79.3. Example Circuit Analysis

As I said that, the analysis part it is done in the previous 2 lectures, and we are going to talk about numerical examples, and we do have primarily differential amplifier using BJT then we do have differential amplifier using MOSFET and then also we do have another example where we do have the differential amplifier, we do have both types of transistor MOSFET as well as BJT.

In this introduction, the instructor outlines that previous lectures covered the theoretical analysis of differential amplifiers, and this session will focus on practical examples using different types of transistors: BJTs (Bipolar Junction Transistors) and MOSFETs (Metal-Oxide-Semiconductor Field-Effect Transistors). This sets the stage for understanding how these components function in real circuits.

Think of learning how to cook a dish. First, you read the recipe (analysis). Then you actually cook the dish (numerical examples). Just as different recipes might use chicken or tofu, different electronics components can be used to achieve similar results in circuit design.

So, this differential amplifier having BJT’s it will be having different perspective; namely, the DC operating point and then small signal parameters, then differential mode gain, common mode gain and then going to the input range and output swing.

Here, the instructor mentions key characteristics of the differential amplifier using BJTs. Each aspect is crucial for designing a functional circuit: The DC operating point ensures transistors operate in the correct region; small signal parameters define how the circuit responds to small changes; differential mode gain represents the amplification of the desired signal; common mode gain indicates how much of the unwanted signal is amplified; and input range and output swing refer to the voltage levels the circuit can handle without distortion.

Imagine managing a loudspeaker system. The DC operating point is like adjusting the volume knob to ensure the speakers work properly, while differential and common mode gains are like the speaker's ability to focus on music (differential) versus picking up background noise (common mode). Ensuring these are balanced is key to a clear sound experience.

So, we do have differential amplifier realized by BJT. So, this is the circuit we have discussed before and you may recall that in our most of our analysis we used to split this resistor R into two identical elements in parallel.

This part refers to the practical assessment of the differential amplifier circuit which uses BJTs. The splitting of the tail resistor R into two components is a method to better understand the circuit behavior under both differential and common mode signals. This technique helps visualize how signals combine and how they are amplified differently in the amplifier.

Think of a garden hose with a splitter at the end. When you separate it into two hoses, you can better control how water flows to different areas. In a similar way, splitting the resistor helps analyze how electrical signals are influenced in the circuit with respect to both the desired input and any noise.

Now here we have picked up the value of this DC voltage well within its range, allowable range. So, with this 2.6 of V_INC, let us try to find the operating point of the transistors.

Selecting a proper DC voltage, here 2.6 V, is essential as it determines the state of the transistors within the circuit. The goal is to ensure that both Q1 and Q2 remain in the active region, where they can amplify signals appropriately, without going into saturation or cutoff misuse regions. The operating point is the specific DC voltage and current value where these conditions are met.

If you've ever used a dimmer switch, you know that in order to get the right brightness (operating point) from a light bulb, you have to find a suitable setting. Too low and the light goes out (cutoff), too high and it flickers or fails (saturation). Finding that middle ground is crucial for optimal performance.

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