83.5.2. Common Source Amplifier

So, we have started the current mirror circuit and today we will be talking about its application.

In this section, we are focused on discussing the applications of the current mirror circuit. A current mirror is an important circuit used in analog electronics, and it functions by mirroring a reference current into another branch of the circuit. The ability to replicate current is particularly useful in amplifiers, where current control is essential for achieving desired performance metrics.

Think of a current mirror like a photocopier that duplicates a document. Just as the photocopier creates exact copies of the original document, a current mirror replicates the input current to another part of the circuit, allowing for consistent and reliable performance, similar to having multiple identical copies of the same document for different uses.

We need to understand the small signal model of current mirror and to go into the small signal model we do have two possible situation.

The small signal model is crucial for analyzing how the current mirror reacts to small variations around a bias point. There are two scenarios: the first is when the current mirror does not carry any signal, known as the DC condition, and the second is when it carries a signal current. For each situation, we can derive the small signal equivalent circuit, which simplifies analysis by focusing on only the AC part of the signals involved.

Imagine you're examining a real-time digital reading, like a speedometer in a car. When the car is stationary, the speedometer reads zero (DC condition). However, when you start driving, the needle fluctuates based on speed (signal current). Analyzing these shifts, or small changes in speed, is akin to working with small signal models—where you're interested in how the system responds to small variations in input.

So, let me talk about the small signal model of current mirror implemented by MOSFET.

When designing with MOSFETs, the small signal model can be expressed with parameters like transconductance (g) and output resistance (r). By analyzing these parameters, one can determine how the circuit behaves under small changes in input voltage or current. The output impedance and the characteristics of the MOSFET are integral to predicting performance, especially in amplifying signals without distortion.

Think of adjusting the volume on a speaker. If you make tiny adjustments to the knob, the sound will respond similarly. The small signal model helps us understand how small tweaks in input can affect the output, much like how slight changes in volume impact sound levels.

So, here we do have; here we do have the current mirror and again we do have a reference current is only DC.

For BJTs, the small signal model differs slightly from MOSFETs due to their intrinsic properties. In BJTs, we often observe parameters such as base-emitter voltage and collector-emitter resistance that influence the small signal analysis. Like MOSFETs, BJTs also allow us to determine how small signal variations can influence the output, being particularly important when intended for amplification tasks.

Imagine tuning a radio. You dial slight adjustments, which can significantly affect the clarity of the sound. This is similar to small signal models: tiny changes (tuning) can lead to a more significant impact (clearer transmission) due to the characteristics of the circuit elements (the radio components).

So, to start with let me consider common emitter or common source amplifier let me see which one I do have yeah and to start with I do have common emitter amplifier with active load.

In common emitter or common source configurations, current mirrors can serve as active loads, enhancing the amplifier's performance. They replicate current and help maintain the necessary biasing conditions while allowing for better signal handling capabilities. This technique is common in analog circuit design to improve efficiency and gain characteristics.

Think of a traffic light system: each light must consistently change to ensure smooth traffic flow. A current mirror helps maintain 'traffic' (current) accurately directed and managed, contributing to overall system efficiency and avoiding overload, just as well-timed traffic lights keep a city running smoothly.

This current mirror it is very similar to NPN transistor based current mirror.

In integrated circuits, matching characteristics of transistors is essential for performance. The current mirror configuration facilitates this by allowing multiple transistors to share common biasing conditions effectively, which aids in achieving consistent current flows within the circuit. This matching capability is vital for maintaining desired functionality across the entire integrated circuit and ensuring that each component operates as intended.

Imagine a team of athletes training together: they must match their pacing to compete effectively. A current mirror serves as a coach, ensuring each member (transistor) maintains the same pace, leading to better results as a cohesive unit in the circuit.

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