77.3.1. Transistor Implementation

So, we shall start with small signal equivalent circuit of differential amplifier both; implemented by BJT as well as MOSFET. And, then we shall talk about small signal analysis.

In this section, we start our discussion on implementing a differential amplifier using two different types of transistors: BJTs and MOSFETs. A differential amplifier is a critical component in many electronic circuits, especially in analog devices, as it amplifies the difference between two input signals. The analysis begins with creating a small signal equivalent circuit, which simplifies the understanding of the amplifier's operation by focusing on small variations around a DC operating point.

Think of a differential amplifier like a referee in a close sports match—his job is to focus on the difference in score between two teams, signaling when one team gains an edge. Similarly, a differential amplifier highlights the difference between two voltage inputs, ignoring their absolute values.

So, for small signal analysis we require small signal equivalent circuit. So, for this circuit we need to have equivalent circuit where, we can drop the DC part and components of this current as well as the voltage and then the linearized circuit which is referred as a small signal equivalent circuit.

The small signal equivalent circuit is constructed by removing DC components from the circuit and focusing solely on AC signals. In this manner, we can model the behavior of the amplifier for small fluctuations, which is crucial for ensuring accurate and effective signal amplification. By linearizing the circuit model, we can apply traditional circuit analysis techniques to derive useful parameters such as gain and frequency response.

Consider a microphone that amplifies sound. When someone speaks quietly, the microphone focuses on amplifying the small differences in sound waves, ignoring the background noise (DC component). Just as the microphone enhances subtle sound differences, the small signal equivalent circuit isolates the crucial signal changes we want to analyze.

First of all, transistor-1 we can replace by its equivalent model; namely g v and so, this current is between its collector and emitter.

In analyzing the differential amplifier, we represent the first transistor (BJT or MOSFET) using its small signal equivalent model, which includes its transconductance (g) and the dependent current source from the collector to emitter. This modeling is critical as it allows a straightforward calculation of the amplifier dynamics, including how input signals get transformed into output signals.

Imagine using a set of scales to weigh two objects. Just as the scales transform the weight (input) into a readable measurement (output), the transistor's model converts the small variations in input voltage into proportional changes in output current, making it easier to analyze.

So, with this model; with this model; what we can do? We can put stimulus and then we can get the subsequent analysis to find the expression of differential mode gain.

Utilizing the previously discussed small signal equivalent model, we can apply a differential stimulus (two opposite signals) to analyze the amplifier's performance. After applying the inputs, we can derive the expression for differential mode gain—how effectively the amplifier boosts the input signal difference. This analysis is key to understanding the gain characteristics of the amplifier when handling real-world signals.

Think of a tuning fork being struck - it vibrates and creates sound waves. Just as a small change in the force applied to the fork changes the sound it produces, applying a differential input to the amplifier leads to a change in the output signal, which we can analyze to understand how 'loud' the amplifier will make it.

We like to get the parameter A or expression of A in this case. We do have another mode of operation called Common mode operation where, at both the inputs we are giving the same signal.

In differential amplifiers, it's essential to distinguish between differential mode operation (where the inputs are different) and common mode operation (where the inputs are the same). Understanding the gain in both modes allows engineers to optimize amplifier performance, ensuring that they amplify the signals they want (differential mode) while minimizing the amplification of noise or unwanted signals (common mode).

Imagine a pair of binoculars designed to focus on two separate objects (differential mode) versus trying to look at the same object with both lenses (common mode). Effectively, in differential mode, you want to enhance the clarity of what the two different lenses see, while in common mode, you want to ensure that both lenses do not let unrelated background noise cloud the view.

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