79.2. Differential Amplifier using BJT

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. And the intention of that was to get more insight of the circuits particularly, to see how the differential signal and common mode signal they are getting propagated from primary input port to the primary output port.

This chunk introduces the differential amplifier made with Bipolar Junction Transistors (BJTs). Here, the BJT circuit configuration is discussed, emphasizing an analysis method where a resistor (denoted as R) is split into two identical resistors in parallel. This simplification helps in understanding how different types of signals (differential and common mode) are processed within the circuit. The goal is to better understand how each component impacts the signals flowing through the amplifier.

Think of this circuit as two highways that represent the two halves of the differential amplifier. By creating two identical and parallel roads, we can better observe traffic patterns—how cars (signals) travel differently when there are different roads available to them. This gives us clearer insights on managing the flow of cars, akin to managing the signal processing in the amplifier.

So here how we do have the different device parameters namely, for BJTs we do have β. In this case, this β may not be having much of use, but for the sake of completeness we are keeping the parameter. And then we do have the V of both the transistors 0.6. In fact, we are considering Q_BE(on) and Q , they are identical and then we also have the early voltage of the 2 transistors = 100 V. And then we do have the supply voltage = 12 V and then the loads R and R , both are equal; and they = 5.2 kΩ and the tail resistor it is 1 kΩ.

This part discusses specific parameters for the transistors in the differential amplifier. Key parameters such as beta (β—indicating current gain), V_BE (the base-emitter voltage) of 0.6 volts, early voltage, supply voltage, and resistor values are highlighted. This information is crucial for calculating the operational characteristics of the circuit, such as how the transistors behave under certain input conditions. In essence, understanding these parameters is fundamental to analyzing the differential amplifier.

Imagine a sports team where every player has specific stats (like speed, agility, and endurance) that affect how well the team performs. In this analogy, the team players are akin to the parameters of the BJTs. Just as understanding a player’s skills helps in forming strategies, knowing the transistor parameters helps us predict how well the amplifier will function.

So, to summarize the DC operating point, we do have 2.6 V is the base voltage and then at the emitter. So here also, it is 2.6 V and at the emitter we do have 2 V. Then, voltage here it is 6.8 V and here also it is 6.8 V and the collector current in both the transistors they are equal and they are 1 mA right.

This chunk explains how to determine the DC operating point for the transistors within the differential amplifier. The base voltage is established at 2.6 V, leading to collector voltages of 6.8 V. Both transistors are operating with the same collector current of 1 mA. It is essential for any amplifier to have a stable DC operating point since it sets the baseline from which all AC signals are amplified. If the operating point is not optimal, it could lead to distortion or clipping of the output signal.

Think of the DC operating point as the starting point of a race. If all racers (transistors) start from the right position (2.6 V base voltage), they have the best chance of successfully navigating the track (amplifying the signal) without crashing or slowing down. Starting too far ahead or behind can lead to problems, similar to how an optimal operating point ensures efficient signal processing.

Now we obtain the small signal parameters of both the transistors. Next thing is we need to find the small signal gain namely, a differential mode gain and common mode gain. So, the differential mode gain A = g R and this is equal to R it is 5.2 and g = , and the common mode gain.

Here, the focus shifts to the calculation of small-signal parameters and gains, which are crucial for understanding how the amplifier responds to AC signals. Differential mode gain (A_d) is calculated based on the transconductance (g_m) and load resistance (R), while common mode gain is also evaluated. This analysis is significant because it enables the determination of how effectively the amplifier can distinguish between useful signals and noise (common mode signals).

Imagine you're trying to listen to your friend speaking at a loud concert. The differential mode gain is like increasing your friend's voice volume to hear them clearly above the noise, while the common mode gain represents the music volume that’s equally affecting both you and your friend. The goal of the differential amplifier is to enhance your friend's voice while minimizing the ‘common noise’ from all around.

So, now we have obtained the differential and common mode component. So, the individual signal now, we can say that say V , it is having the DC part 6.8 V, DC and then, we do have the common mode part. So, that is ‒ 0.52 ( ) and then it is also having half of the differential part.

In the final chunk, the resultant output signals are calculated by combining the differential and common mode components. The setup helps in reflecting the contribution of both signal types towards the final output. The DC component and small-signal components provide a complete picture of how the output behaves. This includes considering how much of the output is useful (differential) versus how much is noise (common mode). This understanding helps in designing amplifiers that can amplify desired signals effectively.

Picture an audio system where your favorite song (differential signal) is played alongside background noise (common mode noise). The output you hear is a mix of both. In this chunk, we learn how to effectively manage that mix to ensure your song is much clearer and louder than the background noise, enhancing your overall listening experience.

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