76.1.2. Equivalent Circuit Description

So, here this is the basic differential amplifier using BJT and here we do have tail resistor called R_T. So, this kind of circuit so far we have not analyzed, but at least to we have analyze something similar particularly, if I consider only half of this circuit something like this and if we feed a signal at the base and if you observe the corresponding output at the collector, we know that this is CE kind of circuit.

In this chunk, we discuss the basic structure of a differential amplifier that utilizes Bipolar Junction Transistors (BJTs). The differential amplifier has a tail resistor, denoted as R_T. While this circuit hasn't been fully analyzed yet, it resembles previous circuits studied, especially the common emitter (CE) configuration. In the CE configuration, the input signal is fed to the base of the transistor, and the output is observed at the collector. The relationship between the input and output is crucial for understanding how signals are amplified.

Imagine a team of two people, where each person (representing a BJT) works together (differential amplifier) to amplify their combined performance. Each individual contributes their strengths (input signals), and the final result (output) is observed after the collaboration. The tail resistor R_T can be thought of as a support system that maintains balance between the two individuals, ensuring they work effectively.

So, instead of using only one R let me consider that 2 R_s are parallelly connected to realize this R_T. So, we are keeping this node connected. So, this circuit as long as this is connected this circuit and this circuit they are same.

To simplify the analysis of the differential amplifier circuit, we can modify it by splitting the resistor R_T into two identical resistors (2 R_s) connected in parallel. This transformation doesn't change the functionality of the circuit. When this transformation is employed, it aids in understanding how input signals affect the output more clearly by allowing the analysis of two identical halves of the circuit, making calculations and predictions easier.

Consider making two identical cups of tea instead of one big pot. Each cup (analogous to parallel resistors) can help you see how much sugar to add for the perfect taste. Analyzing two smaller portions can be simpler than looking at a single, larger pot; similarly, splitting the resistor helps in managing the complexities of the circuit.

Let me consider we are applying a signal here on a meaningful DC voltage and when we say meaningful DC voltage this DC voltage should be sufficiently high so that transistor-1 it is remaining in active region of operation.

In this section, we discuss how to apply input signals to the differential amplifier. A signal is applied on top of a DC voltage (referred to as meaningful DC voltage) that ensures the transistor remains in its active region of operation, which is necessary for the transistor to function efficiently and amplify the input signal. This means that the DC voltage should be high enough (typically around 0.6V for BJTs) to keep the transistor conducting and ready to amplify the incoming signals.

Think of the transistor as a water pump that requires a certain level of energy (DC voltage) to run effectively. If the energy is too low, the pump won't function correctly, and it won’t be able to push the water (input signals) where it needs to go. Thus, ensuring the pump is properly powered (active region) is essential for it to perform its job.

Now, if you see this circuit and if I feed the signal at the base, then at the collector what you are observing this V_o2. And since this emitter node it is degenerated by this resistor, we know that the signal will be getting here say v equals to if I call this is v_o2.

Here, the focus is on what happens at the output of the differential amplifier. When a signal is fed into the base of the transistor, an observable output voltage (V_o2) is generated at the collector. The presence of a resistor connected to the emitter causes degeneration, impacting the output signal. This degeneration typically reduces gain but can improve stability and linearity of the transistor's response, resulting in a more accurate amplification of the input signal.

Imagine playing music through a stereo system. The volume knob (output signal) adjusts the loudness based on how you adjust it (input signal). The resistor in this context acts like a filter that slightly diminishes the loudness, ensuring that the music plays smoothly and without distortion, allowing you to enjoy a clearer sound.

So, now let me again summarize what we said here, it is that if we feed a signal at this point called v on top of a meaningful DC voltage. Let we call V_IN_C so that this meaningful voltage is ensuring that this transistor it is in active reason of operation.

This summary emphasizes the importance of feeding a signal while keeping a meaningful DC voltage to ensure proper operation of the differential amplifier. In differential mode, two opposite input signals are applied to the respective bases of transistors, resulting in differential output that can be measured. The current through each transistor causes an amplified output that reflects the difference between these input signals, a key feature of differential amplifiers.

Consider a seesaw where two children (input signals) sit at opposite ends. When one child pushes down (one signal increases) while the other lifts up (signal decreases), the seesaw tips to one side (differential output). Just like the seesaw reflects the difference in weights, the differential amplifier reflects the difference in input signals, producing a measurable output.

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