77.3.3. Differential Amplifier using MOSFET

In our previous lecture we have talked about the basic structure and working principle of differential amplifier, prior to that we have discussed about single ended signaling versus differential signaling, they are part of the differential amplifier background. And today we are going to discuss more on analysis of differential amplifier.

This part introduces the concept of a differential amplifier, stating that previous discussions have laid the groundwork by explaining its structure and how it functions. It contrasts single-ended signaling (where a signal is measured with respect to a common ground) with differential signaling (where signals are compared against one another). By understanding these basics, students are better prepared for more complex analyses.

Think of a differential amplifier like a pair of ears. Just as your ears pick up sounds from multiple directions and allow you to discern between them, a differential amplifier takes in multiple signals and processes the differences between them. This helps it ignore common noise and enhance the desired signal.

Specifically, for two modes of operation differential mode and common mode operation and their corresponding gain. And, then we will also talk about large signal analysis from where we can get the signal swing possible output signal swing and the range of DC voltage which is referred as Input Common Mode Range.

The differential amplifier works in two main modes: differential mode and common mode. In differential mode, it amplifies the difference between two input signals; this is what makes it so effective in eliminating noise that can affect both signals equally. In contrast, common mode operation signifies that both inputs receive the same signal, which should ideally result in no output. The discussion will extend to large signal analysis, which helps in understanding the amplifier's performance when processing varying signal amplitudes, specifically defining the output swing and common mode range.

Imagine you are trying to hear a conversation in a noisy room. If you focus on the differences in what two people are saying (differential mode), you can understand better despite the noise. However, if both people say the same thing, you are likely to hear nothing interesting (common mode). The range within which you can still hear the conversation without interference represents the input common mode range.

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.

The small signal equivalent circuit simplifies the complex differential amplifier into a more manageable form for analysis. This form focuses on the variations around an operating point (small signals), allowing easier mathematical manipulation to understand amplifier behavior. This involves the use of transistors (BJT and MOSFET) and their respective small-signal parameters to model the amplifier accurately.

Consider a car engine running at a constant speed. If we want to analyze small fluctuations in speed (small signals), it’s easier to look at changes relative to that steady state, rather than examining every small detail of the entire engine. Similarly, the small signal equivalent circuit allows us to study small variations in the amplifier's output based on small input changes.

Specifically, for Differential mode of stimulus or Differential mode of operation then, Common mode stimulus or Common mode operation and then combined one which is referred as generalized stimulus.

We analyze two main operational conditions: differential mode stimulus and common mode stimulus. In differential mode, the input signals differ, allowing the amplifier to produce a substantial output reflecting that difference. In the common mode condition, where the inputs are the same, the output ideally should be zero. A generalized stimulus refers to more complex scenarios where both modes are combined in varying proportions. Understanding these stimuli is crucial for predicting how the amplifier will respond in real-world applications.

Think of a public address system. When a musician plays two different notes (differential mode), the system amplifies the unique sound beautifully. However, if two identical notes are played by different instruments (common mode), the sound should ideally cancel out if timed perfectly, making no distinct output. Generalized stimulus would be like having several instruments play varied notes at once, creating a more complex sound landscape.

So, we shall discuss about DC operating point analysis and then we shall talk about; the input DC voltage range over which devices are in proper region of operation which is commonly known as Input common mode range.

Large signal analysis involves understanding how an amplifier behaves when presented with input signals that significantly change its output, unlike small signal analysis which focuses on slight variations. The DC operating point is the steady-state voltage and current conditions in which the amplifier operates. The operating range for inputs—referred to as the input common mode range—is critical as it defines the limits within which the device will function correctly without distortion.

Imagine a seesaw balanced in the center (the DC operating point). If both sides remain close to that center, it works smoothly (like small signals). However, if one side is pushed up too far (large signal), the seesaw may tip over or malfunction, just as amplifiers can distort if inputs exceed the common mode range.

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