85.1.1. Circuit Representation

this part and if we analyse this circuit it becomes like a common source amplifier, where the load it is diode connected. So, the circuit becomes like this. We do have g , we do have g connected to V and m1 m7 DD then this transistor half of the transistor we may call say M by 2.

This chunk introduces the concept of a common source amplifier, which is a basic type of amplifier used in electronic circuits. In this design, the output is influenced by a transistor configuration and a load that is connected in a diode mode. The mention of 'g' connected to 'V' suggests that we have a voltage source that is affecting the behavior of the transistors (M1 and M7) in the circuit. The term 'M by 2' indicates that we are considering half the characteristics of the transistor for analysis purposes.

Think of a common source amplifier like a water flow system where a pump (transistor) influences the water that flows through pipes (circuit paths). The load connected in a diode format is like a valve that only lets water flow in one direction, allowing the system to amplify the pressure of the water (signal).

At this point we are giving the small signal along with the DC. Of course, this node it is getting the bias similar to this point and we like to know what will be the corresponding output here due to the small signal and this small signal it is v . in_c

In this chunk, we learn about the introduction of a small signal (denoted as 'v_in_c') superimposed on the direct current (DC) biasing voltage applied to the nodes in the amplifier circuit. This means that while the circuit operates at a certain steady-state voltage (DC), we also want to see how it responds to small variations or signals. The output we are interested in will depend on how the circuit processes both DC and this small AC signal.

Imagine turning on a light dimmer switch (DC) and then briefly flicking a switch that activates a strobe light (AC). The light dimmer keeps a steady glow while the strobe adds a quick burst of brightness. The dimmer’s setting impacts how bright the strobe appears, just like the DC bias affects how the circuit amplifies the small signal.

Now, if you analyse this circuit or if you draw the small signal equivalent circuit, you can find that the voltage here v = ‒ g × . So, this into whatever input signal we are applying v . In fact, as I said the voltage here also it will be same.

This chunk discusses how to analyze the output voltage (v) for the amplifier in terms of the conductance (g) and the input signal (v_in). The formula provided indicates that the output voltage is directly proportional to these values, illustrating the amplifying behavior of the circuit. In essence, the output voltage will be negative and scaled by the gain factor (g) times the input signal. It highlights that during operation, if one input changes, the output will respond accordingly.

It’s similar to increasing the volume on a speaker (gain factor) while playing a song (input signal). If you turn the volume up twice as much, the sound (output voltage) coming from the speaker gets louder and inversely affects how the sound waves may modulate.

So, we may say that v under common mode operation is also = v and that gives us v = v common mode.

In this segment, the text emphasizes the concept of common mode operation, where the output voltage (v) remains the same for both inputs. When working under common mode conditions, both input signals (for example, v_o1 and v_o2) produce an equal output voltage. This indicates that the common mode gain has been established, allowing us to analyze how well the circuit can reject any signals that are common to both inputs.

Imagine two identical twins studying together (both inputs being the same) under a single light bulb (common voltage source). They both experience the same amount of light hence, creating a uniform study environment. If someone were to dim the light (input signal), they both would notice the same decrease in brightness (common mode output).

Now, again you can see here because the resistance now it is getting changed. So, this A it is much smaller than the previous common mode gain. To summarize what we can see the advantage of having this current mirror in the load part.

This part highlights how the presence of a current mirror affects the overall gain (designated as A) in the circuit, particularly in common mode operation. The text explains that the current mirror helps to reduce the common mode gain by modifying the circuit impedance. This reduction in gain indicates an improvement in the circuit's ability to reject unwanted signals, an essential characteristic for amplifier design.

Think of a current mirror as a filter on a water supply line that only lets through clean water and blocks impurities (unwanted signals). By doing so, you end up with a clearer output (lower gain) that best represents the water quality you want. This keeps your drinking water safe, similar to how a low common mode gain keeps your audio signals clean.

What do we obtain it is the common mode gain got drastically decreased and hence the corresponding common mode gain if I say A′′ which is defined by = ‒ g × .

As highlighted, the introduction of an active current mirror load significantly decreases the common mode gain (represented as A′′), making it easier for the circuit to filter out noise and unwanted signals. This mathematical representation underscores the importance of the current mirror in improving circuit functionality, as it allows for greater signal integrity.

Imagine trying to listen to music at a concert (common mode signals) with lots of background chatter. A powerful noise-canceling headphone (current mirror) allows you to hear the music clearly, effectively filtering out the noise (decreasing common mode gain) and enhancing your experience.

Since the DC voltage here it is same for the two nodes the DC voltage here it can be directly obtained from this node and since this DC voltage it is with respect to V or to be more precise it is V ‒ V.

This section explains how the DC voltage established across the amplifier sets conditions for the following stages of amplification or signal processing. With the DC voltage being common among the two nodes, it allows for a more straightforward connection to subsequent stages, particularly PMOS transistors, ensuring stable biasing and performance.

You can think of this as setting the temperature of your room (DC voltage) to a comfortable level for everyone before a party (subsequent circuit stages). Once the temperature is set evenly, everyone (transistors) can enjoy the party comfortably without worrying about wild fluctuations in temperature (unstable performance).

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 summary, the discussion wraps up the various uses of current mirrors in amplifier designs, particularly highlighting their roles not only as bias elements but also improving the performance of amplifiers in various platforms (both BJT and MOSFET technology). It summarizes that current mirrors can manage both common mode and differential mode gains, making them crucial in modern circuit design.

Consider a Swiss Army knife, which has multiple tools (current mirror applications) in a single compact object, making it efficient for various tasks. Each tool is designed for a specific function (biasing, amplification) but works together to achieve overall performance, much like how current mirrors enhance circuit functionality.

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