68.1. Analog Electronic Circuits

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.

The introduction sets the stage for today's topic, focusing on amplifiers, particularly those using active loads. An active load is a circuit element that provides a higher load impedance, thus improving the voltage gain of the amplifier circuit. By using active loads, we can maximize the performance of amplifiers, making them more efficient.

Think of an active load as a powerful booster in a relay race. Just as a booster helps a runner maintain speed and efficiency, an active load improves the performance of the amplifier, allowing it to deliver a stronger output with less input signal.

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. So, this is BJT version and this is MOSFET version.

In this part, the lecturer outlines the focus on numerical examples and design principles. CE amplifiers (Common Emitter amplifiers) and common source amplifiers (used in MOSFET technology) will be examined. Understanding the design guidelines is crucial as they guide engineers in creating effective circuits that employ active loads.

Designing an electronic circuit is like planning a recipe. Just as a chef needs a list of ingredients and instructions to create a dish, an engineer needs design guidelines and specific requirements to create effective electronic circuits.

So, here we do have one example. In the previous discussion, we already have mentioned that instead of having a passive load we like to use active load and the purpose of this one we have discussed that to enhance the voltage gain. And here we do have the different parameters of the two transistors for Q1 β is 100, for Q2 β is 200, just for a change we are taking different value of β.

This section emphasizes the need for using active loads instead of passive ones to achieve enhanced voltage gain. Various parameters such as beta (β) of the transistors are listed, indicating how different characteristics can affect circuit performance. Beta is the current gain of a transistor and plays a significant role in determining the efficiency and output of an amplifier.

Consider two athletes with different levels of performance. Athlete Q1 (β=100) is a decent runner, while athlete Q2 (β=200) is exceptional. Using these different types of athletes in a relay can affect how quickly the team finishes the race, just as different transistor parameters can influence an amplifier's performance.

In fact, it is just compensating that so, to achieve that the value of this RB2 is two times of RB1 just to compensate the β difference.

This segment discusses the adjustments made in bias resistors to balance the collector currents of two transistors with different beta values. This balance is essential to ensure consistent performance in amplifying signals, preventing distortion caused by imbalances.

Imagine two friends trying to synchronize their running pace in a race. If one is faster, the slower friend (the resistors) needs to step up their game (the compensation) to keep up. Similarly, adjusting resistor values helps match the performance of transistors.

So, to get the DC voltage at the output node say VOUT, what we can do? we can compare βI × (VCE1) of transistor-1 = βI of transistor-2 × (VCE2) of transistor-2.

In this chunk, the method to calculate the output voltage using the relationship between the collector currents and voltages across the transistors is introduced. Ensuring both collector currents are equal leads to the calculation of the output voltage, which is crucial for amplifier functionality.

Think of the output voltage calculation like balancing a scale. Both sides must weigh the same for the scale to be level. Just as it is essential to balance weights, it’s vital to ensure that the collector currents are equal for proper amplifier operation.

So, we do have I1 = I2 = 2 mA, VCE1 = VCE2 = 6 V.

The operating points for both transistors are defined as equal, establishing a foundation for further calculations. These operating points include crucial parameters like collector current and voltage, which are essential for the amplifier's performance analysis.

Setting the operating points is similar to calibrating a machine. Just as a technician makes sure all settings are correct for optimal performance, engineers must ensure that transistors operate at the right points for efficient signal amplification.

From that we can calculate the small signal parameters of the transistors namely g = thermal equivalent voltage 26 mV.

This portion focuses on calculating small signal parameters like transconductance (g) and output resistance, which are important for analyzing the amplifier’s response to small input signals. Small signal analysis helps predict how the amplifier will behave in typical operating conditions.

Small signal analysis can be likened to the delicate adjustments musicians make to fine-tune their instruments before a concert. Just as fine-tuning improves sound quality, analyzing these small signals enhances amplifier performance.

To find the output resistance what we have to do? We can simulate this circuit by a signal called vx and then we can observe the corresponding current here.

Here, the process of determining input and output resistances using signal simulation is introduced. Understanding resistances is vital for determining how the amplifier interacts with different circuit loads, impacting overall performance.

Calculating resistance is like gauging how much weight a bridge can hold. Just as engineers evaluate a bridge’s tolerances to ensure safety under load, electrical engineers must know input/output resistances to ensure amplifiers handle signals without distortion.

So, in summary what we can say that the cutoff frequency it is getting reduced gain got increased and output resistance also got increased.

This final chunk summarizes key performance parameters when comparing active and passive load circuits. It explains that while active loads increase gain, they also lead to a reduction in bandwidth, which can affect the amplifier's applicability in certain scenarios.

Comparing active and passive circuits is much like evaluating sports equipment. High-performance gear (active loads) may boost performance in one aspect but could compromise versatility; while standard gear (passive loads) might be less efficient but offers more all-around utility.

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