68.1.1. Lecture – 68: Multi-Transistor Amplifiers: Amplifier with Active Load (Contd.) – Numerical Examples (Part A)

Dear students, so welcome back to our NPTEL online certification course on Analog Electronic Circuit, myself Pradip Mandal from E and EC department of IIT Kharagpur. Today we are going to continue Amplifiers with Active Load we have started this topic and today primarily we will be discussing about Numerical Examples.

In this introductory section, the speaker welcomes the students back to the course and gives an overview of what will be discussed in the lecture. The focus will be on amplifiers that use an active load, which are important in increasing the voltage gain in electronic circuits. Active loads are often preferred over passive loads because they can provide better performance, making them a common topic in advanced electronics.

Think of active loads in amplifiers like a turbocharger in a car engine. Just as a turbocharger uses the engine’s exhaust to produce more power without adding much weight, active loads allow amplifiers to achieve higher gains without compromising the circuit's design.

So, the plan for today it is Numerical Examples and inherent design guidelines; while we will be going through the numerical examples we will also be given hint towards how to design a circuit specifically for CE amplifier having active load and then common source amplifier having active load.

Here, the plan for the day is laid out, focusing on practical numerical examples. The speaker emphasizes that, alongside the examples, students will learn about design guidelines for two types of amplifiers: the Common Emitter (CE) amplifier and the Common Source amplifier, both incorporating active loads. This integrates theory with practical application.

Designing an amplifier is much like planning a construction project. You wouldn't just start building without a blueprint; similarly, engineers need guidelines to make effective amplifiers that perform well with active loads.

So, here we do have the different parameters of the two transistors for Q β is 100, for Q β is 200, just for a change we are taking different value 1 2 of β. And V of transistor-1 it is we are approximating it is 0.6 V.

In this part, the speaker describes the parameters of the transistors being used in the example. The β (beta) values are mentioned for two different transistors: Q1 has β = 100 and Q2 has β = 200. Additionally, the base-emitter voltage V_BE for both transistors is approximated to 0.6 V, a typical operating value for silicon-based BJTs. The different beta values are important to note as they affect current gain in the circuit.

Imagine you are baking cookies and have different ovens. One has settings that are very precise (like Q2 with a higher beta) and another has more variation (like Q1 with a lower beta). The precise settings lead to more consistent results, just as a higher beta in a transistor can lead to more reliable amplification.

To make a balance of that the collector current of transistor-1 and collector current of transistor-2 they should be equal and to do so, to get that what we have done it is that the, base current of transistor-1 and base current of transistor-2 we are making it different.

This chunk discusses how to ensure that the collector currents in both transistors are matched. Since the transistors have different beta values, to achieve equal collector currents, adjustments have to be made in the base currents. This is a crucial aspect of amplifier design, as unbalanced currents can lead to distorted outputs.

Think of this like balancing two scales with weights. If one side is more sensitive (like the transistor with a higher beta), you need to adjust the weights on the other side to make sure they balance. In electronics, this means adjusting currents rather than physical weights.

So, to get the DC voltage at the output node say V_OUT what we can do? we can compare β I × (R of transistor-1) = β I of transistor-2 × (R of transistor-2). Now this part and this part we have seen here they are equal.

This section focuses on calculating the output DC voltage (V_OUT) of the amplifier circuit. By equating the products of the beta times collector currents with respect to their respective resistances, the speaker shows how to derive the output voltage. This establishes a vital relationship that determines how signals will behave in the circuit.

Imagine you are calculating the final score of two competing teams in a game. If both teams have equal scoring methods but different strategies, as long as their final scores (like the output voltage) are calculated correctly, the game is balanced. Similarly, in this circuit, balance needs to be maintained for accurate amplification.

So, we do have I = I = 2 mA, V = V = 6 V. And yeah so, the other things we C1 C2 CE1 EC2 already have obtained namely I = 20 µA and I = 10 µA right.

In this part, the speaker summarizes the operating point of the two transistors in the amplifier, identifying the collector currents and voltage at the emitters (or collectors). These operating points are crucial in understanding how the amplifier performs under complete circuit conditions and how much current flows through each transistor.

Consider operating points like the pressure gauges in a water supply system. By knowing the pressure in each section, engineers can better understand how efficiently the system operates. Similarly, knowing the operating points in amplifiers helps in diagnosing performance issues.

From that we can calculate the small signal parameters of the transistors namely in g say g of transistor-1 it is thermal equivalent voltage we can consider that is 26 mV.

This chunk emphasizes the calculation of small signal parameters (like transconductance) which are significant for analyzing how small changes in input signal affect the output in linear operation. The thermal voltage is important in characterizing the response of transistors to small signals.

Think of small signal parameters as the sensitivity settings on a microphone. The more sensitive the mic (or transistor), the better it can respond to tiny sounds (or signals). Just like you’d feel the difference when a mic is properly calibrated, the gain changes in amplifiers are greatly influenced by these small signal parameters.

--