68.1.3. Numerical Examples and Design Guidelines

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 introduction, the professor welcomes students back to a course on Analog Electronic Circuits. He indicates that the focus of the day's lesson will be on numerical examples related to amplifiers with active loads, which they have previously discussed. This sets the tone for practical learning where theory is reinforced with calculations and examples.

Think of this part of the lesson like a chef introducing a new recipe in a cooking class. Just as the chef explains what dish they will be making and how it will build on previous cooking skills learned, the professor is outlining the specific focus on numerical analysis to deepen understanding of amplifier 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.

The professor explains that today's agenda will not only involve numerical examples but also include design guidelines for two types of amplifiers: the Common Emitter (CE) amplifier and the Common Source amplifier. The mention of active load implies strategies to enhance performance, such as higher voltage gain. This prepares students to apply what they learn in practical scenarios.

Imagine you are learning to build a structure. First, you see some blueprints (numerical examples), and then you get the guidelines on how to choose the right materials and tools (design guidelines) to ensure the structure is strong and effective. The session is designed to bridge theoretical knowledge with practical application.

We 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.

The professor emphasizes the transition from passive loads (which do not provide gain) to active loads that improve voltage gain in circuits. Active loads are typically used in amplifier design because they provide better performance by maximizing the use of voltage levels available in the circuit. This concept is critical in analog circuit design since higher gain means better amplification of signals.

Consider a musician using an amplifier to boost sound levels. If the amplifier is designed poorly (like a passive load), the sound will not be very loud. However, if the amplifier is optimized (like an active load), it enhances the music significantly, allowing it to reach broader audiences—just as active loads boost the electrical signals in circuits.

The different parameters of the two transistors for Q1 β is 100, for Q2 β is 200. V_BE(on) for both transistors we are approximating it is 0.6 V. The early voltage of transistor-1 is 100 V; likewise consider early voltage for transistor-2 also 100 V.

This chunk provides specific parameters for two transistors being analyzed in the example. Beta (β) represents the current gain, Voltage V_BE(on) indicates the voltage across the base-emitter junction when conducting, and Early voltage impacts output characteristics of the transistors. These parameters are crucial as they impact how the transistors will behave within the circuit, especially in amplification.

Think of beta (β) in a transistor like the reputation of students in a school — some are more influential in groups (high beta), while others have lesser influence (low beta). This helps determine how they will perform in collaborative projects (like transistor currents in a circuit) based on their influence.

The collector current of transistor-1 (assuming active region of operation) is β × I, and supply voltage minus V_BE divided by R_B1, which gives us 2 mA. For transistor-2, the collector current is β multiplied by the same supply voltage ‒ V_BE divided by R_B2, which also yields 2 mA.

In this chunk, the professor details how to calculate the output current for each transistor. The collector current is derived from the relationship between the transistor's beta value, the voltage supply, and the resistances used in the circuit. Both currents equalize to 2 mA, which is essential for the stable operation of the amplifier.

Imagine each collector current as a flow of water in two parallel pipes. If one pipe has a larger diameter (higher beta), it allows more water through. However, to ensure both pipes carry the same amount of water (2 mA), adjustments need to be made, much like how the design compensates for different beta values to maintain system balance.

To get the DC voltage at the output node, we compare βI × (V_CE1) = βI × (V_CE2), which shows both transistors are equally loaded and gives V_OUT = 6 V.

This section explains calculating the output voltage in the circuit by equating the output states of both transistors. Since both transistors share similar parameters and the system is balanced, their contributions lead to a resultant output voltage of 6 V, which is crucial for the amplifier's operation.

Think of it as two equally skilled cooks preparing a dish; when both contribute the right ingredients in the correct proportions, the final dish (output voltage) turns out perfect. Their collaboration ensures consistency and effectiveness, resulting in a well-balanced meal.

From the obtained parameters, we can calculate the small signal parameters of the transistors namely transconductance g_m and output resistance r_o to determine the voltage gain.

This part introduces the small signal model, where essential parameters such as transconductance (g_m) and output resistance (r_o) are calculated. The small signal model helps in understanding how the amplifier will perform under varying signal conditions. This step is foundational to finding the amplifier's overall gain.

Imagine tuning a musical instrument—small adjustments (small signals) to the tension of the strings affect the sound quality (gain). Similarly, analyzing the small signal parameters reveals how slight changes impact overall circuit performance.

The upper cutoff frequency can be influenced by load capacitance and output resistance, which is calculated using the formula involving these two elements.

The discussion on cutoff frequencies outlines how capacitance and resistance affect the range of frequencies an amplifier can efficiently handle. The professor provides a formula that emphasizes the relationship between these components in determining the upper frequency limit.

Consider a fence that controls how many animals can flow through a gate simultaneously. If the gate is too small (high resistance), fewer animals can pass through (lower cutoff frequency). On the contrary, a wider gate would allow more animals to flow freely, thus enhancing the amplifier's ability to handle higher frequencies.

Summarizing the performance of the CE amplifier with active load, we can compare voltage gain, input resistance, output resistance, and bandwidth with a passive load circuit.

In this concluding chunk, the professor synthesizes all findings to compare the active load amplifier performance against a passive load. Key parameters such as gain, input resistance, and bandwidth are specifically highlighted, demonstrating the advantages of using active configurations in circuits for enhanced performance.

Think of a car (amplifier) performance comparison. One car (active load) has better acceleration (higher gain), while another (passive load) is slower but smoother on the road. The comparison helps you assess which option would better suit your driving needs, much like choosing between amplifier designs based on performance requirements.

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