80.1.1. Device Replacement

So we do have a differential amplifier and also I must say that in this circuit this is the first time we are trying to combine both MOSFET and BJTs together within one amplifier, and this is of course intentional just to give you a confidence that you can mix BJT as well as MOS in a, in your circuit. As long as you are following the fundamental basic guidelines, then you can mix it properly.

In this section, the speaker is discussing the design of a differential amplifier that incorporates both MOSFETs and BJTs. The combination is intentional to illustrate that it is possible to use both types of transistors together in a circuit. By following basic guidelines in electronics, engineers can effectively mix these components without issues.

Think of MOSFETs like sports cars and BJTs like trucks. While sports cars are built for speed and trucks for utility, in certain scenarios (like a hybrid car), both can be used together to achieve optimal performance. Just like in engineering, knowing when and how to use each type can maximize efficiency.

So, here as I said that this R , earlier we used to use R , that has been replaced by this device. In fact, if you see the device characteristic you may see that it is almost working as one ideal current source but it may be having some finite conductance.

The discussion moves to the replacement of a passive resistor (R_T) in the circuit with an active device. This device behaves similarly to an ideal current source, but it has some finite conductance, meaning that it can still provide a stable current while allowing for some variability.

Imagine switching from using a manual faucet (passive resistor) that only lets water flow when you turn it on, to an automatic faucet (active device) that maintains a steady flow but may adjust slightly based on demand. The automatic faucet enhances convenience and provides a more stable output.

This DC current can be obtained by considering its base bias. At the base we do have R_B1 and that is connected to 12 V supply. And R_B1 its value it is given it is 570 kΩ. And if I consider V_BE = 0.6 V then from that we can get, so I_C = 20 µA and then we do have β = 100. So, the corresponding current here it is 2 mA.

In this chunk, the speaker describes how to calculate the DC current flowing through the transistor. Using the base bias resistor (R_B1) and the voltage across the base-emitter junction (V_BE), the current is calculated. Given the transistor's current gain (β), you can find the collector current (I_C).

Think of it like managing an orchestra. The conductor (base current) has a direct influence on how loudly the entire orchestra (total current) plays. If the conductor slightly increases their signal, the whole orchestra picks up the volume, thus allowing more music (current) to flow harmoniously.

Now since, the left branch and right branch they are identical and we do have equal DC voltage coming there V_INC, so we can say that in both the transistors I_C = 1 mA.

The speaker talks about the concept of tail current, which is the total current flowing out of a pair of identical transistors in the differential amplifier. When both branches receive equal DC voltage, the currents in each branch become equal, allowing for balanced operation.

Imagine two identical water pipes (transistors) connected to the same water source (voltage). If both pipes are opened equally, they'll pour out the same amount of water (current), maintaining balance and efficiency in your plumbing system.

In fact it can be shown that even if say this voltage it is rising and hence this voltage it may be rising, but since this transistor it is in active region of operation, its current it is predominantly defined by its base current and the β.

This part explains that while the voltage may vary, the current through the transistor remains stable as long as it operates in the active region. The base current and the current gain (β) are the primary factors determining the collector current, making the operation predictable and stable.

Consider a dimmer switch for lights. Adjusting the dimmer (voltage) doesn't dramatically affect how bright the light is until you reach a certain point. Similarly, the transistor maintains its output current until a specific threshold is reached.

So, in summary what we can say that V_INC, V_INC(max) it is having a nice range, the upper limit it is 9 V and lower limit it is 2.3 V.

In this section, the speaker describes the acceptable range for the input voltage (V_INC) to ensure proper functioning of the amplifier. These limits are crucial as they dictate whether the amplifier will operate correctly without entering undesirable states like saturation.

Think of a car that runs well between certain speed limits. If you drive too fast or too slow, the car may stall or lose power. Similarly, the amplifier needs to stay within its voltage limits for optimal performance.

In fact, let us try to see what the corresponding common mode gain is. So, to calculate the common mode gain we know that the in the expression of common mode gain we do have g_m × R_D and in the denominator we do have (1+2g_m R_T).

This segment discusses the importance of common mode and differential mode gains in evaluating the performance of the differential amplifier. The speaker details how to calculate these gains and how they relate to the circuit's components.

Imagine trying to listen to two musicians playing at once. The common mode gain represents the sound from both, while the differential mode gain represents the unique sound of each musician. A good amplifier allows you to hear the individual musicians clearly while minimizing the background noise.

So, that is the advantage of having this active tail resistor, the biggest advantage of having this tail resistor it is we yet to discuss is that the corresponding common mode gain.

The active tail resistor provides significant benefits by improving common mode rejection and maintaining a strong differential signal. This is crucial for minimizing unwanted noise and interference in the output, leading to clearer amplification.

Think of a noise-cancellation headset. It actively detects and cancels out background noise (common modes) while enhancing the sound you want to hear (differential signals). An active tail resistor works in a similar manner by filtering out noise.

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