Current Matching in Active Load Configurations

So, here the lower part we are retaining same and same circuit we do have the M1, which is receiving the signal at its gate along with the DC voltage. But then it is also having the load, which is instead of having passive load, but it is having a transistor M2. Note that this transistor M2, this is PMOS transistor right. And its source it is connected to VDD and the gate it is receiving DC voltage defined by say VSG with respect to VDD.

In an active load configuration, we replace a passive resistor with an active device such as a PMOS transistor (M2). This allows for better control over the load, which can enhance the performance of the amplifier. The transistor M2 receives its gate voltage from an external DC source, creating a threshold for its operation. It is crucial to understand that the proper biasing of this transistor is essential to maintain functionality and achieve desired results in the amplifier's gain.

Think of the transistor as a gatekeeper. Just like a gatekeeper controls who can enter a venue based on certain criteria (like a ticket or invitation), the voltage applied to the gate of the transistor M2 determines if current can flow through, allowing the signal to be amplified effectively.

Now naturally, then who defines this current? For proper operation, we require both the current should be equal and we need to satisfy some condition to ensure that I_D1 and I_D2 should be equal.

For the amplifier to operate correctly, the currents flowing through both transistors (I_D1 and I_D2) must be equal. This equality is vital for maintaining stability and predictable behavior within the circuit. Even when no other circuit paths are connected at the node, we must ensure that both devices stay in the saturation region, as any imbalance can bring one into triode operation, negatively impacting the amplifier's gain.

Consider a relay team in a race. If all runners run at a similar pace, the team performs well; however, if even one runner slows down significantly (analogous to one transistor entering a different operating region), it can bring the whole team's performance down.

So, whenever we say that these two are equal what we mean is that, the devices are in saturation region and in saturation region whatever the current we do have they are equal.

The operating conditions of the transistors must be closely monitored to keep them in the saturation region, where their current characteristics are well-defined. This ensures that the equations governing their behavior reflect the true operational capability of the amplifier. The saturation region is where the devices can ideally provide maximum gain and linear operation.

Like a car that performs best at a certain speed, transistors also have their 'optimal range' of functioning. Keeping them in this range ensures optimal performance for our amplifier, much like ensuring a car stays in its performance zone for better efficiency and speed.

So, in the next slide we do have the we have drawn the small signal equivalent circuit for the common source amplifier having active load.

To analyze the behavior of the amplifier under small signal conditions—those signals that are minor compared to the DC bias—we use a small signal equivalent circuit. This circuit simplifies the analysis by replacing transistors with their small signal models, allowing the use of linear circuit techniques to predict performance. This aspect is crucial for understanding how the amplifier will respond to varying input signals while maintaining the defined operational parameters.

Imagine analyzing the flow of traffic at a busy intersection during peak hours. Instead of looking at the entire road layout, you might focus on just one lane to determine how signals (cars) navigate through that section. Similarly, a small signal analysis helps simplify complex circuits to evaluate their performance under normal 'noise' conditions.

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