66.1. Analog Electronic Circuits

Dear students welcome back to NPTEL online certification course on Analog Electronic Circuit. Myself Pradip Mandal from E and EC department of IIT Kharagpur. So, to continue this course today’s topic of discussion it is Amplifier with Active Loads. We may be having multiple amplifiers but, primarily we will be talking about common emitter and common source amplifier today.

In this introduction, Professor Pradip Mandal sets the stage for the topic of 'Amplifiers with Active Loads.' He notes that the course will focus on the common emitter (CE) and common source (CS) amplifier configurations, which are essential types of analog amplifiers. Understanding these amplifiers will help students grasp the general principles of analog circuits and their applications.

Think of amplifiers as microphones that capture sounds (signals) and amplify them so everyone can hear clearly. In this case, the common emitter and common source amplifiers serve as different types of microphones with unique characteristics suited for varying environments.

In fact, incidentally when we talk about active load, the amplifier it is having multiple transistors. So, you may say that this is also a special kind of multi transistor amplifiers. But we like to explicitly say that it is having unique characteristics, where the passive load it is getting replaced by a load utilizing MOS transistor or BJT transistor.

This chunk explains that active load amplifiers often utilize multiple transistors, distinguishing them from simpler amplifier designs. The 'active load' refers to using other transistors (like MOSFETs or BJTs) to provide greater performance than traditional passive loads (like resistors). This change enhances the amplifier's efficiency and voltage gain, making it suitable for modern electronic circuit designs.

Imagine replacing a standard bicycle tire (passive load) with an advanced tire technology that improves speed and efficiency (active load). Just as that technology can enhance performance, active loads in amplifiers enable better signal amplification and control.

Now, today what we are planning to cover it is under this active load amplifiers, we do have to start with we do have motivation of using this active load. Then, from that we will be talking about basic operation of amplifier having active load and their corresponding circuit analysis including, small signal model and then finding the gain or maybe intuitively explaining the gain and so and so.

The speaker indicates that the discussion will begin by exploring why active loads are preferred in amplifiers. He outlines that active load amplifiers will be dissected according to their basic operations, circuit analysis, and performance metrics, such as the gain. Understanding these motivations will prepare students to appreciate the practical advantages of employing active loads in circuit designs.

Consider why a company opts for updated machinery (active loads) instead of outdated models (passive loads). While older models may still work, the new machinery can produce better quality products faster and more efficiently. This is similar to how active load amplifiers can perform better than those with passive loads.

This is a recapitulation or recalling whatever we know about CE amplifier and not only we will be talking about CE amplifier. But basic operation of the CE amplifier just to see that, what is its limitation of the voltage gain. In fact, if you recall that this is the main amplifying transistor and it is at the input we do have the signal we are feeding along with the DC component.

Here, the focus shifts to understanding the limitations of the voltage gain in common emitter (CE) amplifiers. The CE amplifier has certain structural benefits but faces constraints in gain due to how the voltage is managed within the circuit. It's crucial for students to recognize these limitations when designing circuits, as knowing these parameters informs amplifier efficiency and effectiveness.

Imagine trying to lift a weight with a pulley system; while the system gives you an advantage, there is still a limit to how much it can lift based on the components' strength and the design. Similar to this, while CE amplifiers enhance signals, they too have maximum limits based on their configuration.

To come to the basic at the base, what we are doing is we are changing the voltage at the base or either you say base voltage or base to emitter voltage. And if you observe the base current it is getting converted into collector current.

This section delves into the operation of the CE amplifier, focusing on how input voltage at the base influences the output current at the collector. When the voltage at the input changes, it causes variations in the base current, which are amplified in the collector current. This mechanism is essential for students to grasp how voltage input translates to amplified current output, laying the groundwork for further understanding of gains.

Think about a water tap connected to a hose: as you increase the water pressure (base voltage), the flow of water out of the hose (collector current) also increases. The more you turn the tap, the greater the flow, just like how increasing the input voltage leads to increased output current in the amplifier.

Now then if we consider the load line characteristic, as you have discussed load line characteristic it is given by essentially I-V characteristic of this R.

In this segment, load line characteristics are reviewed, which illustrate the relationship between the current through and voltage across the load resistor in the CE amplifier. Understanding this relationship is crucial for assessing amplifier performance and achieving desired operational points.

Picture a seesaw: the load line represents the balanced position where one side raises as the other lowers. If you want the seesaw to lift higher (amplifier output) while ensuring balance, you need to strategize the weights and positions (characteristics of the load line).

So, the gain is essentially slope of this mirror multiplied by reciprocal of the slope of the other mirror. Why the reciprocal? That is because, this second mirror it is converting the y-axis into the x-axis.

This chunk explains the method for calculating amplifier gain by relating it to the slopes of the I-V characteristics. The gain can be represented as the ratio of these slopes, where the steepness of one slope affects the gain calculation. Recognizing this relationship is vital for understanding how design choices impact amplifier performance.

Think of how a stair ramp's angle affects how easy it is to push a wheelchair up. A steeper angle (higher slope) requires more effort compared to a gentler slope (lower slope), just like different amplifier designs can greatly affect their gain performance.

So, anyway so what we said is that limitation of the voltage gain or the standard CE amplifier namely CE amplifier with passive load and it is gain it is primarily it is getting restricted by the voltage drop across this resistance divided by V.

This conclusion summarizes that the limitations of the voltage gain in CE amplifiers primarily stem from the voltage drop across passive loads. When designing amplifiers, recognizing these limitations drives engineers to explore alternative solutions such as active loads for better performance.

Imagine a car trying to climb a steep hill with a heavy load. Just as the car’s power is limited when faced with the heavy load (like passive components), the amplifier's gain is capped by its configuration. Finding lighter alternatives lets the vehicle ascend more effectively, akin to using active loads in amplifiers for better gain.

--