85.2.1. Small Signal Model of Current Mirror

This part and if we analyse this circuit it becomes like a common source amplifier, where the load is diode connected.

In this first chunk, we are discussing how the current mirror circuit can be analyzed in a way that resembles a common source amplifier. A common source amplifier is a type of transistor amplifier configuration that provides a significant voltage gain. The mention of a 'diode connected load' suggests that part of the current mirror is used to stabilize and control the current flowing through the amplifier, ensuring it operates correctly.

Think of the common source amplifier like a water pipe system where the water flow needs to be regulated. If part of the pipe is designed to allow a specific amount of flow (like the diode connected load), it ensures that the entire system operates smoothly without too much pressure, similar to how the current mirror maintains stable current in an amplifier.

Now, if you analyse this circuit or if you draw the small signal equivalent circuit, you can find that the voltage here v_o2 = ‒g_m1 × v_in_c.

Here, we are performing a small signal analysis on the circuit. Small signal analysis involves examining how the circuit responds to small changes (or disturbances) in the input signals. The equation provided shows that the output voltage (v_o2) is dependent on the transconductance (g_m1) and the small input voltage (v_in_c). This relationship is crucial for understanding how the amplifier responds to small signal variations.

Imagine you’re standing beside a calm lake. If you drop a small stone into the water, the ripples that form are like the small signals. The behavior of those ripples (the small changes in water level) can give insights into how the lake (the larger body of water, similar to the amplifier) reacts to disturbances, just as the circuit's output responds to small input signals.

We may say that v_o1 under common mode operation is also = v_o2 and that gives us v_common mode. And hence the corresponding common mode gain A″_C = ‒g_m1.

This chunk introduces the concept of common mode gain, which is the gain of the amplifier when the same input signal is applied to both inputs. The equation indicates that the common mode gain is determined by negative transconductance times the input signal. A lower common mode gain signifies better performance in differentiating signals, indicating that the circuit can more effectively reject noise or interference common to both inputs.

Consider a group of friends all talking simultaneously. If one person speaks louder, others might struggle to be heard, creating noise. Common mode gain here is like the voices being amplified in the same way, while good performance means one friend could focus on just the conversation they want, ignoring background noise—much like how the circuit ignores signals that affect both inputs similarly.

To summarize what we can see the advantage of having this current mirror in the load part: the common mode gain got drastically decreased.

The chunk explains that one of the key benefits of implementing a current mirror as part of the load in an amplifier circuit is that it significantly reduces the common mode gain. A decreased common mode gain is advantageous for circuit performance, as it enhances the circuit's ability to reject unintended common signals (noise or interference), leading to cleaner output signals.

Think of a speaker system at a concert. If the common noise from the audience is too high, it drowns out the music. Using a current mirror in the circuit is like adding filters to the speaker system that only let the music through while minimizing the background noise, ensuring a clearer and more enjoyable sound experience for everyone.

Since the DC voltage here it is the same for the two nodes, the DC voltage here can be directly obtained from this node.

In this section, we examine the relationship between the DC voltage levels across different nodes in the circuit. It indicates that the DC voltage remains stable across the circuit, simplifying biasing because the same voltage can be used at multiple points, improving efficiency.

This is similar to a battery powering multiple light bulbs. If the battery provides a consistent voltage, all the bulbs shine equally bright. Similarly, in this circuit, having a stable DC voltage ensures that the transistors operate under consistent conditions, making the circuit's performance reliable and predictable.

We have started with a small signal small signal model of current mirror particularly, under DC condition as well as whenever it is carrying the signal current for both BJT and MOSFET versions.

In the conclusion of the section, a summary of the important topics covered is provided, highlighting the versatility of the current mirror in various applications, such as serving as bias elements in amplifiers and improving performance metrics. It emphasizes the fundamental principle of understanding the small signal model and its applications in practical circuits involving both BJT and MOSFET technology.

Imagine learning to drive a car: first, you learn the basic controls (like the small signal model) before you can navigate your way through the streets (real-world applications). Understanding the small signal model equips you to apply this knowledge effectively in various circuits, just as mastering driving techniques prepares you for a range of driving situations.

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