70.3.4. Small Signal Equivalent Circuit

So, now we do have the small signal equivalent circuit here and here the input signal we are giving with respect to AC ground and output also we are observing this output voltage with respect to ground or AC ground. And since the signal at both the terminals we are comparing with respect to common terminal. So, we can say effectively we do have the signal terminal is only one. So, that is why it is called single ended signaling and the corresponding amplifier it is referred as single ended amplifier.

The small signal equivalent circuit representation focuses on how signals behave when only small changes are considered around a specific operating point. By applying AC ground reference, both input and output signals are compared to this common point. In a single-ended amplifier, there's only one effective signal terminal, which is why this setup is called single-ended signaling.

Think of a person speaking in a quiet room (the AC ground) where only their voice (the input signal) can be heard clearly. If there's only one person speaking, it's straightforward; this represents single-ended signaling. Everyone's attention is on that one voice as compared to other noises in the room.

So, let us see what is the basic difference of the two kinds of circuit. So, for your reference we are keeping this; we are keeping this single ended amplifier and the corresponding small signal model here and now here we do have the differential amplifier. So, we do have the differential amplifier circuit here.

The key difference between single-ended and differential amplifiers lies in their configurations. A single-ended amplifier has one input signal relative to ground, while a differential amplifier has two input signals—each is compared against a common ground. This means a differential amplifier can capture the difference between two signals, providing greater sensitivity to changes in those signals.

Consider two team members discussing their scores in a competition. A single-ended amplifier would track only one person's score (comparing it to a baseline of zero). In contrast, a differential amplifier compares the scores of both individuals to understand the competition's dynamics better—the difference between their scores might reveal the competitive landscape.

The signal for this differential amplifier when you say signal what we what we consider as signal it is the difference of the signal we do have ‒ this signal or to be more precise v which is defined as v ‒ v so this is the input signal.

In a differential amplifier, the input signal is understood as the difference between two signals, v1 and v2. This difference (v_in_d = v1 - v2) represents the overall input that the amplifier processes. The ability to use this kind of differential measurement makes the differential amplifier especially useful in applications that require noise cancellation.

Imagine a weighing scale where you want to measure the weight difference between two bags of groceries. The scale measures the difference in weight (the actual signal) rather than just showing a total weight. This way, you can identify whether one bag is heavier or lighter than the other—just as a differential amplifier highlights the difference between two input signals.

So, we can say that whenever we do have a differential signal pair actually it is having 2 components; one is the differential mode component, another one is the common mode component.

A differential signal pair consists of two aspects: the differential mode component, which reflects the difference between the two signals, and the common mode component, which is the average value of the signals. Differential amplifiers are designed to amplify the differential signal while suppressing the common mode, thus enhancing performance in various applications.

Consider a seesaw in a playground. When both ends are balanced at equal heights, that's the common mode component—the seesaw remains stable. The differential mode is experienced when one child pushes down on one side, causing the other side to lift. The seesaw amplifies the difference in height instead of stabilizing at a fixed point.

So, this gives you an idea that whenever we will be characterizing this circuit in terms of this two important parameter or called gain of the circuit then it is always better to represent the signal in the form of v and v.

Understanding the difference in gain between differential and common mode signals is fundamental in analyzing the performance of a differential amplifier. Differential gain represents how much the amplifier increases the difference between the two input signals, while common mode gain shows how sensitive the circuit is to signals common to both inputs. Ideally, a perfect differential amplifier would have zero common mode gain.

Think of a concert microphone setup. It picks up the singer's voice clearly (differential gain) while ignoring the background audience noise (common mode gain). The objective is to amplify the singer’s voice as much as possible while drowning out any surrounding distractions. This balance is crucial for successful sound engineering.

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